ANNUAL REVIEWS
Further
Quick links to online content
Ann. Rev. Genet 1976. 10:179-207 Copyright © 1976 by Annual Reviews Inc. All rights reserved
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
GENETICS OF SPECIFIC
+3103
COGNITIVE ABILITIES! J. C. DeFries, S. G. Vandenberg, and G. E. MeC/earn Institute for Behavioral Genetics. University of Colorado, Boulder. Colorado
80309
CONTENTS ...................................... . ........... . ..... ..... . .... . ......... Behavioral Phenotypes .............. .............. ................. . . . . . . . . . . . . . . . . . . . . . . . . . . . ............... ....... Historical Development ............................................ . .................................... . ....... ... Parallel Between Classical Test Theory and Quantitative Genetic Theory . ...........
180 180 181 183
. . ........ .. Twin Studies ..... ..... .............. ........... ..... ............ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................ ......... Family Studies ............ ................. ........ . . . . . . . . . . . . . . . . . . ....................... ..................... .......
185 185 188
.... ............ ....................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................... Differential Effects of Mental Retardation Syndromes .......................................... Spatial Ability ... .. . ...... .. .. .. . ......... . .. . ..... . . .... . .. . . .. . ...... . .. . ........ . ........... .. ... . ................... Reading ....................................................................................................................
196 196 198 201
PSYCHOMETRIC CONSIDERATIONS
EVIDENCE FOR THE HERITABILITY OF SPECIFIC COGNITIVE ABILITIES
SPECIAL CASES
PROMISSORY NOTE
.
. .
. .
............ ........... . ............................ .. ..........................................
CONCLUDING REMARKS
.
. .
.
.
.
. .
. .
.
... ..... . .......... .......... ..... ................... . .................. . ...........
202 202
Behavioral genetics was reviewed in 1960 (39), 1966 (78), 1971 (65), and 1974 (IS) for the Annual Review of Psychology and in 1970 (76) for the Annual Review of Genetics. In 1972, human behavioral genetics was reviewed by Childs (20) for the Annual Review ofMedicine. Lewontin (64) authored a critical review entitled "Ge netic Aspects of Intelligence" for the 1975 Annual Review of Genetics, but focused entirely upon human IQ studies. As an indication of the growing interest in this area, a quarterly journ al (Behavior Gen etics) was founded in 1970, and a professional organization (Behavior Genetics Association), which now has more than 300 members, was organized in the same year. The growth of this interdisciplinary field has been so rapid that it would be difficult to review adequately all developments between 1970 and 1976. Therefore, rather than attempting a general review of behavioral genetics, we have chosen to 'Supported in part
by a grant from the Spencer Foundation. 179
180
DEFRIES, VANDENBERG & McCLEARN
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
focus upon only one important behavioral domain, namely, specific cognitive abili ties. Even in this narrowed range, it has been necessary to be quite selective. Our primary objective is to communicate the principle that cognitive ability is far too complex to be assessed by a univariate number such as IQ and that a multivariate approach is required. Toward this end we briefly discuss the history of the develop ment of cognitive test instruments and the operational philosophy of test and measurement theory. This discussion is followed by a review of the evidence con cerning the inheritance of specific cognitive abilities. PSYCHOMETRIC CONSIDERATIONS
Behavioral Phenotypes The phenotypes studied by geneticists have a vast variety of observational require ments. A simple dichotomous classification requires only the ability to perceive differences between the classes. Assessment of continuously distributed characters, however, is more difficult. In addition to the purely statistical considerations, there can be very important problems of measurement. For some "quantitative" charac ters, measurement is relatively straightforward. In dealing with scutellar bristles, for example, it may only be necessary to count. For other characters, such as body height, there may be a fairly obvious application of some fundamental standard of measurement. On the other hand, fundamental comparison operations are not appropriate for some characters, and derived measures are required. Although the problems of derived measurement are by no means limited to the behavioral domain, they are very pronounced in this area. The difficulties have required psychologists to examine the problem of the definition of behavior with great care. As a consequence, certain aspects of operational philosophy and logical positivism have been woven into the fabric of psychological thought. Stevens (108) has succinctly summarized this philosophy as follows: A term denotes something only when there are concrete criteria for its applicability; and a proposition has empirical meaning only when the criteria of its truth or falsity consist of concrete operations which can be performed upon demand. (p. 228)
In many ways, pointing to exemplars of a concept is the most satisfactory sort of definition. But many concepts are not concrete and require definitions which themselves need explication. Herein lie numerous traps for the unwary, because mutuality of understanding of the defined concept depends upon the degree to which the meanings of the definitional words are shared. Or, as Larrabee (61, p. 248) has warned: "talk without mutual understanding is just a means of preventing silence." Our dictionaries may state that intelligence is "the faculty of thought or reason," or that cognition is "the mental process or faculty by which knowledge is acquired." However, unless we have some way of reducing terms such as faculty, thought, reason, mental, and knowledge to operations, these magniloquent definitions are, for scientific purposes, simply gibberish. For research purposes, there are many different operational definitions of cogni tive functions. For example, performance level on a particular test is one definition
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
GENETICS OF SPECIFIC COGNITIVE ABILITIES
18 1
of intelligence; another definition involves correlational analyses. As we shall see from the following review, we should really speak of intelligences, rather than of intelligence. Were cognitive functioning of interest only to specialists, these points would not need elaboration. But intelligence is of concern to the population at large, and everyone has an understanding, however vague, of what it means. The tacit assump tion that there is only one meaning, that it is shared by all, and that it is the same as the meaning employed by the research specialist has created confusion and probably accounts for a considerable part of the controversy that now swirls around the subject. In a brief review such as this, it is not possible to provide all of the operational definitions for all of the cognitive functions discussed. We hope, however, that some appreciation for the diversity of definitions can be gained from the context. Detailed consideration of these matters will require reference to the original works.
Historical Development The idea of a dormant or innate attribute waiting to be developed underlies the concepts of ability, aptitude, and faculty which occur in most European languages (47). The Greek philosophers enumerated many separate attributes such as memory and imagination, and the list was further elaborated by writers of the Renaissance and those of the 18th and 19th centuries. During the latter half of the 19th century, Sir Francis Galton experimented with various measures of sensory discrimination, including bisecting a line and judging differences between small weights. From his research, Galton (40, p. 20) concluded the following: "The trials I have as yet made on the sensitivity of different persons confirms the reasonable expectation that it would on the whole be highest among the intellectually ablest." This view was a logical outcome of the British empiricist philosophy which held that all knowledge was acquired through the senses. James McKeen Cattell, who first used the term mental test in 1890, worked with Galton in England and later brought his ideas to the United States. Cattell experi mented with measures such as keenness of eyesight and hearing, reaction time, afterimages, color vision, perception of pitch, and judging a to-second interval ( 19). Other early mental tests were developed during the latter part of the 19th century by Oehrn (90), Munsterberg (86), Kraepelin (60), and Ebbinghaus (29). In 1904, Alfred Binet, a gifted experimental psychologist at the Sorbonne, was appointed to a commission responsible for devising a national system of education for the mentally retarded. The commission concluded that no child should be sent to a special school unless an examination revealed that "his intellectual state renders him unable to profit in the ordinary measure from the instruction given in the regular schools" (9, p. 163). Binet & Simon developed scales (9- 1 1) which were highly effective for this purpose. The test was later revised for administration in the United States by Terman ( 1 10) and by Terman & Merrill ( 1 1 1) at Stanford Univer sity. Success of the Binet-Simon test led to increased interest in mental testing. In 1917, the US War Department began a program of mental testing for the classification of
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
182
DEFRIES, VANDENBERG & McCLEARN'
military men, The test employed, the Army Alpha (128), was to a large extent a creation of Otis, a graduate student of Terman's. It was a "paper and pencil" test which could be administered by relatively unskilled personnel to large groups. Eventually, over 1,750,000 men were tested. A second version, the Army Beta, was developed for use with illiterates or non-English speaking recruits. After World War I, there was a rapidly increasing interest in the use of intelligence tests in industry, education, and civil service; it is likely that this was due at least in part to the fact that so many people had been exposed to mental tests during the war. In the meantime, Charles Spearman, an English army officer who later became a professor at the University of London, attempted to provide a theoretical basis for the concept of intelligence He was influenced by Galton and reasoned that even weak tests might be useful ifit could be shown that a combination of such tests could predict a criterion such as school grades. Spearman developed a statistical test to determine whether different tests were measuring something in common and some thing specific to each test. In this two-factor theory (104), the first factor was ascribed to general intelligence (or g), whereas the second was a specific factor unique to each test. Spearman's two-factor theory did not provide a solution to the problem of the nature of intelligence. Later investigators (58) proposed the existence of other "group factors" in addition to g. For example, in 1935, EI Koussy (30) identified a spatial group factor which he symbolized "k:" After several such group factors were recognized, a hierarchical theory of intelligence was proposed by Burt (17) and Vernon (121). In this model, illustrated in Figure I, four kinds of factors are assumed: (a) a general factor; (b) two major group factors (verbal-education and spatial-mechanical); (c) an undetermined number of minor group factors, some of which can be still further subdivided; and (d) factors specific to each test. The most prominent advocate of multiple ability factors was Thurstone (11311S). In addition to developing a shortcut method for extracting factors from a correlation matrix, Thurstone (116) advocated rotating these factors to a new posi tion which could provide simple structure, where: (a) each variable is identified with only one or a small proportion of the factors; (b) the number of variables loaded on (correlated with) a factor is minimized; (c) the variance accounted for by the major unrotated factors is distributed across the rotated factors; and (d) each rotated factor is more or less identified with a distinct cluster of correlated variables g
Major group factors
I
v:ed
I
I
k:m
� � I fJn rll � I r=t-,
Minor group factor
Specific factors Figure 1
1111111111111111111111111111111111111111
Hierarchical structure of human abilities. Reprinted from Reference 121.
GENETICS OF SPECIFIC COGNITIVE ABILITIES
183
(see Rummel, 98). Using these techniques, Thurstone identified seven uncorrelated factors which were found to recur across a number of large studies: verbal meaning or vocabulary (V); verbal fluency (W); numerical ability (N); spatial ability (S); inductive reasoning (I or R); memory (M); and perceptual speed (P). When later searching for simple structure by plotting test loadings on pairs of factors, Thurstone noted that more near-zero values could be obtained when factors were allowed to correlate. Such axes and the resulting factor structure are said to be "oblique," in contrast to the orig inal orthogonal" factor structure. Once factors are allowed to correlate, reSUlting factor correlation matrices may again be factor analyzed to produce higher order factor solutions. Thus, a compromise was reached between Spearman's theory of one general plus many specific factors and Thur stone's multiple factor theory. With the advent of electronic computers, the full characteristic equation repre sented by the correlations between n tests could be solved readily to yield n roots and, hence, n factors. Thurstone's concepts of rotation, simple structure, and oblique factors have survived this technical revolution. For a detailed account of factor analysis and the various methods of approximating simple structure, see Harman (45) and Rummel (98). In contrast to Thurstone, Guilford (43) insisted upon uncorrelated factors and attempted to broaden the content and format of psychological tests. Guilford recog nized five psychological processes which can be performed on four types of materi als, where the information can be packaged into six kinds of products. Because these three classification criteria are postulated to be independent, there would be 5 X 4 X 6 120 uncorrelated ability factors (see Figure 2). While Guilford's theory has stimulated considerable research, few investigators have accepted his plethora of factors. What consensus there is seems to favor the hierarchical view (49).
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
"
=
Parallel Between Classical Test Theory and Quantitative Genetic Theory It is interesting to note that a marked similarity exists between the mathematical models of psychological test theory and quantitative genetics, as well as between the parameters derived from these models. In classical test theory (see 67), the following linear model is assumed: X=T+E,
1.
where X is the observed test score of an individual, T is the true score, and E is the error of measurement. In quantitative genetic theory (see 32), a similar model is assumed:
P= G +E.
2.
where P is the observed phenotypic value, G is the genotypic value (or the value conferred upon the individual by its genotype), and E is the environmental devia tion. Two tests are said to be parallel if they yield identical true scores and if the errors are uncorrelated and have equal variances. The analogous situation in quantitative genetics is the case of identical twins reared in random environments. In such a case,
184
DEFRIES, VANDENBERG & McCLEARN
OPERATIONS Cognition Memory
Diver ent Production Convergent Production Evaluation
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
PRODUCTS
Units Classes Relations Systems Transformations mplications
CONTENTS Figural Symbolic Semantic
Behavioral
Gui lford' s ability factors.
Figure 2
members of the twin pair would have identical genotypic values and the environ mental deviations would be uncorrelated. From the above model of test theory, the following result obtains: Vx
=
Vr + VE>
3.
where V symbolizes variance. The proportion of the variance in observed scores owing to variation in true scores is defined as the reliability of a test. It may be shown that reliability has the following equivaknt forms:
reliability = VTI Vx =
r
iT
=
bTX
=
'XX�
4.
where 'XT is the correlation between true and observed scores, bTX is the regression of true score on observed score, and 'XX' is the correlation between performances
on parallel forms of a test. From the model of quantitative genetics, 5.
when G and E are uncorrelated. The proportion of the phenotypic variance owing to variation in genotypic values is defined as "heritability in the broad sense" (68) and has the following equivalent forms: heritability = VGI Vp = r
�p
= bGP = rppl
6.
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
GENETICS OF SPECIFIC COGNITIVE ABILITIES
185
where P and P' are measures on members of identical twin pairs reared in random environments. In test theory, the correlation between performances on the same test adminis tered at two different times provides a measure of "test-retest" reliability. In quanti tative genetics, the correlation between successive measurements on an individual is referred to as repeatability, and it provides an upper-bound estimate of heritabil ity. Thus, it is seen that the basic models and concepts of psychological test theory and quantitative genetics are highly similar. EVIDENCE FOR THE HERITABILITY OF SPECIFIC COGNITIVE ABILITIES Twin Studies
In his 1975 Annual Review of Genetics article concerning the heritability of IQ, Lewontin (64) completely dismissed the results of twin studies: I have quite deliberately not discussed the large number of simple comparisons of dizy gotic and monozygotic twins . ..., because any such comparisons must assume that the environmental similarities for dizygotic twins are the same
as
for monozygotic [italics
added]. The environmental dissimilarities will be completely confounded with genetic differences. (p. 396)
But how much weight should be given to the scanty evidence, as contrasted to intuition, of greater environmental dissimilarities within fraternal (DZ) than within identical (MZ) twin pairs? The most recent treatments of this topic have been by Scarr (99) and Vandenberg (120). After reviewing the evidence, Scarr concluded that "genetic relatedness" of twins determines the similarity of their treatment and that this is not necessarily evidence for environmental bias. Evidence that some abilities maybe more heritable than others is presented in Tables 1 through 4. F ratios of the within-pair variances of DZ and MZ twins are tabulated for subtests of the Primary Mental Abilities Test in four studies, for the Differential Aptitude Test in two studies, for the Wechsler Adult Intelligence Scale (WAIS) in one study, and for a number of separate tests in the Swedish study. It may be seen in these tables that the rank order of F ratios (from highest to lowest) tends to be as follows: spatial, vocabulary, word fluency, speed in simple arithmetic, and reasoning. Further evidence for the differential heritabilities of specific cognitive abilities may be found in Vandenberg (119). Is there evidence for the existence of genetically independent cognitive abilities? Modest attempts to answer this question have been made by Vandenberg (118), by Loehlin & Vandenberg (66), and by Bock & Vandenberg (14). If we can assume that the environmental influences produce the same degree of covariance for pair differ ences in the two types of twins, one can subtract the MZ from the DZ covariances to obtain an estimate of the genetic covariances (66), or one can generalize the F test that is used in univariate studies to determine the significance of the differ ence between the MZ and DZ within-pair variances (14). Instead of evaluating . . 2 / 2 F = (TwDZ (T wMZ, one ascertains h ow many elgen va1ues are sign! " ficant In ' the characteristic equation, .
186
DEFR IES, VANDENBERG & McCLEARN
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
7.
In practice, this number is usually equal to the number of eigen values greater than or equal to unity, although with increasing sample sizes more eigen values will acquire statistical significance. Using the procedure of subtracting MZ from DZ covariances, Loehlin & Vanden berg (66) concluded (on the basis of rather small samples) that the same ability factors appeared in the environmental and in the genetic covariation among 15 scales of Thurstone's Primary Mental Abilities Test. However, there were three oblique higher-order factors (number, space, and verbal-educational) in the environ mental covariation, while only one general factor appeared in the genetic covariance. Table 1 F ratios of the within-pair variances of dizygous and monozygous twins for sub tests of the Primary Mental Abilities ( P MA) Testa
Name of P MA subtest
----�."
Verbal Space Number Reasoning Word fluency Memory
Blewett 1954 3.13b 2.04c 1.07 2.7Sb 2.7Sb not used 26 DZ
26 MZ a Reprinted from Reference 119. bp < 0.01. cp < 0.05.
Thurstone 1955 2. S1b 4.19b
Vandenberg ( Michigan) 1962 2.65b
Vandenberg (Louisville) 1966
1.52 1.35 2.47b 1.62 53 DZ
1.77c 2.5Sb 1.40 2.57b 1.26 37 DZ
1.74c 3.51b 2.26b 1. 10 2.24b not used 36 DZ
45 MZ
45 MZ
76 MZ
Table 2 F ratios of the within-pair variances of dizygous and monozygous twins for the Differential Aptitude Testa
Verbal reasoning Numerical ability Abstract reasoning Space relations Mechanical reasoning Clerical speed and accuracy Language useI: Spelling Langu age use II: Sentences
a Reprinted from Reference 119. b p < 0.01.
1961 2.29b
1965 2.3Sb
1.39 1.47 1.67 1.36 2.54b 3.64b 3. 06b 25 DZ
1.37 1.23 2.19b 1.463.13b 2.58b 2.00b 86 DZ
47 MZ
109MZ
GENE TICS OF S PEC IF IC C OGN IT IVE A BIL IT IES
187
Table 3 F ratios of the within-pair variances of 60 dizygous and 60 monozygous twins for the scaled scores of II subtests of the Wechsler Adult Intelligence Scalea F
Subtest
3.88b 2.2Sc
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
Information Comprehension
2.78b
'Arithmetic Similarities Digit span Vo cabu lary Digit symbol
3.14b 2.06c
Picture completion Block d esign Picture arrangement
l.74d
1.8Id
1.S3d
LSD 2.3Sc
Object assembly
1.36
Verbal score Performance score Tot al s cor e
3.3Sb
3.41b 3.47b
aReprinted from Reference 119. bp < 0.001. c p < om. dp < 0.05.
Table 4 F ratios of the within-pair variances of like-sexed dizygous and monozygous twins for 14 psychological tests. administered to Swedish twins of elementary and high school agea
Simplex C-test Verbal analysis Form perception Picture perception Number perception Number series Number analysis Numerical classification Numerical reasoning Routine simple arithmetic Memory for 2-digit numbers (recall) Memory for 3-dig it numbers (recognition) Paired associates word-numb er memory
Boys
Girls
1.98b
2.84b
1.14
0.96
2.41b I.Slc
1.17
4.3Sb
1.23 I.S4c
All
cases 2.38b
3.37b
1.12
1.34c 1.36c
1.58c
1.91b
1.59b
1.61c
1.68c
1.63b
2.83b
1.96b
2.18b
1.15
1.39
1.24
2.37b
1.47
1.87b 0.94 1.32
1.7Dc
1.64c 1.Sle 1.34
2.01b l.57b
1.68b 1.17
66 DZ
1.00 75 DZ
141 DZ
66MZ
62MZ
128MZ
a Reprinted from Reference 119, ased on Reference 122. b bp < 0.01. cp < 0.05.
1.16
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
188
DEFR IES, VANDEN BERG & McCLEARN
Applying the second method to twin-pair differences on the Differential Aptitude Test, three dimensions were significant in the genetic covariance for boys and two for girls ( 1 4). In a large Finnish study of adult twins (91), a set of tests measuring six cognitive abilities (verbal, word fluency, space, number, memory, and reasoning) was adminis tered. A multivariate generalization of the F test led to the conclusion that the genetic covariance in the six abilities had six significant roots, that is, there were six independent genetic factors. On the other hand, when Eaves & Gale (28) reanalyzed the data of Loehlin & Vandenberg (66) with a method by which the additive genetic and dominance components of covariance could be estimated, they conclUded that a single genetic component fit well enough to suggest an absence of strong major gene effects for the abilities tested by the Primary Mental Abilities Test. It is clear that future studies will have to utilize larger samples, and more tests, to answer conclusively the question concerning the existence of genetically indepen dent abilities.
Family
Studies
Another methodology which has been employed to study the genetics of specific cognitive abilities is the family study. Although relatively few such studies have been reported to date, a major report was published by Willoughby (125) in the I 920s. He administered a battery of 11 individual tests of verbal and nonverbal abilities to 14 1 children between 12.5 and 13.5 years of age and to members of their families (90 fathers, 100 mothers, and 280 sibs). It may be seen from Table 5 that the average correlation for the verbal tests exceeded that for the nonverbal tests for all parent offspring combinations and for two of the three sib combinations. Since there is no difference between the spouse correlations, the greater parent-offspring and sib resemblance for verbal ability may be due either to its higher heritability and/or to a larger environmental correlation between relatives. Five years later, a family study of arithmetic and vocabulary abilities was reported by Carter ( 1 8). Four arithmetic subtests and two vocabulary tests were administered to members of 108 families in which both parents and one or more children over 12 years of age were available for testing and to 3 1 incomplete family groups. From Table 5
Familial correlations for verbal and nonverbal abilitiesa Verbal average
Nonverbal average
0.41
0.29
Mother-daughter
0.41
0.31
Mother-son
0.38
0.35
0.34
0.26
Father-son
Father-daughter Brother-sister
0.38
0.35
Brother-brother
0.50
0.37
0.40
0.52
0.44
0.44
Sister-sister Husband-wife aReprinted from Reference 125.
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
GENETICS OF SPECIFIC COGNITIVE ABILITIES
189
the resulting familial correlations presented in Table 6, it may be seen that the vocabulary tests yielded higher correlations for three of the four parent-offspring combinations and for all sib combinations. The greater spouse correlations for performance on the vocabulary tests may account at least in part for the higher parent-offspring and sib correlations. However, as in the Willoughby ( 1 25) study, a higher heritability and/or greater environmental correlation between relatives for verbal ability may also be involved. Three family studies of specific cognitive abilities have only recently been re ported. Utilizing a western Canadian sample, Williams (123) compared WISC subscale scores of 55 sons, 10 years of age, to WAIS scores of their parents. Although the same tests were not administered to both age groups, Williams (124) has shown that the two test batteries measure similar abilities. Spouse correlations and regressions of offspring on midparent value for the 11 subscales, for the verbal and performance IQ aggregates obtained from these subscales, and for total IQ are presented in Table 7. In agreement with previous evidence, the parent-son regres sions and spouse correlations tend to be higher on the verbal tests (subscales 1-6) than on the performance tests (subscales 7-11). This contrast is especially clear between verbal IQ and performance IQ. Note that the regression of offspring on midparent, which provides an upper-bound estimate for heritability, is only 0.46 for total IQ. Two other studies, the Hawaii Family Study of Cognition and the Boulder Family Study, are currently in progress. In both of these studies, data are being collected on 15 cognitive tests (see Table 8), various environmental indices, and blood group and enzyme systems. There are two major ethnic groups included in the Hawaii Study, Americans of Japanese ancestry (AJA) and Americans of European ancestry (AEA). Results of multivariate analyses have revealed that the cognitive structures of these two groups are nearly identical (26). Phenotypic correlations among the 15 cognitive variables were obtained for both AJA and AEA groups (N = 262 and 782, respectively), and principal component analyses with varimax rotations were performed. Four readily interpretable factors emerged from these analyses: spatial visualization, verbal, perceptual speed and accuracy, and visual memory. Common Table 6
Familial correlations for vocabulary and arithmetic abilitya Vocabulary
Arithmetic
n Father-son
n
0.21
116
0.10
116
Mother-daughter
0.34
136
0.24
136
Mother-son
0.07
126
0.12
127
Father-daugh ter
0.26
114
114
Brother-sister
0.31
III
0.04
Brother-brother
0.43
46
Sister-sister Husband-wife a Reprinted from Reference 18.
0.17 0.21
III 48
0.32
53
0.26
53
0.21
108
-0.04
108
190
DEFRIES, VANDENBERG & McCLEARN
Table 7 Spouse correlations and regressions of offspring on midparent for the Wechsler Intelligence ScalesB
Regression of offsp ring on midparent Spouse correlations
WISC/WAfS subscales 1.
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Information Comprehension Arithmetic Similarities Digit span Vocabulary Digit symbol (coding) Picture completion Block design Picture arrangement Object assembly Verbal fQ PerformanceIQ TotalIQ
BReprinted from Reference
Estimate
Standard error
0.65
0.25
0.13
0.48
0.26
0.13
0.19
0.45
0.12
0.41
0.36
0.15
0.06
0.13
0.15
0.58
0.53
0.10 0.15
0.20
0.56
0.37
-0.12
0.20
0.19
0.34
0.19
0.30
0.07
0.18
-0.07
0.02
0.24
0.62
0.55
0.35 0.61
0.22
0.09 0.17
0.46
0.10
123.
factor loadings of the 15 cognitive tests on these four varimax-rotated principal components are presented for both ethnic groups in Figure 3. The marked similarity of the factor-loading profiles for AJA and AEA subjects is obvious from this illustration. Coefficients of congruence between the two groups were 0.96 or larger for each of the four factors. When these analyses were subsequently repeated with a larger data set (N for AJA 648, N for AEA 1971), similar results were obtained (126). In fact, congruence coefficients calculated from the larger data set were all 0.99 or larger. Zonderman and associates (131) have subjected the data from the Boulder Family Study to principal components analysis and have obtained the same four varimax rotated factors. Coefficients of congruence between this mainland AEA sample and the Hawaiian AEA sample were all greater than 0.94. Taken together, these data demonstrate that the 15 cognitive variables provide measures ofthe same underlying abilities in Hawaiian AJA, Hawaiian AEA, and mainland AEA samples. Assortative marriage for specific cognitive abilities has been assessed in both the Hawaii and Boulder family studies (57, 131). Previous studies of assortative mar riage for cognitive ability were recently reviewed by Jensen (53), who reported that the unweighted mean of 43 spouse correlations for various tests of mental ability was +0.45. However, at least some of these correlations may have been inflated. When cross-sectional data concerning mental ability are examined, some change occurs as a function of age (see 126). Thus, when both older and younger couples =
=
GENETICS OF SPECIFIC COGNITIVE ABILITIES
191
Table 8 Administration time and reliabilities for tests utilized in the Hawaii Family Studyof Cognition and the Boulder Family Studya Cognitive Test
factor
Test time
Reliability
V
3min
0.96
Primary Mental Abi lities (PM A) Vocabulary
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
Visual Memory (immediate)
M
Things (a fluency test)
V
I-min exposure/
0.58
2parts/3min
0.74
I-min recall each
Sheppard-Metzler Mental Rotations
S
10min
0.88
(modified for group testing by Vandenberg) Subtraction and Multiplication
P
2parts/2min
0.96
each Elithorn Mazes ("lines and dots"),
S
5 min
0.89
V
2 parts/3min
0.71
shortened form Educational Testing Service (ETS)
each
Word Beginnings and Endings ETS Card Rotations
S
2 parts/3 min
0.88
each Visual Memory (delayed recall)
M
1min
0.62
PM A Pedigrees (a reasoning test)
V,S,P
4min
0.72
S
2parts/2min
0.92
ETS Hidden Patterns
eaeh S
3 min
0.84
ETS Number Comparisons
P
2parts/1.5 min
0.81
Whiteman Testof SocialPerception
V
10min
0.69
Raven's Progressive Matrices,
S
20min
0.86
Paper Form Board
each
modified form a Reprinted
from Reference 126. b Varimax rotated factor loading;;' 04 . 0
ceptual speed and accuracy, M
visual memory).
=
are included in a sample, some of the observed spouse similarity in mental ability is due to their similarity in age. For this reason, it is essential that age adjustment be employed in studies of assortative marriage for cognitive ability. As discussed below, method of test administration may also influence spouse correlations. Age-adjusted spouse correlations for couples included in the Hawaii and Boulder family studies are presented in Table 9. With regard to the Hawaiian samples, AEA spouse correlations are higher than those for AJA subjects on 13 of the 15 individual tests. However, the magnitude of the differences is not large (median AEA and AlA spouse correlations were +0.13 and +0.06, respectively). In addition, spouse corre lations for factor scores were highest for the verbal factor in both ethnic groups.
DEFRIES, VANDENBERG & M cCLEARN
192 1.0 .8
SPATIAL
VERBAL
.6 .4 .2
�
�
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
c:::i
a -.2 -.4
� ...,J
-.5
� "
1.0
(",)
�
.8
I I I PERCEPTUAL
--AEA --------
SPEED
AJA
VISUAL
MEMORY
.6 .4 .2 0 -2
4
-.
-!j
I I
3
Figure 3
7
9
13
15
I I
3
5
7
9
"
13
I I
15
Loadings of 15 cognitive tests on fou r varimax-rotated principal components in
Americans of Japanese ancestry (AJA) and Americans of European ancestry (AEA). Re printed from Reference 26.
Thus, it does not appear that ethnicity is a major influence on the extent and nature of assortative marriage in Hawaii. When the mainland AEA sample is compared with the Hawaiian samples, the mainland spouse correlations are somewhat higher (median value of +0.22). This difference, however, may be due in part to a difference in method of test administration. In the Hawaii study, tests are administered to large groups under highly standardized conditions (automated tape and slide presentation of instructions, etc). In contrast, tests in the Boulder study are usually administered to individual nuclear family units by various testers, so that differences among testers could cause increased spouse correlations in this study. Nevertheless, results of both studies suggest relatively less assortative marriage for cognitive abilities than previously reported. When effects of educational attainment were statistically re moved, resulting partial correlations became even smaller. Thus, there is very little assortative marriage for that part of cognitive ability which is independent of educa tional level (57, 131). Parent-offspring resemblance for specific cognitive abilities has also been exam ined in both the Hawaii and Boulder family studies (25, 105). When environmental effects are randomized among subjects, the regression of offspring's score on midpar ent value provides a direct estimate of heritability (32). However, because environ mental influences on at least some measures of cognitive ability may be common to parents and their offspring, regressions of offspring on midparent value for such characters should be regarded as measures of phenotypic similarity (genetic and/or environmental) and only as upper-bound estimates of heritability (77).
1 93
GENE TICS OF SPECIFIC C OGNI TI VE ABILI TIES Table
9
Age-adjusted spouse correlations Hawaii a AEA (N 555 couples)
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
=
Tests Vocabulary Visual Memory (immediate) Things MentalRotations Subtraction and Multiplication Elithorn Mazes ("lines & dots") Word Beginnings and Endings Card Rotations Visual Memory (delayed) Pedigrees Hidden Patterns Paper Form Board Number Comparisons Social Perception Progressive Matrices. Factors Spatial Verbal Perceptual speed and accuracy Visual memory First principal component (unrotated) a Reprinted from Reference 57. AEA icans of Japanese ancestry . b Reprinted from Reference 131.
0.27
(N
=
148
couples)
Mainland b AEA (N 1 23 couples)
0.16
=
0.36
0.07
0.15
0.09
0.22
0.08
-0.12
0.11
0.04
0.03
0.10
0.18
0.07
0.00 0.12 0.10
0.13
0.02
0.12
0.18
0.04
0.03
0.10
0.41
0.06
0.26
0.06
0.28
0.16
om
0.10
0.21
0.22
0.Q3
0.20
0.00
0.23
-0.05
0.13
0.19
0.20 0.13
0.22
0.13
=
Hawaii a AJA
0.20
0.00
0.00
0.13
0.28
0.13
Americans of European ancestry; AJA
0.36 0.35
0.09
0.35
0.41 0.05
0.35
=
Amer
Regressions of midchild on midparent obtained in the Hawaii study are presented in Table 10. These regressions have been adjusted for lack of perfect test reliability (an adjustment which is essential when estimates from tests with different reliabili ties are to be compared). Standard errors for the regression coefficients are suffi ciently small to suggest that at least some of the variability among tests is real. Another indication of this differential parent-offspring resemblance is the marked congruence between ethnic groups. The Spearman rank correlation between the 1 5 AEA and AlA regressions for the individual tests was +0.77 (p
Further
Quick links to online content
Ann. Rev. Genet 1976. 10:179-207 Copyright © 1976 by Annual Reviews Inc. All rights reserved
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
GENETICS OF SPECIFIC
+3103
COGNITIVE ABILITIES! J. C. DeFries, S. G. Vandenberg, and G. E. MeC/earn Institute for Behavioral Genetics. University of Colorado, Boulder. Colorado
80309
CONTENTS ...................................... . ........... . ..... ..... . .... . ......... Behavioral Phenotypes .............. .............. ................. . . . . . . . . . . . . . . . . . . . . . . . . . . . ............... ....... Historical Development ............................................ . .................................... . ....... ... Parallel Between Classical Test Theory and Quantitative Genetic Theory . ...........
180 180 181 183
. . ........ .. Twin Studies ..... ..... .............. ........... ..... ............ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................ ......... Family Studies ............ ................. ........ . . . . . . . . . . . . . . . . . . ....................... ..................... .......
185 185 188
.... ............ ....................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................... Differential Effects of Mental Retardation Syndromes .......................................... Spatial Ability ... .. . ...... .. .. .. . ......... . .. . ..... . . .... . .. . . .. . ...... . .. . ........ . ........... .. ... . ................... Reading ....................................................................................................................
196 196 198 201
PSYCHOMETRIC CONSIDERATIONS
EVIDENCE FOR THE HERITABILITY OF SPECIFIC COGNITIVE ABILITIES
SPECIAL CASES
PROMISSORY NOTE
.
. .
. .
............ ........... . ............................ .. ..........................................
CONCLUDING REMARKS
.
. .
.
.
.
. .
. .
.
... ..... . .......... .......... ..... ................... . .................. . ...........
202 202
Behavioral genetics was reviewed in 1960 (39), 1966 (78), 1971 (65), and 1974 (IS) for the Annual Review of Psychology and in 1970 (76) for the Annual Review of Genetics. In 1972, human behavioral genetics was reviewed by Childs (20) for the Annual Review ofMedicine. Lewontin (64) authored a critical review entitled "Ge netic Aspects of Intelligence" for the 1975 Annual Review of Genetics, but focused entirely upon human IQ studies. As an indication of the growing interest in this area, a quarterly journ al (Behavior Gen etics) was founded in 1970, and a professional organization (Behavior Genetics Association), which now has more than 300 members, was organized in the same year. The growth of this interdisciplinary field has been so rapid that it would be difficult to review adequately all developments between 1970 and 1976. Therefore, rather than attempting a general review of behavioral genetics, we have chosen to 'Supported in part
by a grant from the Spencer Foundation. 179
180
DEFRIES, VANDENBERG & McCLEARN
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
focus upon only one important behavioral domain, namely, specific cognitive abili ties. Even in this narrowed range, it has been necessary to be quite selective. Our primary objective is to communicate the principle that cognitive ability is far too complex to be assessed by a univariate number such as IQ and that a multivariate approach is required. Toward this end we briefly discuss the history of the develop ment of cognitive test instruments and the operational philosophy of test and measurement theory. This discussion is followed by a review of the evidence con cerning the inheritance of specific cognitive abilities. PSYCHOMETRIC CONSIDERATIONS
Behavioral Phenotypes The phenotypes studied by geneticists have a vast variety of observational require ments. A simple dichotomous classification requires only the ability to perceive differences between the classes. Assessment of continuously distributed characters, however, is more difficult. In addition to the purely statistical considerations, there can be very important problems of measurement. For some "quantitative" charac ters, measurement is relatively straightforward. In dealing with scutellar bristles, for example, it may only be necessary to count. For other characters, such as body height, there may be a fairly obvious application of some fundamental standard of measurement. On the other hand, fundamental comparison operations are not appropriate for some characters, and derived measures are required. Although the problems of derived measurement are by no means limited to the behavioral domain, they are very pronounced in this area. The difficulties have required psychologists to examine the problem of the definition of behavior with great care. As a consequence, certain aspects of operational philosophy and logical positivism have been woven into the fabric of psychological thought. Stevens (108) has succinctly summarized this philosophy as follows: A term denotes something only when there are concrete criteria for its applicability; and a proposition has empirical meaning only when the criteria of its truth or falsity consist of concrete operations which can be performed upon demand. (p. 228)
In many ways, pointing to exemplars of a concept is the most satisfactory sort of definition. But many concepts are not concrete and require definitions which themselves need explication. Herein lie numerous traps for the unwary, because mutuality of understanding of the defined concept depends upon the degree to which the meanings of the definitional words are shared. Or, as Larrabee (61, p. 248) has warned: "talk without mutual understanding is just a means of preventing silence." Our dictionaries may state that intelligence is "the faculty of thought or reason," or that cognition is "the mental process or faculty by which knowledge is acquired." However, unless we have some way of reducing terms such as faculty, thought, reason, mental, and knowledge to operations, these magniloquent definitions are, for scientific purposes, simply gibberish. For research purposes, there are many different operational definitions of cogni tive functions. For example, performance level on a particular test is one definition
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
GENETICS OF SPECIFIC COGNITIVE ABILITIES
18 1
of intelligence; another definition involves correlational analyses. As we shall see from the following review, we should really speak of intelligences, rather than of intelligence. Were cognitive functioning of interest only to specialists, these points would not need elaboration. But intelligence is of concern to the population at large, and everyone has an understanding, however vague, of what it means. The tacit assump tion that there is only one meaning, that it is shared by all, and that it is the same as the meaning employed by the research specialist has created confusion and probably accounts for a considerable part of the controversy that now swirls around the subject. In a brief review such as this, it is not possible to provide all of the operational definitions for all of the cognitive functions discussed. We hope, however, that some appreciation for the diversity of definitions can be gained from the context. Detailed consideration of these matters will require reference to the original works.
Historical Development The idea of a dormant or innate attribute waiting to be developed underlies the concepts of ability, aptitude, and faculty which occur in most European languages (47). The Greek philosophers enumerated many separate attributes such as memory and imagination, and the list was further elaborated by writers of the Renaissance and those of the 18th and 19th centuries. During the latter half of the 19th century, Sir Francis Galton experimented with various measures of sensory discrimination, including bisecting a line and judging differences between small weights. From his research, Galton (40, p. 20) concluded the following: "The trials I have as yet made on the sensitivity of different persons confirms the reasonable expectation that it would on the whole be highest among the intellectually ablest." This view was a logical outcome of the British empiricist philosophy which held that all knowledge was acquired through the senses. James McKeen Cattell, who first used the term mental test in 1890, worked with Galton in England and later brought his ideas to the United States. Cattell experi mented with measures such as keenness of eyesight and hearing, reaction time, afterimages, color vision, perception of pitch, and judging a to-second interval ( 19). Other early mental tests were developed during the latter part of the 19th century by Oehrn (90), Munsterberg (86), Kraepelin (60), and Ebbinghaus (29). In 1904, Alfred Binet, a gifted experimental psychologist at the Sorbonne, was appointed to a commission responsible for devising a national system of education for the mentally retarded. The commission concluded that no child should be sent to a special school unless an examination revealed that "his intellectual state renders him unable to profit in the ordinary measure from the instruction given in the regular schools" (9, p. 163). Binet & Simon developed scales (9- 1 1) which were highly effective for this purpose. The test was later revised for administration in the United States by Terman ( 1 10) and by Terman & Merrill ( 1 1 1) at Stanford Univer sity. Success of the Binet-Simon test led to increased interest in mental testing. In 1917, the US War Department began a program of mental testing for the classification of
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
182
DEFRIES, VANDENBERG & McCLEARN'
military men, The test employed, the Army Alpha (128), was to a large extent a creation of Otis, a graduate student of Terman's. It was a "paper and pencil" test which could be administered by relatively unskilled personnel to large groups. Eventually, over 1,750,000 men were tested. A second version, the Army Beta, was developed for use with illiterates or non-English speaking recruits. After World War I, there was a rapidly increasing interest in the use of intelligence tests in industry, education, and civil service; it is likely that this was due at least in part to the fact that so many people had been exposed to mental tests during the war. In the meantime, Charles Spearman, an English army officer who later became a professor at the University of London, attempted to provide a theoretical basis for the concept of intelligence He was influenced by Galton and reasoned that even weak tests might be useful ifit could be shown that a combination of such tests could predict a criterion such as school grades. Spearman developed a statistical test to determine whether different tests were measuring something in common and some thing specific to each test. In this two-factor theory (104), the first factor was ascribed to general intelligence (or g), whereas the second was a specific factor unique to each test. Spearman's two-factor theory did not provide a solution to the problem of the nature of intelligence. Later investigators (58) proposed the existence of other "group factors" in addition to g. For example, in 1935, EI Koussy (30) identified a spatial group factor which he symbolized "k:" After several such group factors were recognized, a hierarchical theory of intelligence was proposed by Burt (17) and Vernon (121). In this model, illustrated in Figure I, four kinds of factors are assumed: (a) a general factor; (b) two major group factors (verbal-education and spatial-mechanical); (c) an undetermined number of minor group factors, some of which can be still further subdivided; and (d) factors specific to each test. The most prominent advocate of multiple ability factors was Thurstone (11311S). In addition to developing a shortcut method for extracting factors from a correlation matrix, Thurstone (116) advocated rotating these factors to a new posi tion which could provide simple structure, where: (a) each variable is identified with only one or a small proportion of the factors; (b) the number of variables loaded on (correlated with) a factor is minimized; (c) the variance accounted for by the major unrotated factors is distributed across the rotated factors; and (d) each rotated factor is more or less identified with a distinct cluster of correlated variables g
Major group factors
I
v:ed
I
I
k:m
� � I fJn rll � I r=t-,
Minor group factor
Specific factors Figure 1
1111111111111111111111111111111111111111
Hierarchical structure of human abilities. Reprinted from Reference 121.
GENETICS OF SPECIFIC COGNITIVE ABILITIES
183
(see Rummel, 98). Using these techniques, Thurstone identified seven uncorrelated factors which were found to recur across a number of large studies: verbal meaning or vocabulary (V); verbal fluency (W); numerical ability (N); spatial ability (S); inductive reasoning (I or R); memory (M); and perceptual speed (P). When later searching for simple structure by plotting test loadings on pairs of factors, Thurstone noted that more near-zero values could be obtained when factors were allowed to correlate. Such axes and the resulting factor structure are said to be "oblique," in contrast to the orig inal orthogonal" factor structure. Once factors are allowed to correlate, reSUlting factor correlation matrices may again be factor analyzed to produce higher order factor solutions. Thus, a compromise was reached between Spearman's theory of one general plus many specific factors and Thur stone's multiple factor theory. With the advent of electronic computers, the full characteristic equation repre sented by the correlations between n tests could be solved readily to yield n roots and, hence, n factors. Thurstone's concepts of rotation, simple structure, and oblique factors have survived this technical revolution. For a detailed account of factor analysis and the various methods of approximating simple structure, see Harman (45) and Rummel (98). In contrast to Thurstone, Guilford (43) insisted upon uncorrelated factors and attempted to broaden the content and format of psychological tests. Guilford recog nized five psychological processes which can be performed on four types of materi als, where the information can be packaged into six kinds of products. Because these three classification criteria are postulated to be independent, there would be 5 X 4 X 6 120 uncorrelated ability factors (see Figure 2). While Guilford's theory has stimulated considerable research, few investigators have accepted his plethora of factors. What consensus there is seems to favor the hierarchical view (49).
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
"
=
Parallel Between Classical Test Theory and Quantitative Genetic Theory It is interesting to note that a marked similarity exists between the mathematical models of psychological test theory and quantitative genetics, as well as between the parameters derived from these models. In classical test theory (see 67), the following linear model is assumed: X=T+E,
1.
where X is the observed test score of an individual, T is the true score, and E is the error of measurement. In quantitative genetic theory (see 32), a similar model is assumed:
P= G +E.
2.
where P is the observed phenotypic value, G is the genotypic value (or the value conferred upon the individual by its genotype), and E is the environmental devia tion. Two tests are said to be parallel if they yield identical true scores and if the errors are uncorrelated and have equal variances. The analogous situation in quantitative genetics is the case of identical twins reared in random environments. In such a case,
184
DEFRIES, VANDENBERG & McCLEARN
OPERATIONS Cognition Memory
Diver ent Production Convergent Production Evaluation
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
PRODUCTS
Units Classes Relations Systems Transformations mplications
CONTENTS Figural Symbolic Semantic
Behavioral
Gui lford' s ability factors.
Figure 2
members of the twin pair would have identical genotypic values and the environ mental deviations would be uncorrelated. From the above model of test theory, the following result obtains: Vx
=
Vr + VE>
3.
where V symbolizes variance. The proportion of the variance in observed scores owing to variation in true scores is defined as the reliability of a test. It may be shown that reliability has the following equivaknt forms:
reliability = VTI Vx =
r
iT
=
bTX
=
'XX�
4.
where 'XT is the correlation between true and observed scores, bTX is the regression of true score on observed score, and 'XX' is the correlation between performances
on parallel forms of a test. From the model of quantitative genetics, 5.
when G and E are uncorrelated. The proportion of the phenotypic variance owing to variation in genotypic values is defined as "heritability in the broad sense" (68) and has the following equivalent forms: heritability = VGI Vp = r
�p
= bGP = rppl
6.
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
GENETICS OF SPECIFIC COGNITIVE ABILITIES
185
where P and P' are measures on members of identical twin pairs reared in random environments. In test theory, the correlation between performances on the same test adminis tered at two different times provides a measure of "test-retest" reliability. In quanti tative genetics, the correlation between successive measurements on an individual is referred to as repeatability, and it provides an upper-bound estimate of heritabil ity. Thus, it is seen that the basic models and concepts of psychological test theory and quantitative genetics are highly similar. EVIDENCE FOR THE HERITABILITY OF SPECIFIC COGNITIVE ABILITIES Twin Studies
In his 1975 Annual Review of Genetics article concerning the heritability of IQ, Lewontin (64) completely dismissed the results of twin studies: I have quite deliberately not discussed the large number of simple comparisons of dizy gotic and monozygotic twins . ..., because any such comparisons must assume that the environmental similarities for dizygotic twins are the same
as
for monozygotic [italics
added]. The environmental dissimilarities will be completely confounded with genetic differences. (p. 396)
But how much weight should be given to the scanty evidence, as contrasted to intuition, of greater environmental dissimilarities within fraternal (DZ) than within identical (MZ) twin pairs? The most recent treatments of this topic have been by Scarr (99) and Vandenberg (120). After reviewing the evidence, Scarr concluded that "genetic relatedness" of twins determines the similarity of their treatment and that this is not necessarily evidence for environmental bias. Evidence that some abilities maybe more heritable than others is presented in Tables 1 through 4. F ratios of the within-pair variances of DZ and MZ twins are tabulated for subtests of the Primary Mental Abilities Test in four studies, for the Differential Aptitude Test in two studies, for the Wechsler Adult Intelligence Scale (WAIS) in one study, and for a number of separate tests in the Swedish study. It may be seen in these tables that the rank order of F ratios (from highest to lowest) tends to be as follows: spatial, vocabulary, word fluency, speed in simple arithmetic, and reasoning. Further evidence for the differential heritabilities of specific cognitive abilities may be found in Vandenberg (119). Is there evidence for the existence of genetically independent cognitive abilities? Modest attempts to answer this question have been made by Vandenberg (118), by Loehlin & Vandenberg (66), and by Bock & Vandenberg (14). If we can assume that the environmental influences produce the same degree of covariance for pair differ ences in the two types of twins, one can subtract the MZ from the DZ covariances to obtain an estimate of the genetic covariances (66), or one can generalize the F test that is used in univariate studies to determine the significance of the differ ence between the MZ and DZ within-pair variances (14). Instead of evaluating . . 2 / 2 F = (TwDZ (T wMZ, one ascertains h ow many elgen va1ues are sign! " ficant In ' the characteristic equation, .
186
DEFR IES, VANDENBERG & McCLEARN
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
7.
In practice, this number is usually equal to the number of eigen values greater than or equal to unity, although with increasing sample sizes more eigen values will acquire statistical significance. Using the procedure of subtracting MZ from DZ covariances, Loehlin & Vanden berg (66) concluded (on the basis of rather small samples) that the same ability factors appeared in the environmental and in the genetic covariation among 15 scales of Thurstone's Primary Mental Abilities Test. However, there were three oblique higher-order factors (number, space, and verbal-educational) in the environ mental covariation, while only one general factor appeared in the genetic covariance. Table 1 F ratios of the within-pair variances of dizygous and monozygous twins for sub tests of the Primary Mental Abilities ( P MA) Testa
Name of P MA subtest
----�."
Verbal Space Number Reasoning Word fluency Memory
Blewett 1954 3.13b 2.04c 1.07 2.7Sb 2.7Sb not used 26 DZ
26 MZ a Reprinted from Reference 119. bp < 0.01. cp < 0.05.
Thurstone 1955 2. S1b 4.19b
Vandenberg ( Michigan) 1962 2.65b
Vandenberg (Louisville) 1966
1.52 1.35 2.47b 1.62 53 DZ
1.77c 2.5Sb 1.40 2.57b 1.26 37 DZ
1.74c 3.51b 2.26b 1. 10 2.24b not used 36 DZ
45 MZ
45 MZ
76 MZ
Table 2 F ratios of the within-pair variances of dizygous and monozygous twins for the Differential Aptitude Testa
Verbal reasoning Numerical ability Abstract reasoning Space relations Mechanical reasoning Clerical speed and accuracy Language useI: Spelling Langu age use II: Sentences
a Reprinted from Reference 119. b p < 0.01.
1961 2.29b
1965 2.3Sb
1.39 1.47 1.67 1.36 2.54b 3.64b 3. 06b 25 DZ
1.37 1.23 2.19b 1.463.13b 2.58b 2.00b 86 DZ
47 MZ
109MZ
GENE TICS OF S PEC IF IC C OGN IT IVE A BIL IT IES
187
Table 3 F ratios of the within-pair variances of 60 dizygous and 60 monozygous twins for the scaled scores of II subtests of the Wechsler Adult Intelligence Scalea F
Subtest
3.88b 2.2Sc
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
Information Comprehension
2.78b
'Arithmetic Similarities Digit span Vo cabu lary Digit symbol
3.14b 2.06c
Picture completion Block d esign Picture arrangement
l.74d
1.8Id
1.S3d
LSD 2.3Sc
Object assembly
1.36
Verbal score Performance score Tot al s cor e
3.3Sb
3.41b 3.47b
aReprinted from Reference 119. bp < 0.001. c p < om. dp < 0.05.
Table 4 F ratios of the within-pair variances of like-sexed dizygous and monozygous twins for 14 psychological tests. administered to Swedish twins of elementary and high school agea
Simplex C-test Verbal analysis Form perception Picture perception Number perception Number series Number analysis Numerical classification Numerical reasoning Routine simple arithmetic Memory for 2-digit numbers (recall) Memory for 3-dig it numbers (recognition) Paired associates word-numb er memory
Boys
Girls
1.98b
2.84b
1.14
0.96
2.41b I.Slc
1.17
4.3Sb
1.23 I.S4c
All
cases 2.38b
3.37b
1.12
1.34c 1.36c
1.58c
1.91b
1.59b
1.61c
1.68c
1.63b
2.83b
1.96b
2.18b
1.15
1.39
1.24
2.37b
1.47
1.87b 0.94 1.32
1.7Dc
1.64c 1.Sle 1.34
2.01b l.57b
1.68b 1.17
66 DZ
1.00 75 DZ
141 DZ
66MZ
62MZ
128MZ
a Reprinted from Reference 119, ased on Reference 122. b bp < 0.01. cp < 0.05.
1.16
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
188
DEFR IES, VANDEN BERG & McCLEARN
Applying the second method to twin-pair differences on the Differential Aptitude Test, three dimensions were significant in the genetic covariance for boys and two for girls ( 1 4). In a large Finnish study of adult twins (91), a set of tests measuring six cognitive abilities (verbal, word fluency, space, number, memory, and reasoning) was adminis tered. A multivariate generalization of the F test led to the conclusion that the genetic covariance in the six abilities had six significant roots, that is, there were six independent genetic factors. On the other hand, when Eaves & Gale (28) reanalyzed the data of Loehlin & Vandenberg (66) with a method by which the additive genetic and dominance components of covariance could be estimated, they conclUded that a single genetic component fit well enough to suggest an absence of strong major gene effects for the abilities tested by the Primary Mental Abilities Test. It is clear that future studies will have to utilize larger samples, and more tests, to answer conclusively the question concerning the existence of genetically indepen dent abilities.
Family
Studies
Another methodology which has been employed to study the genetics of specific cognitive abilities is the family study. Although relatively few such studies have been reported to date, a major report was published by Willoughby (125) in the I 920s. He administered a battery of 11 individual tests of verbal and nonverbal abilities to 14 1 children between 12.5 and 13.5 years of age and to members of their families (90 fathers, 100 mothers, and 280 sibs). It may be seen from Table 5 that the average correlation for the verbal tests exceeded that for the nonverbal tests for all parent offspring combinations and for two of the three sib combinations. Since there is no difference between the spouse correlations, the greater parent-offspring and sib resemblance for verbal ability may be due either to its higher heritability and/or to a larger environmental correlation between relatives. Five years later, a family study of arithmetic and vocabulary abilities was reported by Carter ( 1 8). Four arithmetic subtests and two vocabulary tests were administered to members of 108 families in which both parents and one or more children over 12 years of age were available for testing and to 3 1 incomplete family groups. From Table 5
Familial correlations for verbal and nonverbal abilitiesa Verbal average
Nonverbal average
0.41
0.29
Mother-daughter
0.41
0.31
Mother-son
0.38
0.35
0.34
0.26
Father-son
Father-daughter Brother-sister
0.38
0.35
Brother-brother
0.50
0.37
0.40
0.52
0.44
0.44
Sister-sister Husband-wife aReprinted from Reference 125.
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
GENETICS OF SPECIFIC COGNITIVE ABILITIES
189
the resulting familial correlations presented in Table 6, it may be seen that the vocabulary tests yielded higher correlations for three of the four parent-offspring combinations and for all sib combinations. The greater spouse correlations for performance on the vocabulary tests may account at least in part for the higher parent-offspring and sib correlations. However, as in the Willoughby ( 1 25) study, a higher heritability and/or greater environmental correlation between relatives for verbal ability may also be involved. Three family studies of specific cognitive abilities have only recently been re ported. Utilizing a western Canadian sample, Williams (123) compared WISC subscale scores of 55 sons, 10 years of age, to WAIS scores of their parents. Although the same tests were not administered to both age groups, Williams (124) has shown that the two test batteries measure similar abilities. Spouse correlations and regressions of offspring on midparent value for the 11 subscales, for the verbal and performance IQ aggregates obtained from these subscales, and for total IQ are presented in Table 7. In agreement with previous evidence, the parent-son regres sions and spouse correlations tend to be higher on the verbal tests (subscales 1-6) than on the performance tests (subscales 7-11). This contrast is especially clear between verbal IQ and performance IQ. Note that the regression of offspring on midparent, which provides an upper-bound estimate for heritability, is only 0.46 for total IQ. Two other studies, the Hawaii Family Study of Cognition and the Boulder Family Study, are currently in progress. In both of these studies, data are being collected on 15 cognitive tests (see Table 8), various environmental indices, and blood group and enzyme systems. There are two major ethnic groups included in the Hawaii Study, Americans of Japanese ancestry (AJA) and Americans of European ancestry (AEA). Results of multivariate analyses have revealed that the cognitive structures of these two groups are nearly identical (26). Phenotypic correlations among the 15 cognitive variables were obtained for both AJA and AEA groups (N = 262 and 782, respectively), and principal component analyses with varimax rotations were performed. Four readily interpretable factors emerged from these analyses: spatial visualization, verbal, perceptual speed and accuracy, and visual memory. Common Table 6
Familial correlations for vocabulary and arithmetic abilitya Vocabulary
Arithmetic
n Father-son
n
0.21
116
0.10
116
Mother-daughter
0.34
136
0.24
136
Mother-son
0.07
126
0.12
127
Father-daugh ter
0.26
114
114
Brother-sister
0.31
III
0.04
Brother-brother
0.43
46
Sister-sister Husband-wife a Reprinted from Reference 18.
0.17 0.21
III 48
0.32
53
0.26
53
0.21
108
-0.04
108
190
DEFRIES, VANDENBERG & McCLEARN
Table 7 Spouse correlations and regressions of offspring on midparent for the Wechsler Intelligence ScalesB
Regression of offsp ring on midparent Spouse correlations
WISC/WAfS subscales 1.
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Information Comprehension Arithmetic Similarities Digit span Vocabulary Digit symbol (coding) Picture completion Block design Picture arrangement Object assembly Verbal fQ PerformanceIQ TotalIQ
BReprinted from Reference
Estimate
Standard error
0.65
0.25
0.13
0.48
0.26
0.13
0.19
0.45
0.12
0.41
0.36
0.15
0.06
0.13
0.15
0.58
0.53
0.10 0.15
0.20
0.56
0.37
-0.12
0.20
0.19
0.34
0.19
0.30
0.07
0.18
-0.07
0.02
0.24
0.62
0.55
0.35 0.61
0.22
0.09 0.17
0.46
0.10
123.
factor loadings of the 15 cognitive tests on these four varimax-rotated principal components are presented for both ethnic groups in Figure 3. The marked similarity of the factor-loading profiles for AJA and AEA subjects is obvious from this illustration. Coefficients of congruence between the two groups were 0.96 or larger for each of the four factors. When these analyses were subsequently repeated with a larger data set (N for AJA 648, N for AEA 1971), similar results were obtained (126). In fact, congruence coefficients calculated from the larger data set were all 0.99 or larger. Zonderman and associates (131) have subjected the data from the Boulder Family Study to principal components analysis and have obtained the same four varimax rotated factors. Coefficients of congruence between this mainland AEA sample and the Hawaiian AEA sample were all greater than 0.94. Taken together, these data demonstrate that the 15 cognitive variables provide measures ofthe same underlying abilities in Hawaiian AJA, Hawaiian AEA, and mainland AEA samples. Assortative marriage for specific cognitive abilities has been assessed in both the Hawaii and Boulder family studies (57, 131). Previous studies of assortative mar riage for cognitive ability were recently reviewed by Jensen (53), who reported that the unweighted mean of 43 spouse correlations for various tests of mental ability was +0.45. However, at least some of these correlations may have been inflated. When cross-sectional data concerning mental ability are examined, some change occurs as a function of age (see 126). Thus, when both older and younger couples =
=
GENETICS OF SPECIFIC COGNITIVE ABILITIES
191
Table 8 Administration time and reliabilities for tests utilized in the Hawaii Family Studyof Cognition and the Boulder Family Studya Cognitive Test
factor
Test time
Reliability
V
3min
0.96
Primary Mental Abi lities (PM A) Vocabulary
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
Visual Memory (immediate)
M
Things (a fluency test)
V
I-min exposure/
0.58
2parts/3min
0.74
I-min recall each
Sheppard-Metzler Mental Rotations
S
10min
0.88
(modified for group testing by Vandenberg) Subtraction and Multiplication
P
2parts/2min
0.96
each Elithorn Mazes ("lines and dots"),
S
5 min
0.89
V
2 parts/3min
0.71
shortened form Educational Testing Service (ETS)
each
Word Beginnings and Endings ETS Card Rotations
S
2 parts/3 min
0.88
each Visual Memory (delayed recall)
M
1min
0.62
PM A Pedigrees (a reasoning test)
V,S,P
4min
0.72
S
2parts/2min
0.92
ETS Hidden Patterns
eaeh S
3 min
0.84
ETS Number Comparisons
P
2parts/1.5 min
0.81
Whiteman Testof SocialPerception
V
10min
0.69
Raven's Progressive Matrices,
S
20min
0.86
Paper Form Board
each
modified form a Reprinted
from Reference 126. b Varimax rotated factor loading;;' 04 . 0
ceptual speed and accuracy, M
visual memory).
=
are included in a sample, some of the observed spouse similarity in mental ability is due to their similarity in age. For this reason, it is essential that age adjustment be employed in studies of assortative marriage for cognitive ability. As discussed below, method of test administration may also influence spouse correlations. Age-adjusted spouse correlations for couples included in the Hawaii and Boulder family studies are presented in Table 9. With regard to the Hawaiian samples, AEA spouse correlations are higher than those for AJA subjects on 13 of the 15 individual tests. However, the magnitude of the differences is not large (median AEA and AlA spouse correlations were +0.13 and +0.06, respectively). In addition, spouse corre lations for factor scores were highest for the verbal factor in both ethnic groups.
DEFRIES, VANDENBERG & M cCLEARN
192 1.0 .8
SPATIAL
VERBAL
.6 .4 .2
�
�
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
c:::i
a -.2 -.4
� ...,J
-.5
� "
1.0
(",)
�
.8
I I I PERCEPTUAL
--AEA --------
SPEED
AJA
VISUAL
MEMORY
.6 .4 .2 0 -2
4
-.
-!j
I I
3
Figure 3
7
9
13
15
I I
3
5
7
9
"
13
I I
15
Loadings of 15 cognitive tests on fou r varimax-rotated principal components in
Americans of Japanese ancestry (AJA) and Americans of European ancestry (AEA). Re printed from Reference 26.
Thus, it does not appear that ethnicity is a major influence on the extent and nature of assortative marriage in Hawaii. When the mainland AEA sample is compared with the Hawaiian samples, the mainland spouse correlations are somewhat higher (median value of +0.22). This difference, however, may be due in part to a difference in method of test administration. In the Hawaii study, tests are administered to large groups under highly standardized conditions (automated tape and slide presentation of instructions, etc). In contrast, tests in the Boulder study are usually administered to individual nuclear family units by various testers, so that differences among testers could cause increased spouse correlations in this study. Nevertheless, results of both studies suggest relatively less assortative marriage for cognitive abilities than previously reported. When effects of educational attainment were statistically re moved, resulting partial correlations became even smaller. Thus, there is very little assortative marriage for that part of cognitive ability which is independent of educa tional level (57, 131). Parent-offspring resemblance for specific cognitive abilities has also been exam ined in both the Hawaii and Boulder family studies (25, 105). When environmental effects are randomized among subjects, the regression of offspring's score on midpar ent value provides a direct estimate of heritability (32). However, because environ mental influences on at least some measures of cognitive ability may be common to parents and their offspring, regressions of offspring on midparent value for such characters should be regarded as measures of phenotypic similarity (genetic and/or environmental) and only as upper-bound estimates of heritability (77).
1 93
GENE TICS OF SPECIFIC C OGNI TI VE ABILI TIES Table
9
Age-adjusted spouse correlations Hawaii a AEA (N 555 couples)
Annu. Rev. Genet. 1976.10:179-207. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 01/24/15. For personal use only.
=
Tests Vocabulary Visual Memory (immediate) Things MentalRotations Subtraction and Multiplication Elithorn Mazes ("lines & dots") Word Beginnings and Endings Card Rotations Visual Memory (delayed) Pedigrees Hidden Patterns Paper Form Board Number Comparisons Social Perception Progressive Matrices. Factors Spatial Verbal Perceptual speed and accuracy Visual memory First principal component (unrotated) a Reprinted from Reference 57. AEA icans of Japanese ancestry . b Reprinted from Reference 131.
0.27
(N
=
148
couples)
Mainland b AEA (N 1 23 couples)
0.16
=
0.36
0.07
0.15
0.09
0.22
0.08
-0.12
0.11
0.04
0.03
0.10
0.18
0.07
0.00 0.12 0.10
0.13
0.02
0.12
0.18
0.04
0.03
0.10
0.41
0.06
0.26
0.06
0.28
0.16
om
0.10
0.21
0.22
0.Q3
0.20
0.00
0.23
-0.05
0.13
0.19
0.20 0.13
0.22
0.13
=
Hawaii a AJA
0.20
0.00
0.00
0.13
0.28
0.13
Americans of European ancestry; AJA
0.36 0.35
0.09
0.35
0.41 0.05
0.35
=
Amer
Regressions of midchild on midparent obtained in the Hawaii study are presented in Table 10. These regressions have been adjusted for lack of perfect test reliability (an adjustment which is essential when estimates from tests with different reliabili ties are to be compared). Standard errors for the regression coefficients are suffi ciently small to suggest that at least some of the variability among tests is real. Another indication of this differential parent-offspring resemblance is the marked congruence between ethnic groups. The Spearman rank correlation between the 1 5 AEA and AlA regressions for the individual tests was +0.77 (p
