Gender Differences in Cognitive Abilities of Opposite-Sex and Same-Sex Twin Pairs With Reading Disabilty Valerie S. Knopik, John C. DeFries, and Maricela Alarc6n University of Colorado Boulder, Colorado
In order to compare the pattern of gender differences for cognitive measures in opposite-sex twin pairs to that in independent samples of twins from same-sex pairs, psychometric test data were obtained from four research-identified samples of children: (1) 96 pairs of oppositesex fraternal twins in which at least one member of each pair is reading disabled; (2) 62 pairs of opposite-sex fraternal twins with no history of reading problems; (3) 167 males and 155 females from samesex identical and same-sex fraternal twin pairs in which at least one member of each pair is reading disabled; and (4) a comparison sample of 126 males and 132 females from same-sex twin pairs with no history of reading problems. Results of multivariate analyses indicate that gender differences for cognitive measures are similar in twin pairs with and without reading disabilities. Moreover, a highly similar pattern of gender differences occurs for opposite-sex twin pairs who shared both prenatal and early postnatal influences and for independent samples of children from different families. In her comprehensive review of the literature concerning gender differences in children with learning disabilities (LD), Vogel (1990) found that compared to males with LD, females with LD have a lower average IQ and more severe academic achievement Annals of Dyslexia, Vol. 46, 1996 Copyright©1996 by The Orton Dyslexia Society ISSN 0736-9387
241
242
COGNITIVE AND NEUROLOGICAL FACTORS IN DYSLEXIA
deficits, together with somewhat better visual-motor abilities, spelling, and written language mechanics. More specifically, with regard to the WISC-R (Wechsler 1974) and WAIS-R (Wechsler 1981) Verbal and Performance Scales, Bradbury et al. (1975) found that males with LD have higher Verbal IQ scores than females with LD. Eno and Woehlke (1980) also found evidence of higher Verbal IQ scores in males with LD, as well as significantly higher mean subtest scores on WISC-R Information, Comprehension, and Similarities. In their study of WISC-R subtest scores of students with LD, Mayes and Roberts (1985) found that males with LD have significantly higher mean scores on Picture Completion and Object Assembly, whereas females with LD have higher average Coding scores. Therefore, in general males with LD tend to perform better on verbal and visual-spatial abilities than females with LD (Vogel 1990). Vogel (1990) also noted that samples of children with LD are ascertained mainly from system-identified populations and that such children may not be representative of those with LD in the general population. Compelling evidence for ascertainment bias in referred samples of boys and girls with reading disability was first reported by Naiden (1976) and more recently by Shaywitz et al. (1990). Based upon her review, Vogel (1990) concluded that research concerning gender differences in children with LD is in its infancy, but has significant implications for screening, diagnosis, intervention, and prognosis for all individuals with learning disabilities. Also in 1990, DeFries, Wadsworth, and Gillis summarized results obtained from a preliminary study of gender differences in cognitive abilities of twins with reading disabilities. With regard to same-sex twin pairs, highly significant, but relatively small, gender differences were found for Mathematics and Spelling subtests of the Peabody Individual Achievement Test (Dunn and Markwardt 1970), and Verbal and Performance IQ, with males obtaining higher average scores on Mathematics and both Verbal and Performance IQ, and females obtaining higher average scores for Spelling. With regard to the WISC-R subtests, males achieved higher average scores on Information, Similarities, Vocabulary, Picture Completion, Block Design, and Object Assembly, but lower scores on Coding. The opposite-sex twin sample was relatively small (only 34 pairs in which at least one member of each pair was reading disabled); therefore, fewer differences were significant. However, the pattern of gender differences for the opposite-sex pairs was similar to that observed for the same-sex sample.
GENDER DIFFERENCES
243
The primary objectives of the present study are threefold: (1) to assess gender differences for cognitive variables in a larger sample of opposite-sex twin pairs with reading-disability (RD); (2) to compare this pattern of differences to that observed in a non-RD sample of opposite-sex twin pairs; and (3) to compare the profile of gender differences in opposite-sex twin pairs to that manifested in independent samples of males and females from same-sex pairs. For the present study, data were obtained from a sample of opposite-sex twin pairs with RD and a comparison sample without RD, thereby facilitating multivariate analyses of variance of data from opposite-sex twin pairs comparable to those previously reported for same-sex pairs (DeFries et al. 1990). METHODS PARTICIPANTS The twin pairs were ascertained from 27 school districts in the state of Colorado. In order to minimize the possibility of ascertainment bias, all twin pairs in a school were identified, without regard to reading status, by school administrators. Parental permission was then sought to review the twins' school records for evidence of reading problems (e.g., low reading achievement test scores, referral to a reading therapist, etc.). Twin pairs in which at least one member of the pair had a positive school history of reading problems and a non-RD comparison sample were invited to laboratories at the University of Colorado to complete an extensive battery of psychometric tests. The sample of twins with reading disability includes pairs in which at least one member of the pair has a positive school history of reading problems and is classified as reading disabled by the discriminant score (described below). In addition, twins were required to have an IQ score of at least 90 on the Verbal or Performance Scale of the WISC-R or WAIS-R, no serious neurological, emotional, or behavioral problems, and no uncorrected deficits in visual or auditory acuity. For the opposite-sex twin analysis, data from both members of each pair were analyzed in order to assess gender differences in males and females who had shared prenatal and early postnatal environmental influences. Therefore, both concordant and discordant twin pairs are included in the reading disabled sample. In the same-sex analysis, data from only the firstborn twin of each pair were analyzed in order to compare the pattern of differences found in opposite-sex pairs to that of independent samples of males and females raised
244
COGNITIVE AND NEUROLOGICALFACTORS IN DYSLEXIA
in different environments. This ascertainment procedure resuited in a same-sex RD sample that is comparable to that of the opposite-sex RD twin sample (i.e., both affected and unaffected members of RD twin pairs are included). Twins without RD were matched, where possible, to the sample of twins with RD on the basis of age, gender, twin type (opposite- versus same-sex), and zygosity; however, both members of pairs without RD have a negative school history for reading problems and were classified as unaffected using discriminant function scores described below. All twin pairs included in this analysis were raised in Englishspeaking, predominantly middle-class homes, and ranged in age from 8 to 20 years, with a mean age of 11.85, at the time of testing. Zygosity of the same-sex twin pairs was determined by selected items from the Nichols and Bilbro (1966) questionnaire, which has a reported accuracy of 95 percent. In cases of doubtful zygosity, blood samples were analyzed. MEASURES
The test battery included the Wechsler Intelligence Scale for Children-Revised (WISC-R; Wechsler 1974) or the Wechsler Adult Intelligence Scale-Revised (WAIS-R; Wechsler 1981), the Peabody Individual Achievement Test (PIAT; Dunn and Markwardt 1970), the Primary Mental Abilities Spatial Relations Test (PMA; Science Research Associates 1965), the Colorado Perceptual Speed Test (CPS; DeFries et al. 1981), the Educational Testing Service Identical Pictures Test (ETS; French, Elstrom, and Price 1963), Rapid Automatic Naming (RAN; Denckla and Rudel 1976), and Expressive Verbal Fluency (EVF; Decker and Vandenburg 1985). A discriminant function score (DISCR) was also computed for each individual using data from the PIAT Reading Recognition, Reading Comprehension, and Spelling subtests and weights obtained from a discriminant analysis of an independent sample of 140 reading disabled and 140 control nontwin children (DeFries et al. 1991). Measures were age adjusted using regression deviation scores obtained from an unselected sample of twins and then standardized by expressing each individual's score as a deviation from the corresponding control mean and dividing by the control standard deviation. Data from both opposite- and same-sex control twin pairs were used to compute the control means and standard deviations. ANALYSES
For the present study, the Wechsler Verbal and Performance IQ, PIAT, Verbal Fluency, Spatial Relations, Perceptual Speed, Iden-
GENDER DIFFERENCES
245
tical Pictures, and RAN scores of individuals from the oppositesex samples were subjected to a 2 x 2 multivariate analysis of variance (MANOVA) that included group (twins from pairs with RD versus those without RD) and gender as main effects plus their interaction. WISC-R subtest scores were also subjected to a MANOVA using the same model. Corresponding analyses were conducted on data from the same-sex sample. In a preliminary analysis, the interaction involving gender and zygosity was not significant for any of the measures; therefore, zygosity was not included in the model. In addition, post hoc gender difference scores were computed within group for each measure (i.e., male score minus female score) and tested for statistical significance using paired and independent sample t-tests for the oppositeand same-sex samples, respectively. Because of the number of tests of significance conducted for this study, p < .01 was employed as the criterion for statistical significance.
RESULTS OPPOSITE-SEX PAIRS
The mean cognitive scores for the Wechsler Verbal and Performance Scales, the PIAT subtests, Verbal Fluency, Spatial Relations, Perceptual Speed, Identical Pictures, and RAN subtests of opposite-sex twin pairs with and without reading disability are presented in table I. Both of the main effects for group and gender are highly significant for the multivariate comparison F(1,292) = 14.40, p < .001; F(1,292) = 3.29, p s .001, respectively). Except for Spatial Relations, the group difference is also highly significant for each of the individual measures. The largest reading-group differences occur for Verbal IQ (-1.33 standard deviations), Reading Recognition (-2.01), Reading Comprehension (-1.36) and Spelling (-1.56). As can be seen from table I, the main effect due to gender is significant for Verbal IQ, Spelling, Mathematics, and Spatial Relations, and that for RAN Colors and Perceptual Speed approaches significance (p = .011 and p = .018, respectively). However, the magnitude of these gender differences (average male score minus average female score, pooled across RD and non-RD groups) is again relatively small, ranging from a 0.35 standard deviation unit advantage in favor of males for Mathematics to a 0.35 standard deviation unit advantage in favor of females for Spelling. In contrast, the interaction between group and gender is not sig-
-1.84 -1.39 -1.18 -1.13
-.57 -.48 -.83 -.48 -.50 -.45
-.59 -.51 -.52 -.82
-.61 -.37 -.75 -.36 -.90 -.54
-.45 -.65 -.66 -.99
.14 -.14 -.14 -.17
-.04 .11 .08 .12 -.40 -.09
-.26 .02 -.55 .16
Twins with RD Diff. n = 96 -1.33 .17 -.86 .22
Females
-2.10 -1.37 -1.73 -.97
-1.16 -.64
n= 96
Males
1.01 -.91 -1.09 -1.02
-.30 .97 .57 1.13 -3.01" -.84
-1.32 .13 -3.29** 1.12
1.35 1.68
t(95)
-.12 -.26 -.18 -.18
-.09 -.16 -.05 .12 -.04 -.03
.10 .13 .02 .25
Males n = 62 .32 .09
.21 .18 .15 .30
-.04 .00 .12 -.42 .12 .10
-.03 -.16 .17 -.28
-.33 -.44 -.33 -.48
-.05 -.16 -.17 .54 -.16 -.13
.13 .29 -.15 .53
-1.80 -2.87* -2.09 -2.48
-.41 -1.16 -1.15 3.43** -.96 -.90
.96 1.79 -.89 3.41"*
Twins without RD Females Diff. t(61) n = 62 -.17 .49 3.88** -.02 .11 .93
21.59"* 20.65** 21.82** 50.89**
26.06** 12.35"* 44.90** 4.34 38.94** 24.33**
184.60"* 114.81"* 140.07"* 76.10"*
.90 6.50 4.63 5.40
.18 .01 .25 6.96* 5.66 1.21
.12 1.47 6.93* 8.33*
3.16 1.36 .71 .83
.00 1.43 1.17 2.28 1.30 .02
2.47 1.71 3.19 2.99
F-Values, df = 1,307 Gender Group x Gender 150.10"* 8.50* 2.26 4Z52"* 1.94 .14 Group
1Verb IQ a n d Perf IQ (Wechsler Verbal and P e r f o r m a n c e Scales, respectively); P I A T ( P e a b o d y I n d i v i d u a l A c h i e v e m e n t Test subtests); Fluency (Expressive Verbal Fluency subtests); Spatial (Primary M e n t a l Abilities Spatial Relations Test); S p e e d (Colorado Perceptual Speed Test); Pictures (Educational Testing Service Identical Pictures Test); R A N (Rapid A u t o m a t i c N a m i n g subtests). *p s .01 **p .~ .001
Verb IQ Perf IQ PlAT Rec Comp Spell Math Fluency Left Cat Rhy Spatial Speed Pictures RAN Num Col Pic Lett
Test
Table I. M e a n Cognitive Test Scores 1 (in S t a n d a r d D e v i a t i o n Units) of Opposite-Sex T w i n Pairs W i t h a n d Without Reading Disabilitiy
o~
t~
t~
(3 (3
D~
GENDER DIFFERENCES
247
nificant for the multivariate comparison F(1,292) = 1.21, p -~ .255) or for any of the individual variables, indicating that gender differences for these measures in opposite-sex twin pairs with reading disability do not differ substantially from those in opposite-sex twin pairs without reading disability. Table I also presents post hoc within group gender differences and results from paired t-tests for opposite-sex twin pairs. The largest gender differences (males minus females) within the sample of twins with RD occur for Spelling (-0.55 standard deviation units) and Perceptual Speed (-0.40), with females, on average, performing better than males. Within the sample of opposite-sex twins without RD, the largest gender differences occur for Spatial Relations (0.54), Mathematics (0.53), and Verbal IQ (0.49), with males, on average, performing better than females. Mean WISC-R subtest scores of opposite-sex twin pairs with and without RD are presented in table II. For the multivariate comparisons, both of the main effects for group and gender are again highly significant F(1,264) = 15.33, p a .001; F(1,264) = 5.82, p a .001, respectively). The group difference is also highly significant for each of the individual subtests, with the largest differences being for Vocabulary (-1.34 standard deviations) and Information (-1.15). The main effect for gender is highly significant (p a .001) for Information and Coding, and approaches significance (p = .011) for Block Design. As expected, based on results obtained from prior research (Eno and Woehlke 1980; Mayes and Roberts 1985), males obtained higher scores for Information (0.63 standard deviations) and Block Design (0.32), whereas females obtained higher scores for Coding (0.41 standard deviations). The group by gender interaction is not significant for the multivariate comparison F(1,264) -- 0.88, p a .560) or for any of the WISC-R subtests, again indicating that gender differences are similar for opposite-sex twins with and without reading disability. This finding supports Vogel's (1990) conclusion that gender differences in children with RD are similar to those in normally achieving samples. Post hoc within-group gender differences for the WISC-R subtests are also presented in table II. The largest gender differences in twin pairs with RD occur for Information and Coding, with males obtaining higher scores (0.44 standard deviations) for Information, and females obtaining higher scores (0.36 standard deviations) for Coding. In twins without RD, gender differences range from a 0.83 standard deviation unit advantage in favor of males for Information to a 0.47 standard deviation unit advantage in favor of females for Coding.
-1.01 -1.16 -.72 -.92 -.94 -.62 -.62 -.16 -.37 -.60 -.71
n=87
Males
**p a .001
*p a .01
ObA PArr PComp Cod DS
BD
Arith Voc Comp Sim Info
Test
Table If.
-1.01 -1.16 -.68 -.88 -1.38 -.73 -.83 -.50 -.65 -.24 -.48
.00 .00 -.04 -.04 .44 .11 .21 .34 .28 -.36 -.23
Twins with RD Females Diff. n = 87 ,01 ,01 -.28 -,28 3~45"* .81 1.56 2.09 1.87 -3.09* - I .74
t(86) -.06 .39 .28 .14 .41 .22 .07 .15 .13 -.26 .05
Males n = 53 -.34 -.03 .18 -,04 -.42 -.30 -.05 .14 -.16 .21 .02
.28 .42 .10 .18 .83 .52 ,12 .01 .29 -.47 .03
Twins without RD Females Diff. n = 53 1.43 3.30* .66 1,08 4.65** 3.51"* ,74 .10 1.65 -3.23* .24
t(52)
1.35 3.15 .04 .29 29.73** 6.55 1.50 1.08 4.68 15.24"* .52
1.00 2,78 .29 .69 2.53 2.72 .09 .85 .00 .29 1.06
F-Values, df = 1,274 Gender Group x Gender
39.19"* 118.19"* 52.45** 56.59** 94.32** 27.03** 33.62** 11.32"* 14.67"* 14.37"* 30.73**
Group
M e a n W I S C - R S u b t e s t S c o r e s (in S t a n d a r d D e v i a t i o n U n i t s ) of o p p o s i t e - S e x T w i n P a i r s With and Without Reading Disability
(3 c3
C~ C3
-2.08 -1.51 -1.30 -1.23
-.58 -.51 -.75 -.57 -.72 -.51
-.55 -.48 -.63 -.86
-2.39 -1.67 -1.77 -1.19
-.73 -.65 -.88 -.42 -1.27 -.74
-.91 -.88 -.83 -1.31
-.36 -.40 -.20 -.45
-.15 -.14 -.13 .15 -.55 -.23
-.31 -.16 -.47 .04
Twins with RD Females Diff. n = 155 -1.42 .15 -.74 .06
Males n = 167 -1.27 -.68
-3.37** -3.60** -1.71 -3.53**
-1.63 -1.35 -1.23 1.42 -5.09** -2.38
-2.36 -1.46 -4.55** .36
1.34 .55
t(320)
-.02 -.07 -.04 -.01
.05 .00 -.01 .20 .00 .05
.20 .33 --.05 .36
.05 ,22 .09 .16
.07 .09 .13 -.06 .11 .11
-.05 -.06 .11 -.12
-.07 -.29 -.13 -.17
-.02 -.09 -.14 .26 -.11 -.06
.25 .39 -.16 .48
-.52 -2.34* -1.01 -1.40
-.16 -.64 -1.13 2.31 -.91 -.41
2.21 3.37** -1.38 4.02**
Twins without RD Males Females Diff. t(256) n = 126 n = 132 .26 -.11 .37 3.03* .18 .06 .12 .97
80.27** 80.75** 73.43** 162.23"*
91.58"* 55.88** 110.18"* 51.86"* 165.68"* 72.46**
666.10"* 451.03** 418.08"* 283.95**
6.63* 16.59"* 3.78 12.28"*
1.14 1.61 2.41 7.92* 16.13"* 3.03
.09 1.99 18.29"* 9.47*
3.33 .36 .15 2.61
.60 .09 .01 .35 6.90* 1.08
9.91' 11.32"* 3.97 7.23*
F-Values, df = 1,571 Gender Group x Gender 282.62** 10.14" 1.51 102.30"* 1.19 .05 Group
W e r b IQ a n d Perf IQ (Wechsler Verbal a n d P e r f o r m a n c e Scales, respectively); P I A T (Peabody I n d i v i d u a l A c h i e v e m e n t Test subtests); Fluency (Expressive Verbal Fluency subtests); Spatial (Primary M e n t a l Abilities Spatial Relations Test); Speed (Colorado Perceptual Speed Test); Pictures (Educational Testing Service Identical Pictures Test); R A N (Rapid A u t o m a t i c N a m i n g subtests) *p a .01 **p ,~ .001
Verb IQ Perf IQ PIAT Rec Comp Spell Math Fluency Lett Cat Rhy Spatial Speed Pictures RAN Num Col Pic Lett
Test
Table III. Mean Cognitive Test Scores I (in Standard Deviation Units) of Same-Sex T w i n Pairs W i t h a n d Without Reading DisabUitiy
t,o
In order to compare the pattern of gender differences for cognitive measures in opposite-sex twin pairs to that in independent samples of twins from same-sex pairs, psychometric test data were obtained from four research-identified samples of children: (1) 96 pairs of oppositesex fraternal twins in which at least one member of each pair is reading disabled; (2) 62 pairs of opposite-sex fraternal twins with no history of reading problems; (3) 167 males and 155 females from samesex identical and same-sex fraternal twin pairs in which at least one member of each pair is reading disabled; and (4) a comparison sample of 126 males and 132 females from same-sex twin pairs with no history of reading problems. Results of multivariate analyses indicate that gender differences for cognitive measures are similar in twin pairs with and without reading disabilities. Moreover, a highly similar pattern of gender differences occurs for opposite-sex twin pairs who shared both prenatal and early postnatal influences and for independent samples of children from different families. In her comprehensive review of the literature concerning gender differences in children with learning disabilities (LD), Vogel (1990) found that compared to males with LD, females with LD have a lower average IQ and more severe academic achievement Annals of Dyslexia, Vol. 46, 1996 Copyright©1996 by The Orton Dyslexia Society ISSN 0736-9387
241
242
COGNITIVE AND NEUROLOGICAL FACTORS IN DYSLEXIA
deficits, together with somewhat better visual-motor abilities, spelling, and written language mechanics. More specifically, with regard to the WISC-R (Wechsler 1974) and WAIS-R (Wechsler 1981) Verbal and Performance Scales, Bradbury et al. (1975) found that males with LD have higher Verbal IQ scores than females with LD. Eno and Woehlke (1980) also found evidence of higher Verbal IQ scores in males with LD, as well as significantly higher mean subtest scores on WISC-R Information, Comprehension, and Similarities. In their study of WISC-R subtest scores of students with LD, Mayes and Roberts (1985) found that males with LD have significantly higher mean scores on Picture Completion and Object Assembly, whereas females with LD have higher average Coding scores. Therefore, in general males with LD tend to perform better on verbal and visual-spatial abilities than females with LD (Vogel 1990). Vogel (1990) also noted that samples of children with LD are ascertained mainly from system-identified populations and that such children may not be representative of those with LD in the general population. Compelling evidence for ascertainment bias in referred samples of boys and girls with reading disability was first reported by Naiden (1976) and more recently by Shaywitz et al. (1990). Based upon her review, Vogel (1990) concluded that research concerning gender differences in children with LD is in its infancy, but has significant implications for screening, diagnosis, intervention, and prognosis for all individuals with learning disabilities. Also in 1990, DeFries, Wadsworth, and Gillis summarized results obtained from a preliminary study of gender differences in cognitive abilities of twins with reading disabilities. With regard to same-sex twin pairs, highly significant, but relatively small, gender differences were found for Mathematics and Spelling subtests of the Peabody Individual Achievement Test (Dunn and Markwardt 1970), and Verbal and Performance IQ, with males obtaining higher average scores on Mathematics and both Verbal and Performance IQ, and females obtaining higher average scores for Spelling. With regard to the WISC-R subtests, males achieved higher average scores on Information, Similarities, Vocabulary, Picture Completion, Block Design, and Object Assembly, but lower scores on Coding. The opposite-sex twin sample was relatively small (only 34 pairs in which at least one member of each pair was reading disabled); therefore, fewer differences were significant. However, the pattern of gender differences for the opposite-sex pairs was similar to that observed for the same-sex sample.
GENDER DIFFERENCES
243
The primary objectives of the present study are threefold: (1) to assess gender differences for cognitive variables in a larger sample of opposite-sex twin pairs with reading-disability (RD); (2) to compare this pattern of differences to that observed in a non-RD sample of opposite-sex twin pairs; and (3) to compare the profile of gender differences in opposite-sex twin pairs to that manifested in independent samples of males and females from same-sex pairs. For the present study, data were obtained from a sample of opposite-sex twin pairs with RD and a comparison sample without RD, thereby facilitating multivariate analyses of variance of data from opposite-sex twin pairs comparable to those previously reported for same-sex pairs (DeFries et al. 1990). METHODS PARTICIPANTS The twin pairs were ascertained from 27 school districts in the state of Colorado. In order to minimize the possibility of ascertainment bias, all twin pairs in a school were identified, without regard to reading status, by school administrators. Parental permission was then sought to review the twins' school records for evidence of reading problems (e.g., low reading achievement test scores, referral to a reading therapist, etc.). Twin pairs in which at least one member of the pair had a positive school history of reading problems and a non-RD comparison sample were invited to laboratories at the University of Colorado to complete an extensive battery of psychometric tests. The sample of twins with reading disability includes pairs in which at least one member of the pair has a positive school history of reading problems and is classified as reading disabled by the discriminant score (described below). In addition, twins were required to have an IQ score of at least 90 on the Verbal or Performance Scale of the WISC-R or WAIS-R, no serious neurological, emotional, or behavioral problems, and no uncorrected deficits in visual or auditory acuity. For the opposite-sex twin analysis, data from both members of each pair were analyzed in order to assess gender differences in males and females who had shared prenatal and early postnatal environmental influences. Therefore, both concordant and discordant twin pairs are included in the reading disabled sample. In the same-sex analysis, data from only the firstborn twin of each pair were analyzed in order to compare the pattern of differences found in opposite-sex pairs to that of independent samples of males and females raised
244
COGNITIVE AND NEUROLOGICALFACTORS IN DYSLEXIA
in different environments. This ascertainment procedure resuited in a same-sex RD sample that is comparable to that of the opposite-sex RD twin sample (i.e., both affected and unaffected members of RD twin pairs are included). Twins without RD were matched, where possible, to the sample of twins with RD on the basis of age, gender, twin type (opposite- versus same-sex), and zygosity; however, both members of pairs without RD have a negative school history for reading problems and were classified as unaffected using discriminant function scores described below. All twin pairs included in this analysis were raised in Englishspeaking, predominantly middle-class homes, and ranged in age from 8 to 20 years, with a mean age of 11.85, at the time of testing. Zygosity of the same-sex twin pairs was determined by selected items from the Nichols and Bilbro (1966) questionnaire, which has a reported accuracy of 95 percent. In cases of doubtful zygosity, blood samples were analyzed. MEASURES
The test battery included the Wechsler Intelligence Scale for Children-Revised (WISC-R; Wechsler 1974) or the Wechsler Adult Intelligence Scale-Revised (WAIS-R; Wechsler 1981), the Peabody Individual Achievement Test (PIAT; Dunn and Markwardt 1970), the Primary Mental Abilities Spatial Relations Test (PMA; Science Research Associates 1965), the Colorado Perceptual Speed Test (CPS; DeFries et al. 1981), the Educational Testing Service Identical Pictures Test (ETS; French, Elstrom, and Price 1963), Rapid Automatic Naming (RAN; Denckla and Rudel 1976), and Expressive Verbal Fluency (EVF; Decker and Vandenburg 1985). A discriminant function score (DISCR) was also computed for each individual using data from the PIAT Reading Recognition, Reading Comprehension, and Spelling subtests and weights obtained from a discriminant analysis of an independent sample of 140 reading disabled and 140 control nontwin children (DeFries et al. 1991). Measures were age adjusted using regression deviation scores obtained from an unselected sample of twins and then standardized by expressing each individual's score as a deviation from the corresponding control mean and dividing by the control standard deviation. Data from both opposite- and same-sex control twin pairs were used to compute the control means and standard deviations. ANALYSES
For the present study, the Wechsler Verbal and Performance IQ, PIAT, Verbal Fluency, Spatial Relations, Perceptual Speed, Iden-
GENDER DIFFERENCES
245
tical Pictures, and RAN scores of individuals from the oppositesex samples were subjected to a 2 x 2 multivariate analysis of variance (MANOVA) that included group (twins from pairs with RD versus those without RD) and gender as main effects plus their interaction. WISC-R subtest scores were also subjected to a MANOVA using the same model. Corresponding analyses were conducted on data from the same-sex sample. In a preliminary analysis, the interaction involving gender and zygosity was not significant for any of the measures; therefore, zygosity was not included in the model. In addition, post hoc gender difference scores were computed within group for each measure (i.e., male score minus female score) and tested for statistical significance using paired and independent sample t-tests for the oppositeand same-sex samples, respectively. Because of the number of tests of significance conducted for this study, p < .01 was employed as the criterion for statistical significance.
RESULTS OPPOSITE-SEX PAIRS
The mean cognitive scores for the Wechsler Verbal and Performance Scales, the PIAT subtests, Verbal Fluency, Spatial Relations, Perceptual Speed, Identical Pictures, and RAN subtests of opposite-sex twin pairs with and without reading disability are presented in table I. Both of the main effects for group and gender are highly significant for the multivariate comparison F(1,292) = 14.40, p < .001; F(1,292) = 3.29, p s .001, respectively). Except for Spatial Relations, the group difference is also highly significant for each of the individual measures. The largest reading-group differences occur for Verbal IQ (-1.33 standard deviations), Reading Recognition (-2.01), Reading Comprehension (-1.36) and Spelling (-1.56). As can be seen from table I, the main effect due to gender is significant for Verbal IQ, Spelling, Mathematics, and Spatial Relations, and that for RAN Colors and Perceptual Speed approaches significance (p = .011 and p = .018, respectively). However, the magnitude of these gender differences (average male score minus average female score, pooled across RD and non-RD groups) is again relatively small, ranging from a 0.35 standard deviation unit advantage in favor of males for Mathematics to a 0.35 standard deviation unit advantage in favor of females for Spelling. In contrast, the interaction between group and gender is not sig-
-1.84 -1.39 -1.18 -1.13
-.57 -.48 -.83 -.48 -.50 -.45
-.59 -.51 -.52 -.82
-.61 -.37 -.75 -.36 -.90 -.54
-.45 -.65 -.66 -.99
.14 -.14 -.14 -.17
-.04 .11 .08 .12 -.40 -.09
-.26 .02 -.55 .16
Twins with RD Diff. n = 96 -1.33 .17 -.86 .22
Females
-2.10 -1.37 -1.73 -.97
-1.16 -.64
n= 96
Males
1.01 -.91 -1.09 -1.02
-.30 .97 .57 1.13 -3.01" -.84
-1.32 .13 -3.29** 1.12
1.35 1.68
t(95)
-.12 -.26 -.18 -.18
-.09 -.16 -.05 .12 -.04 -.03
.10 .13 .02 .25
Males n = 62 .32 .09
.21 .18 .15 .30
-.04 .00 .12 -.42 .12 .10
-.03 -.16 .17 -.28
-.33 -.44 -.33 -.48
-.05 -.16 -.17 .54 -.16 -.13
.13 .29 -.15 .53
-1.80 -2.87* -2.09 -2.48
-.41 -1.16 -1.15 3.43** -.96 -.90
.96 1.79 -.89 3.41"*
Twins without RD Females Diff. t(61) n = 62 -.17 .49 3.88** -.02 .11 .93
21.59"* 20.65** 21.82** 50.89**
26.06** 12.35"* 44.90** 4.34 38.94** 24.33**
184.60"* 114.81"* 140.07"* 76.10"*
.90 6.50 4.63 5.40
.18 .01 .25 6.96* 5.66 1.21
.12 1.47 6.93* 8.33*
3.16 1.36 .71 .83
.00 1.43 1.17 2.28 1.30 .02
2.47 1.71 3.19 2.99
F-Values, df = 1,307 Gender Group x Gender 150.10"* 8.50* 2.26 4Z52"* 1.94 .14 Group
1Verb IQ a n d Perf IQ (Wechsler Verbal and P e r f o r m a n c e Scales, respectively); P I A T ( P e a b o d y I n d i v i d u a l A c h i e v e m e n t Test subtests); Fluency (Expressive Verbal Fluency subtests); Spatial (Primary M e n t a l Abilities Spatial Relations Test); S p e e d (Colorado Perceptual Speed Test); Pictures (Educational Testing Service Identical Pictures Test); R A N (Rapid A u t o m a t i c N a m i n g subtests). *p s .01 **p .~ .001
Verb IQ Perf IQ PlAT Rec Comp Spell Math Fluency Left Cat Rhy Spatial Speed Pictures RAN Num Col Pic Lett
Test
Table I. M e a n Cognitive Test Scores 1 (in S t a n d a r d D e v i a t i o n Units) of Opposite-Sex T w i n Pairs W i t h a n d Without Reading Disabilitiy
o~
t~
t~
(3 (3
D~
GENDER DIFFERENCES
247
nificant for the multivariate comparison F(1,292) = 1.21, p -~ .255) or for any of the individual variables, indicating that gender differences for these measures in opposite-sex twin pairs with reading disability do not differ substantially from those in opposite-sex twin pairs without reading disability. Table I also presents post hoc within group gender differences and results from paired t-tests for opposite-sex twin pairs. The largest gender differences (males minus females) within the sample of twins with RD occur for Spelling (-0.55 standard deviation units) and Perceptual Speed (-0.40), with females, on average, performing better than males. Within the sample of opposite-sex twins without RD, the largest gender differences occur for Spatial Relations (0.54), Mathematics (0.53), and Verbal IQ (0.49), with males, on average, performing better than females. Mean WISC-R subtest scores of opposite-sex twin pairs with and without RD are presented in table II. For the multivariate comparisons, both of the main effects for group and gender are again highly significant F(1,264) = 15.33, p a .001; F(1,264) = 5.82, p a .001, respectively). The group difference is also highly significant for each of the individual subtests, with the largest differences being for Vocabulary (-1.34 standard deviations) and Information (-1.15). The main effect for gender is highly significant (p a .001) for Information and Coding, and approaches significance (p = .011) for Block Design. As expected, based on results obtained from prior research (Eno and Woehlke 1980; Mayes and Roberts 1985), males obtained higher scores for Information (0.63 standard deviations) and Block Design (0.32), whereas females obtained higher scores for Coding (0.41 standard deviations). The group by gender interaction is not significant for the multivariate comparison F(1,264) -- 0.88, p a .560) or for any of the WISC-R subtests, again indicating that gender differences are similar for opposite-sex twins with and without reading disability. This finding supports Vogel's (1990) conclusion that gender differences in children with RD are similar to those in normally achieving samples. Post hoc within-group gender differences for the WISC-R subtests are also presented in table II. The largest gender differences in twin pairs with RD occur for Information and Coding, with males obtaining higher scores (0.44 standard deviations) for Information, and females obtaining higher scores (0.36 standard deviations) for Coding. In twins without RD, gender differences range from a 0.83 standard deviation unit advantage in favor of males for Information to a 0.47 standard deviation unit advantage in favor of females for Coding.
-1.01 -1.16 -.72 -.92 -.94 -.62 -.62 -.16 -.37 -.60 -.71
n=87
Males
**p a .001
*p a .01
ObA PArr PComp Cod DS
BD
Arith Voc Comp Sim Info
Test
Table If.
-1.01 -1.16 -.68 -.88 -1.38 -.73 -.83 -.50 -.65 -.24 -.48
.00 .00 -.04 -.04 .44 .11 .21 .34 .28 -.36 -.23
Twins with RD Females Diff. n = 87 ,01 ,01 -.28 -,28 3~45"* .81 1.56 2.09 1.87 -3.09* - I .74
t(86) -.06 .39 .28 .14 .41 .22 .07 .15 .13 -.26 .05
Males n = 53 -.34 -.03 .18 -,04 -.42 -.30 -.05 .14 -.16 .21 .02
.28 .42 .10 .18 .83 .52 ,12 .01 .29 -.47 .03
Twins without RD Females Diff. n = 53 1.43 3.30* .66 1,08 4.65** 3.51"* ,74 .10 1.65 -3.23* .24
t(52)
1.35 3.15 .04 .29 29.73** 6.55 1.50 1.08 4.68 15.24"* .52
1.00 2,78 .29 .69 2.53 2.72 .09 .85 .00 .29 1.06
F-Values, df = 1,274 Gender Group x Gender
39.19"* 118.19"* 52.45** 56.59** 94.32** 27.03** 33.62** 11.32"* 14.67"* 14.37"* 30.73**
Group
M e a n W I S C - R S u b t e s t S c o r e s (in S t a n d a r d D e v i a t i o n U n i t s ) of o p p o s i t e - S e x T w i n P a i r s With and Without Reading Disability
(3 c3
C~ C3
-2.08 -1.51 -1.30 -1.23
-.58 -.51 -.75 -.57 -.72 -.51
-.55 -.48 -.63 -.86
-2.39 -1.67 -1.77 -1.19
-.73 -.65 -.88 -.42 -1.27 -.74
-.91 -.88 -.83 -1.31
-.36 -.40 -.20 -.45
-.15 -.14 -.13 .15 -.55 -.23
-.31 -.16 -.47 .04
Twins with RD Females Diff. n = 155 -1.42 .15 -.74 .06
Males n = 167 -1.27 -.68
-3.37** -3.60** -1.71 -3.53**
-1.63 -1.35 -1.23 1.42 -5.09** -2.38
-2.36 -1.46 -4.55** .36
1.34 .55
t(320)
-.02 -.07 -.04 -.01
.05 .00 -.01 .20 .00 .05
.20 .33 --.05 .36
.05 ,22 .09 .16
.07 .09 .13 -.06 .11 .11
-.05 -.06 .11 -.12
-.07 -.29 -.13 -.17
-.02 -.09 -.14 .26 -.11 -.06
.25 .39 -.16 .48
-.52 -2.34* -1.01 -1.40
-.16 -.64 -1.13 2.31 -.91 -.41
2.21 3.37** -1.38 4.02**
Twins without RD Males Females Diff. t(256) n = 126 n = 132 .26 -.11 .37 3.03* .18 .06 .12 .97
80.27** 80.75** 73.43** 162.23"*
91.58"* 55.88** 110.18"* 51.86"* 165.68"* 72.46**
666.10"* 451.03** 418.08"* 283.95**
6.63* 16.59"* 3.78 12.28"*
1.14 1.61 2.41 7.92* 16.13"* 3.03
.09 1.99 18.29"* 9.47*
3.33 .36 .15 2.61
.60 .09 .01 .35 6.90* 1.08
9.91' 11.32"* 3.97 7.23*
F-Values, df = 1,571 Gender Group x Gender 282.62** 10.14" 1.51 102.30"* 1.19 .05 Group
W e r b IQ a n d Perf IQ (Wechsler Verbal a n d P e r f o r m a n c e Scales, respectively); P I A T (Peabody I n d i v i d u a l A c h i e v e m e n t Test subtests); Fluency (Expressive Verbal Fluency subtests); Spatial (Primary M e n t a l Abilities Spatial Relations Test); Speed (Colorado Perceptual Speed Test); Pictures (Educational Testing Service Identical Pictures Test); R A N (Rapid A u t o m a t i c N a m i n g subtests) *p a .01 **p ,~ .001
Verb IQ Perf IQ PIAT Rec Comp Spell Math Fluency Lett Cat Rhy Spatial Speed Pictures RAN Num Col Pic Lett
Test
Table III. Mean Cognitive Test Scores I (in Standard Deviation Units) of Same-Sex T w i n Pairs W i t h a n d Without Reading DisabUitiy
t,o
