J. Child Psychol. Psychiai. Vol. 33, No. 7, pp. 1229-1239, 1992 Printed in Great Britain.

0O21-963O/92S5.0O + O.00 Pergamon Press Ltd © 1992 Association for Ciiild Psychology and Psychiatry

Gender Ratios Among Reading-Disabled Children and Their Siblings as a Function of Parental Impairment Sally J. Wadsworth,* J. C. DeFries,* Jim Stevenson,''' Jeffrey W. Gilger* and B. F. Pennington* Abstract—Gender ratios are reported for 374 reading-disabled probands and their 530 siblings included in five independent studies of reading disability. Ratios were tabulated for each study as a function of parental impairment (neither parent affected, mother only affected, father only affected, and both parents affected). Results reveal a small excess of male probands in referred and clinic samples of reading-disabled children, but not in researchidentified samples. Gender ratios among siblings of reading-disabled probands are approximately 1:1. In addition, combined results indicate that gender ratios of neither probands nor their siblings vary substantially as a function of parental impairment. Keywords: Reading disability, learning disability, gender, family studies

Introduction Referred samples of language- and learning-disabled children contain more males than females, and gender ratios of three or four to one are often reported (Funucci & Childs, 1981; Vogel, 1990). Results obtained from two recent studies of readingdisabled children (DeFries, Olson, Pennington & Smith, 1991; Shaywitz, Shaywitz, Fletcher & Escobar, 1990), however, suggest that gender ratios in research-identified samples are approximately one to one. Thus, the greater number of males invariably found in system-identified and clinic samples of reading-disabled children may be due at least in part to a referral bias (Vogel, 1990). However, other factors may also influence gender ratios, such as diagnostic criteria (e.g. language vs reading disorder, use of age- vs IQ-discrepancy formulas, and severity), composition of samples (e.g. twin vs non-twin populations, and age range of participants), and family history, including parental impairment. It is this last factor, the effect of parental impairment on gender ratios, which is the major focus of the present study. Accepted manuscript received 30 October 1991

'Institute for Behavioral Genetics, University of Colorado, Boulder, CO, U.S.A. 'University of Surrey, Guildford, U.K. ^University of Denver, CO, U.S.A. Requests for reprints to:}. C. DeFries, Institute for Behavioral Genetics, Campus Box 447, University of Colorado, Boulder, CO 80309, U.S.A.

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S. J. Wadsworthrta/.

In a recent family study of 62 children with developmental language/learning disorders, Tallal, Ross and Curtiss (1989a,b) observed a significant excess of medes (44 males and 18 females, a ratio of 2.4 : 1). However, the gender ratio in this referred proband sample varied as a function of parental impairment: 1.9 : 1, when neither parent was affected; 1.8 : 1, when only the father was affected; 4.0 : 1, when only the mother was affected; and 5.0 : 1, when both parents were affected. Tallal et al. (1989b) also found that the language-impaired probands had almost twice as many brothers as sisters (65 and 35, respectively), and that this tendency for a greater number of males than females occurred among both affected (2.5 : 1) and unaffected (1.6 : 1) siblings. However, the gender ratio among siblings of language-impaired children also varied as a function of parental impairment. In families with an affected mother, the gender ratio was 3 : 1 , whereas in families with an affected father, it was 1 : 1 . Moreover, this difference in gender ratio of sibs as a function of parental impairment was greater for affected sibs. Families of affected mothers contained eight times as many affected non-proband sons as daughters, whereas families of affected fathers contained equal numbers of language-impaired sons and daughters. For those families with affected parents, male and female offspring were equally likely to be language-impaired, but families with affected mothers had disproportionately more male offspring. Thus, Tallal et ai (1989b) concluded that the unexpected gender ratio among affected offspring in families with language/learning-impaired mothers was a consequence of the greater number of males born in such families. Tailed et al. (1989b) also cited results obtained from two earlier family studies as corroborative evidence for a disproportionate number of male births in families of language-impaired children. In a reanalysis of data from Hallgren's (1950) classic family study of developmental dyslexia, Sladen (1970) previously noted that dyslexic mothers had 70 sons, but only 34 daughters. However, this includes 35 male and 7 female probands; thus, the gender ratio among siblings of probands in families of dyslexic mothers was 1.3 : 1. The 89 male and 27 female probands included in the total sample had 84 brothers and 91 sisters, a gender ratio of .92 : 1. More recently, Robinson (1987) reported that 75 probands in a family study of language-impaired children had 71 brothers and 49 sisters, a gender ratio of 1.45 : 1. Thus, results of these previous family studies suggest that disproportionate gender ratios may occur in families of children with developmental language/learning disorders. The primary objective of the present study was to test the hypothesis of Tallal et al. (1989b) that gender ratios of children with developmental language/learning disorders and their siblings vary as a function of parental impairment. Gender ratios in five independent studies of reading disability will be reported for probands, affected siblings, and unaffected siblings as a function of parental impairment (neither parent affected, mother only affected, father only affected, and both parents affected) for each individual study, as well as for the combined sample of 374 probands and 530 siblings. Methods Gender ratios were computed from data of five independent studies of reading disability: the Colorado Family Reading Study (DeFries & Decker, 1982); families of twin pairs tested in the Colorado Reading

Gender ratios

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Project (DeFries, Fulker & LaBuda, 1987); families of reading-disabled twin pairs included in the London Twin Study (Stevenson, Graham, Fredman & McLoughlin, 1984); the Iowa Family Study of Learning Disability (Gilger, in press); and families tested in the Linkage Study of Smith, Kimberling. Pennington and Lubs (1983). Each sample was independently ascertained, using the criteria outlined below. Colorado Eamily Reading Study

Reading-disabled children were ascertained by referral from the Boulder Valley and St. Vrain Valley school districts in Colorado. Children were invited to participate in the study if they met the following criteria for inclusion in the proband sample: a chronological age between 7.5 and 12 years; reading performance less than or equal to one-half of grade-level expectancy (e.g. a 9-year-old child in fourth grade, with a reading level equivalent to that expected for a 7-year-old child in second grade); a standardized IQtest score of at least 90, no known emotional, behavioral, or neurological impairments; and no uncorrected visual or auditory acuity deficits. Ghildren and parents were tested at the Institute for Behavioral Genetics, University of Golorado, Boulder. The test battery included the Reading Recognition, Reading Gomprehension, Spelling, and Mathematics subtests ofthe Peabody Individual Achievement Test (PIAT; Dunn & Markwardt, 1970), as well as various other cognitive tests (DeFries, Singer, Foch & Lewitter, 1978). All families were middle to upper middle class Gaucasian, and English was the primary language spoken in the home. Parents of subjects completed questionnaires concerning their own reading habits, preferences, and abilities. One item asked if they had encountered any serious difficulty in learning to read. Because self-report data have been shown to provide a valid index of reading problems (Decker, Vogler & DeFries, 1989), responses to this question were used to assess parental impairment in the present study. Questionnaire data were available for both parents of 125 probands (95 males and 30 females). Diagnosis of 159 siblings of probands was based upon a discriminant function score computed for each individual, using the Reading Recognition, Reading Gomprehension, and Spelling subtests ofthe PIAT, and discriminant weights estimated from an independent analysis of 140 reading-disabled and 140 control children (DeFries, Gillis & Wadsworth, in press). The Iowa Family Study of Learning Disability

As part of an ongoing family study of dyslexia. The Iowa Family Study includes three-generation data from 39 families. These families were selected through a dyslexic proband ascertained by the University of Iowa Pediatric Psychology Glinic. Individuals were included in the proband sample using the Diagnostic and Statistical Manual III—Revised (DSM III—R; American Psychiatric Association, 1987) criteria. A further requirement was that all probands demonstrate the "memory deficit" subtype ofthe University of Iowa diagnostic scheme (Richman, 1983; Lindgren, Richman & Eiiason, 1986). Ghildren possessing the memory subtype are distinguishable from other reading-disabled children by their characteristic pattern of memory deficts despite normal functioning in general intelligence, perceptual-motor skills, and associative reasoning. In addition to the classic symptoms of dyslexia, individuals in the memory-disordered group are characterized by the following: a verbal I Q within 11 points of their performance IQ; a verbal and performance IQof at least 90; scores at least one standard deviation below average on more than one memory test (e.g. short- or long-term verbal and visual memory tests); and no observable deficits in associative reasoning and visuo-perceptual skills. All index cases were between 9 and 18 years of age at the time of participation in the study. Once an individual was identified as a proband, the participation of relatives was solicited through the mail, and interviews were conducted by telephone. Although the study included siblings, aunts, uncles, grandparents, and cousins of affected individuals, only data from nuclear families were included in the present analyses. Questionnaire data for parents and siblings include items relating to physical and socio-emotioneil development, and the presence of symptoms suggestive of learning disabilities and behavioral disorders. Adequate validity and reliability of self and parent reports have been demonstrated for a variety of the questionnaire items used in the Iowa study (Gilger, in press; Gilger, Geary & Eisele, 1991). Archival objective test data (national and state percentile scores) were also obtained for all individuals included in the study. Such data were made available through the University of Iowa Testing Program, which has maintained extensive records of the Iowa Test of Basic Skills since their inception in the early 194O's (Hieronymus & Hoover, 1986; Iowa Testing Program, 1987).

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For the present analysis, parents and siblings were classified as reading disabled using an algorithm based on the survey and test data. The current report includes data from 39 families of affected individuals. The linkage kindreds

Probands in the dyslexia linkage analysis of Smith et al. (1983) were ascertained through clinics or referred from clinical sources. Only those families with pedigrees suggestive of autosomal dominant transmission of dyslexia (e.g. a three-generation history of familial reading problems) were asked to participate. Mean proband age in years was 18.9, with a standard deviation of 8.9. Subjects were interviewed and administered a battery of psychometric tests. Questionnaire data were obtained, including a Reading History Survey (Finucci, Isaacs, Whitehouse & Childs, 1982) as well as information pertaining to handedness, medical and socio-emotional history, and other general information. Blood samples were collected for linkage analysis. Subjects were diagnosed as reading-disabled on the basis of information obtained from the Reading History Survey. On the rare occasion that self-report data were not available, a subject was diagnosed as reading-disabled if personal interviews with the subject or information from a blood relative positively indicated reading difficulties. Proband, parent, and sibling data from 17 families of affected individuals were analysed for the present report. The Colorado Reading Project

In order to minimize the possibility of referral bias, twin pairs in the ongoing Colorado Reading Project are systematically ascertained through cooperating school districts. Without regard to reading status, twin pairs are identified by administrators of school districts within a 150-mile radius of Boulder, Colorado, and permission is then sought from parents to review the school records of both members of each pair for evidence of reading problems. Such evidence includes low reading-achievement test scores, referral to resource rooms or reading therapists because of poor reading performance, reports by classroom teachers or school psychologists, and parental interviews (Gillis & DeFries, 1989). Pairs of twins in which at least one member has a positive school history of reading problems are then invited to be tested at the University of Colorado, where they are administered an extensive psychometric test battery which includes the Wechsler Intelligence Scale for Children-Revised (WISC—R; Wechsler, 1974) or the Wechsler Adult InteUigence Scale—Revised (WAIS-R; Wechsler, 1981) and the Peabody Individual Achievement Test (PIAT; Dunn & Markwardt, 1970). A discriminant function score is then computed for each member of a pair using the method described for the Colorado Family Reading Study. In order for a pair to be included in the proband sample, at least one member of the pair must have a positive school history for reading problems and Eilso be classified as affected by the discriminant score. Other diagnostic criteria include a verbal or performance I Q score of at least 90; no known neurological, emotional, or behavioral problems; and no uncorrected visual or auditory acuity deficits. Twins range in age from 7.5 to 21 years. For the present analysis, questionnaire data were used to determine affected status of siblings and parents. In order to maximize comparability with studies of non-twin probands, each twin birth was considered as one gestation; thus, if both members ofa pair met the criteria for inclusion in the proband sample, only one member was counted as being affected. In addition, the cotwin, whether affected or not, was omitted from the analysis of sibling data. To avoid any possible bias in gender ratios for opposite-sex twins, only data from same-sex twin pairs were analysed. The present sample includes data from 151 families of reading-disabled twin pairs for which data are available on both parents (69 male probands and 82 female probands). The London Twin Study

The London Twin Study obtained standardized measures of intelligence and of reading and spelling performance from an unselected sample of twins. This sample of 285 pairs of 13-year-old twins was obtained in one of two ways: approximately one-half of the twins were ascertained through screening of hospital records in five London boroughs, and the other half through primary schools in the London area. T'wins were administered a test battery that included the WISC—R, the Neale Analysis of Reading Ability (Neale, 1967), and the Schonell Graded Word Reading and Spelling Tests (Schonell & Schonell,

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1960). Twins were then diagnosed for reading or spelling "backwardness" or "retardation". Backwardness was identified by the presence of reading or spelling performance 18 months below that expected based on chronologicjil age, whereas retardation was defined by marked underachievement in reading or spelling (also by 18 months) relative to that predicted from I Q a n d chronological age. For the purposes of the present study, individuals were considered "affected" only if they met the criteria for specific reading retardation. Parent reading status was determined in the course of a structured interview which took place in the home, usually with the mother. The mother was asked if either she or the children's father had had problems learning to read. Similar questions were asked about any full siblings or half-siblings in the family. Only data on biological parents and full siblings are reported in the present study. As a check on the self-report of reading problems in the parents, literacy abilities were measured directly by a variety of tests including the Schonell Spelling Test. However, to maintain comparability across studies, parent reading status was determined solely by self-report data. As in the Colorado Reading Project, the twins were considered as a single gestation; thus, in the case of same-sex concordant pairs only one twin was counted as being affected, and sibling analyses included only nontwin children. Again, only data from families of same-sex twin pairs were analysed, a total of 42 families.

Results For each study, gender ratios were computed separately for probands, affected siblings of probands, and unaffected siblings, as a function of parental impairment (neither parent affected, mother only affected, father only affected, both parents affected). Overall gender ratios were also computed for each study, without regard to affected status of parents, and for each parental impairment type across studies. A chi-square test was computed for each gender ratio to determine the significance of its deviation from the expected 1 : 1 ratio, including correction for continuity for small samples. Due to the large number of significance tests performed, an a level of .001 would have been appropriate for the present study. However, in order to increase the likelihood of detecting effects such as those reported by Tallal et ai (1989) in their study of language/learning-disabled children, a more liberal a level of .05 was employed. Table 1 presents gender ratios for probands in the five independent studies of reading disability. It is noteworthy that only in those studies of referred or clinical populations (the Colorado Family Reading Study, the Iowa Family Study, and the Linkage Study) Table 1. Gender ratios of probands (number of males/number of females) Affected parent Study Colorado (Family) Iowa (Family) Linkage (Family) Colorado (Twin) London (Twin) Combined Sample 'p < .05. **/)< .001.

Neither

Mother only

Father only

Both

Overall ratio

60/13 = 4 . 6 " 13/6 = 2.2 — 32/47 = 0.7 15/15=1.0 120/81 = 1.5*'

7/6 = 1.2 2/5 = 0.4 6/0* 8/8=1.0 0/2 23/21 = 1.1

21/8 = 2.6* 10/2 = 5.0* 3/4 = 0.8 25/23 = 1.1 5/3 = 1.7 64/40=1.6*

7/3 = 2.3 1/0 3/1=3 .0 4/4= 1 .0 1/1 = 1.0 16/9 = 1.8

95/30 = 3.2*** 26/13 = 2.0* 12/5 = 2.4 69/82 = 0.8 21/21 = 1.0 223/151 = 1.4***

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do the overall proband gender ratios differ substantially from 1 : 1 , and only in the Colorado Family Study (3.2 : 1, X^ = 33.80, d.f. = i,p< .001) and the Iowa Family Study (2.0 : 1, X^ = 4-33, d.f. = 1, p< .05) are those deviations significant. The two research-identified samples (the Colorado and London twin studies) do not contain an excess of male probands. It is of particular interest that the London twin study (the first study in which reading-disabled probands were ascertained from an unselected sample of twins) reports an overall gender ratio of exactly 1 : 1 . The combined sample contains a significant excess of male probands (1.4 : 1, x^=13.86, d.f. = 1, p < .001), but this is entirely due to the inclusion of data from the referred and clinical samples. When the gender ratios are expressed as a function of parental impairment, it may be seen that the Colorado Family Study contains an excess of males when neither parent was affected and when only fathers were affected. It is interesting to note that relatively high ratios were also obtained for these two conditions in the Iowa family study. However, the two research-identified samples show no such deviations. Data combined across studies also manifest a significant deviation when neither parent was affected (1.5 : 1, X^ = 7.57, d.f. = 1, p< .01), and when only the father was affected (1.6 : 1, X^ = 5.54, d.f. = 1, /> < .05). However, again this deviation is due largely to the inclusion of data from the referred and clinical samples. The Linkage Study data contain a significant excess of males when only mothers were affected, but the gender ratio in the combined sample is approximately 1 : 1 for this parental impairment type (x^ = .09, d.f. = 1, /) > 0.7). Table 2 presents gender ratios of affected siblings of reading-disabled probands. Sample sizes are very small, especially when both parents were affected; thus these gender ratios may not be highly reliable. As may be seen in Table 2, two studies contain an excess of affected male siblings irrespective of parental impairment status. Table 2. Gender ratios of affected siblings (number of males/number of females) Affected parent Study Colorado (Family) Iowa (Family) Linkage (Family) Colorado (Twin) London (Twin) Combined Sample

Neither 21/12 = 1.8 2/4 = 0.5 14/11 = 1.3 5/4=1.3 42/31 = 1.4

Mother only

Father only

Both

Overall ratio

5/2 = 2.5 4/3 = 1.3 2/3 = 0.7 8/1 =8.0* 0/0 19/9 = 2.1

12/6 = 2.0 6/1=6.0 3/6 = 0.5 16/4 = 4.0* 1/2 = 0.5 38/19 = 2.0'

4/3 = 1.3 1/0 3/3 = 1.0 1/1 = 1.0 1/0 10/7 = 1.4

42/23 = 1.8» 13/8=1.6 8/12 = 0.7 39/17 = 2.3' 7/6= 1.2 109/66=1.7"

*p.O5). Table 3 presents gender ratios for unaffected siblings of reading-disabled probands. In none of the five studies is there a significant overall excess of unaffected male siblings. In fact, most of the gender ratios are 1 : 1 or less, both in the individual studies and the combined sample, which exhibits an overall ratio which is significantly less than 1 : 1(0.8 : 1, X^ = 4.28, d.f. = 1, pK Table 3. Gender ratios of unaffected siblings (number of males/number of females)

Study Colorado (Family) Iowa (Family) Linkage (Family) Colorado (Twin) London (Twin) Combined Sample

Neither 24/41=0.6* 11/12 = 0.9 — 38/35 = 1.1 19/30 = 0.6 92/118 = 0.8

Affected parent Father Mother only only 4/6 = 0.7 2/2 = 1.0 1/1 = 1.0 4/8 = 0.5 2/1=2.0 13/18 = 0.7

17/8 = 0.9 8/7 = 1.1 2/3 = 0.7 23/36 = 0.6 4/1=4.0 44/55 = 0.8

Both

Overall ratio

3/1=3.0 1/1 = 1.0 0/1 5/2 = 2.5 0/1 9/6 = 1.5

38/56 = 0.7 22/22 = 1.0 3/5 = 0.6 70/81=0.9 25/33 = 0.8 158/197 = 0.8*

*p

Gender ratios among reading-disabled children and their siblings as a function of parental impairment.

Gender ratios are reported for 374 reading-disabled probands and their 530 siblings included in five independent studies of reading disability. Ratios...
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