Journal of Speech and Hearing Research, Volume 35, 1086-1094, October 1992

A Study of Developmental Speech and Language Disorders in Twins Barbara A. Lewis Department of Pediatrics Case Western Reserve University Cleveland, OH Lee A. Thompson Department of Psychology Case Western Reserve University Cleveland, OH

Fifty-seven same-sex twin sets (32 monozygotic and 25 dizygotic) were examined for concordance of speech and language disorders. Results showed monozygotic twins to have higher concordance than dizygotic twins. Inaddition, monozygotic twins were more similar inthe types of disorders they presented than dizygotic twins. Positive family histories for speech, language, and learning disorders were reported inthe nuclear families of the twins. KEY WORDS: twins, genetic, speech/language disorder, learning disabilities

Growing evidence suggests that at least some developmental speech and language disorders are familial in nature (Kidd, 1983; Lewis, Ekelman, & Aram, 1989; Felsenfeld, McGue & Broen, 1992; Tallal, Ross, & Curtiss, 1989; Tomblin, 1989). These studies have documented a familial concentration of speech, language, and learning disorders among family members of individuals with such disorders. While these studies have been invaluable in laying the foundation for further genetic studies, such studies cannot separate environmental and genetic factors. In fact, a genetic component to the disorder cannot be demonstrated by a family study alone. Familial aggregation of a disorder may be due to a shared family environment, rather than genetic factors. Classical twin methodology provides a tool for establishing a genetic contribution to speech and language disorders.

Twin Methodology The classical twin design compares the phenotypes (i.e., the characteristics that the individual presents) of identical and same-sex fraternal twin pairs. As both types of twins share the same family environment, differences between the members of a twin set are attributed to genetic factors. Identical, or monozygotic (MZ), twins are genetically alike, whereas fraternal, or dizygotic (DZ), twins share on the average 50% of segregating genes (Plomin, DeFries, & McClearn, 1990). If heredity is affecting a particular trait, then MZ twins should be twice as similar as DZ twins on the measure. If MZ and DZ twins' correlations do not differ for the measure, then heredity cannot have a major influence on the measure. The twin design has been useful in establishing a genetic component to dyslexia (Bawkin, 1974; Hallgren, 1950; Hermann, 1959; Weinschenk, 1965) and stuttering (Howie, 1981A; Howie, 1981B; Kidd, 1983; Nelson, Hunter, & Walter, 1945; Seemann, 1959). Without exception, MZ twins have been found to be more concordant for these disorders than same-sex DZ twins, thus establishing a genetic component to each. Concordance rates for dyslexia have been estimated at .92 for MZ twins, and .49 for DZ twins (Vandeberg, 1967). Concordance rates for stuttering range from .53 to .83 for MZ twins, and -.09 to .19 for DZ twins (Howie, 1981a; Howie, 1981b; Nelson, Hunter, & Walter, 1945). © 1992, American Speech-Language-Hearing Association

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Lewis & Thompson: Speech-Language Disordered Twins

Normal Speech and Language n Twins To date, with the exception of a few case reports (Bentley, 1988; Levi & Bemabei, 1976), no studies have examined concordance for speech and language disorders in twins. However, many studies have explored normal speech and language abilities in twin sets (Hay & O'Brien, 1983; Lenneberg, 1967; Locke & Mather, 1989; Matheny & Bruggemann, 1973; Mather & Black, 1984; Mittler, 1970; Munsinger & Douglaus, 1976; Osborne, Gregor, & Miele, 1968). Studies of normal speech and language skills in twins have had two main goals. One group of studies has sought to use a twin design to estimate the relative contributions of genetics and environment to speech and language by employing the methodology described above (Locke & Mather, 1989; Matheny & Bruggemann, 1973; Munsinger & Douglaus, 1976; Osborne, Gregor & Miele, 1968). The other has attempted to describe speech and language development in twins and highlight differences between twins and singletons (Conway, Lytton, & Pysh, 1980; Hay, Prior, Collett, & Williams, 1987; Koch, 1966; Mittler, 1970). These studies have examined the unique impact twinning has on speech and language development. Some large longitudinal twins projects, such as the Louisville Twin Study (Matheny & Bruggemann, 1973) and the La Troube Twin Study (Hay & O'Brien, 1983), have incorporated both of these goals. Twin studies have unanimously demonstrated a higher concordance for normal MZ than DZ twins on a variety of speech and language measures (Hay & O'Brien, 1983; Lenneberg, 1967; Locke & Mather, 1989; Matheny & Bruggemann, 1973; Mather & Black, 1984; Mittler, 1970; Munsinger & Douglaus, 1976; Osborne, Gregor, & Miele, 1968). Estimates of heritability of normal speech and language skills employing twins are around .78 (Osborne et al., 1968). Environmental contributions were estimated at .10 (Munsinger & Douglaus, 1976). Several explanations for differences between MZ and DZ twins have been proposed. One study by Locke and Mather (1989) specifically examined articulation skills and reported that shared articulatory errors were exhibited by 82% of MZ twins and by 56% of DZ twins. The authors suggested that MZ twins are more likely to demonstrate shared errors due to shared phonetic constraints of the articulatory mechanism. That is, MZ twins are more similar in their articulatory structures than DZ twins and, thus, are more likely to develop similar speech sound pattems. Another study (Matheny & Bruggemann, 1973) suggested that not only motor proficiency, but also similar cognitive skills, accounted for greater similarity in articulation skills between MZ than DZ twins. MZ twins are more similar than DZ twins in specific cognitive abilities, especially verbal skills (Thompson, Detterman, & Plomin, 1991). MZ twins with speech and language disorders may be more similar in verbal skills than DZ twins. Studies whose purpose has been to describe the unique characteristics of speech and language development in twins have generally reported a speech and language delay for both male and female twins, with about a 6 month greater lag in males than females (Conway et al., 1980; Hay et al., 1987; Koch, 1966; Lenneberg, 1967; Mittler, 1970). However, twins reportedly catch up in their speech and language develop-

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ment by 3 to 4 years of age, with females more advanced than males (Hay et al., 1987; Mittler, 1970). Explanations for this delayed speech and language acquisition are perinatal factors such as prematurity and oxygen deprivation and the unique environment that twinning creates, including styles of mother-child interactions (Conway et al., 1980), and the twin idioglossia reported in the literature (Bakker, 1987). Most of the language delay experienced by twins has been attributed to environmental rather than biological factors (Mittler, 1970; 1976; Tomasello, Mannle, & Kruger, 1986). Mittler (1970) reports that twins do not demonstrate unique linguistic patterns, but rather an overall immaturity in verbal skills. Twins reportedly speak less and employ shorter utterances (Conway et al., 1980). This speech and language delay is reported equally in MZ and DZ twins (Mittler, 1970). Environmental factors cited as contributing to this delay are less frequent mother-child interactions, as well as the mother of twins employing shorter, less complex, and more directive utterances (Conway et al., 1980). A second environmental factor is the presence of a second child (i.e., the twin) acquiring language at exactly the same time. Twin idioglossia, or autonomous languages, are well documented. Bakker (1987) reviewed the literature on autonomous languages and concluded that twins do not create a language, but rather employ onomatopoeic expressions, some invented words, and adult language adapted to the child's phonological constraints-not unlike the singleton child. However, in the presence of another child acquiring language at the same time, communicative attempts are reinforced. Autonomous languages are mostly unintelligible and lack morphology; however, the twins understand each other. They are reported in 40% of twin pairs, slightly more in MZ than DZ twins, and disappear quickly. In utilizing the twin methodology to document a genetic component to speech and language disorders, a delay in speech and language acquisition due to the twin environment must be separated from a speech and language disorder with a possible genetic basis. As twins reportedly catch up to singletons by 3 to 4 years of age, twins with speech and language disorders that persist into grade school may have a genetic component involved in their disorder.

Twins With Speech and Language Disorders Case reports of speech and language disorders in twins are confounded by the inclusion of twins with other developmental disorders, such as cerebral palsy (Bentley, 1988), and the lack of standardized speech and language testing (Levi & Bemabei, 1976). Some evidence of concordance for disorders is found in these reports. Borges-Osorio and Salzano (1985) reported on three sets of MZ twins with learning disabilities who were concordant for their disabilities. However, to date, no study has examined a large group of speech and language disordered twins, both MZ and DZ, for concordance of speech and language disorders. Such a study would provide evidence that not only are some developmental speech and language disorders familial in nature, but such disorders are in part genetic.

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TABLE 1. Subject group demographics for four twin groups. One twin with speech & language disorders (n = 50)

Both twins with speech & language disorders (n = 64) Subject variables Male (%) Female (%) Age at test (years)

MZ (n = 48)

DZ (n = 16)

MZ (n = 16)

DZ (n = 34)

75 25

87.5 12.5

75 25

64.7 35.3

M

9.42

SD

1.71

M

9.04

SD

1.79

M

10.09

The present study examines speech and language disorders in twins drawn from a larger project, the Western Reserve Twin Project (WRTP). The study, designed as a preliminary investigation, employed questionnaires rather than direct testing of subjects. The following questions were addressed: Do MZ twins have higher concordance for speech and language disorders than DZ twins? Are MZ twins more similar in the nature of their speech and language problems than DZ twins? Do speech and language disordered twins present positive family histories for speech, language, and learning disorders?

Method Subjects The subjects were 131 same-sex twin sets that had participated in a previous study, the Western Reserve Twin Project (Thompson, et al., 1991) that had involved extensive intelligence and achievement testing. Twins were between 6 and 12 years of age (M = 9.88; SD = 1.65) upon initial participation in the WRTP study. Zygosity was determined by means of a zygosity questionnaire devised by Nichols and Bilbro (1966) assessing physical characteristics with a reported accuracy of 95%. When zygosity was uncertain, blood sampling was performed. Twins with histories of speech and language disorders were identified through a letter explaining the study. The letter was mailed to the parents of 300 twin sets. Parents returned a postcard indicating if one, both, or neither of the twins had received speech-language treatment. One hundred forty-nine (49.7%) of the parents of twins returned the postcard questionnaire. Of these, 57 (38.2%) indicated that one or both twins had received speech-language treatment, and 92 (61.7%) indicated that neither twin had received treatment. (It was felt the percentages did not reflect the actual percentage of twins enrolled in speech-language treatment, as it seemed that parents whose children had received treatment were more prone to return the questionnaire.) Families indicating that one or both twins had received speech-language treatment participated in a 20-min phone interview conducted by a licensed and certified speechlanguage pathologist. The purpose of the interview was to determine the nature of the speech and language disorder, the presence or absence of accompanying learning problems, the severity of the problems, and family history for Downloaded From: http://jslhr.pubs.asha.org/ by a La Trobe Univ User on 10/25/2016 Terms of Use: http://pubs.asha.org/ss/rights_and_permissions.aspx

SD

1.48

M

8.97

Total (N = 114) DZ (n = 50)

MZ (n = 64) 72.41 27.59

SD

1.95

M

9.35

SD

1.76

speech and language disorders (see Appendix for follow-up phone questionnaire). Following the selection process, the sample consisted of 57 sets of same-sex twins (32 MZ, 25 DZ) in which at least one of the twins had received speech-language treatment. Forty-two twin sets were male and 15 sets were female. This represents a 2.8 : 1 male-to-female ratio. Sex ratios commonly reported in speech and language disordered populations are about 2.4 males to 1.5 females (Leske, 1981). The mean age at the time of testing was 9.35 years (SD = 1.76). Socioeconomic (SES) status was determined through use of the Hollingshead Four Factor Index of Social Class (Hollingshead, 1975). No significant differences between MZ and DZ twins were found in sex (X2 = 0.00; p > 1.0); SES (X2 = 6.22; p > .10); IQ (t = .83; p > .41); or age (t = 1.83; p > .07). See Table 1 for subject group demographics. Scores from the standard test battery administered previously by the Western Reserve Twin Project on the WISC-R, the Peabody Picture Vocabulary Test-Revised (PPVT-R; Dunn & Dunn, 1981), and the spelling and reading subtests of the Wide Range Achievement Tests-Revised (WRAT-R; Jastak & Wilkinson, 1984) were obtained for each of the four subject groups (MZ with both twins affected; DZ with both twins affected; MZ with one twin affected; DZ with one twin affected). As shown inTable 2, mean full-scale IQ scores, as well as mean verbal and performance subscale IQ scores, fell within the normal range for all four groups. Similarly, mean performance on the PPVT-R, the WRAT reading subtest, and the WRAT spelling subtests were within normal limits for all four groups. However, the range of test scores revealed much variability in IQs and reading and spelling skills with some subjects' scores falling below average. Due to the large differences in ns among the four groups, parametric statistics were not employed.

Data Analysis Data analyses were designed to answer the three questions posed earlier. First, concordances for speech and language disorders in MZ and DZ twins were compared. Second, the types of speech, language, and learning disorders reported by the parents were tallied. Data were examined for similarities in disorder types within twin sets. Finally, family history data for speech, language, and learning disor-

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TABLE 2. Standard scores on tests for twin groups. Both twins with disorder Measures Full Scale WISC-R

Verbal IQ Performance IQ PPVT WRAT Read WRAT Spelling

MZ (n = 48)

DZ (n = 16)

MZ (n = 16)

DZ (n = 32)

103.62

102.50

112.68

104.00

16.04 89-133 111.44 12.99 90-131 111.00 16.24 91-132 105.69 14.60 72-131 102.62 17.51 77-138 102.12 14.34 73-136

16.48 66-139 103.16 18.55 58-141 102.94 15.53 73-133 100.75 13.50 61-129 99.47 14.47 65-132 99.28 16.75 53-137

M

SD Range M SD Range M SD Range M SD Range M SD Range M SD Range

17.50 69-135 100.46 17.02 66-133 107.62 15.82 72-135 100.15 16.64 48-127 92.46 14.56 50-116 92.70 16.29 62-119

ders for first-degree relatives of the twins (i.e., parents and siblings) were compiled. Results . .___.._

Concordance for Speech and Language Disorders As shown in Table 3, 24 sets of MZ and 8 sets of DZ twins comprised the group in which both twins had received speech-language treatment. In contrast, 8 sets of MZ and 17 sets of DZ twins comprised the group in which only one twin had received speech and language treatment. Twin concordance rates can be calculated using either a pairwise or a probandwise approach. The manner in which the probands are ascertained determines which approach is appropriate. If it is possible to ascertain only one member per twin pair as a proband, then the pairwise concordance rate is the appropriate choice. However, as is the case inthe current study, when both members of the twin pair can be ascertained, pairwise concordance underestimates both the MZ and DZ twin concordance, but affects the DZ twin concordance rate to a greater degree, thus producing an overestimate of the importance of genetic influences (DeFries & Gillis, 1991). Therefore, the correct approach is to calculate concordances based on the proband method, which involves dividing the total number of affected individuals in concordant pairs by the total number of affected individuals across both concordant and discordant pairs (Plomin et al., 1990). For the current study, 24 concordant and 8 TABLE 3. Probandwise concordance for speech and language disorders In monozygotic and dizygotic twin sets. Measures Monozygotic twins Dizygotic twins

One twin with disorder

Concordant pairs

Discordant pairs

Ratio

24 8

8 17

0.86 0.48

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14.95 80-128 99.06 13.49 77-127 105.56 17.25 75-138 96.31 16.61 75-132 83.94 17.35 50-105 85.25 14.28 59-103

discordant MZ twin pairs were identified. Probandwise concordance equals .86 for identical twins [48/(48 + 8) = .86]. Similarly, 8 concordant and 17 discordant DZ twin pairs were identified, yielding a probandwise concordance of .48 [16/(16 + 17) = .48]. Thus, MZ twins were more concordant for speech and language disorders than DZ twins, suggesting a genetic component to some speech and language disorders.

Types of Speech and Language Disorders Reported Follow-up phone interviews conducted by a speech-language pathologist were employed to determine the nature of the speech, language, and learning disorders in terms of the following categories: articulation, language, voice, delayed onset of speech, tongue thrust, lisp, stuttering, hearing loss, and learning disorders. As shown inTable 4, the most frequently reported problem was an articulation disorder, with 79.7% of the speechlanguage disordered twins reporting this problem. The second most frequent problem was learning disorders, with 39.3% reported; delayed onset of speech followed, with 15.7% of the disordered children reporting this problem. MZ twins differed from DZ twins in the percentage of articulation disorders reported, with MZ twins demonstrating more articulation difficulties (chi-square = 5.133; p = .023). MZ and DZ twins did not differ significantly on other disorder types. Twin pairs were examined for concordance of disorder type. As shown inTable 5, MZ twins were more concordant in the type of disorder presented than DZ twins. Articulation disorders represented the most frequently reported disorder. Twenty MZ twins were concordant for articulation disorders, with only 1 set of MZ twins discordant. In contrast, only 4 sets of DZ twins were concordant for articulation disorders, with 14 sets discordant. Although the incidence of the remaining disorders was low, in each case MZ twins were found to be more similar in the type of disorders they presented than DZ twins. However, as direct

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TABLE 4. Frequency of reported disorders types. Both twins MZ (n = 48) Articulation disorder Learning disorders Delayed onset of speech Language disorders Stuttering Tongue thrust Lisp Hearing loss Voice disorder

One twin

DZ (n = 16)

DZ (n = 17)

(N = 89)

Freq.

%

Freq.

%

Freq.

%

Freq.

%

Freq.

%

41 16 11 6 3 5 4 6 2

85.4 33.3 27.5 15.8 7.5 13.2 10.5 15.8 5.3

8 6 0 3 3 2 0 0 0

50.0 37.5 0.0 25.0 25.0 16.7 0.0 0.0 0.0

6 7 1 0 1 0 2 0 1

75.0 87.5 12.5 0.0 12.5 0.0 25.0 0.0 12.5

14 6 2 2 2 1 2 0 0

82.3 35.3 11.8 11.8 11.8 5.9 11.8 0.0 0.0

71 35 14 11 9 8 8 6 3

79.7 39.3 15.7 12.3 10.1 8.9 8.9 6.7 3.4

speech and language testing was not performed, it is not known if twins were similar in the severity of the disorders they demonstrated or the type of speech sounds in error. Family Histories for Speech and Language Disorders Family histories for speech, language, and leading disorders were obtained through parent interviews. As shown in the Appendix, parents were first asked to list all nuclear family members and their relation to the proband. The parent indicated the last grade each family member had completed and whether or not each member demonstrated reading problems inschool or had received speech-language treatment. Finally, the parent was questioned as to whether each member exhibited any of the TABLE 5. Concordance for type of disorders reported. Concordant palrs Articulation Learning disorders Delayed speech onset

MZ (n = 8)

Total

Discordant pairs

Ratio

MZ

DZ

MZ

DZ

MZ

DZ

20 8 5

4 3 0

1 7 2

14 6 2

.95 .53 .71

.22 .33 .0

15 disorders listed on the questionnaire. As shown in Table 6, when twins were concordant for speech and language disorders, a higher incidence of first-degree family members reported problems than when twins were discordant. Incidence for speech and language disorders was 25% of first-degree relatives of concordant twins (23% MZ; 27% DZ); 13.5% of first-degree relatives for discordant twins. Learning disorders were reported as follows: 23.5% of first-degree relatives of concordant pairs (16.4% MZ; 30.7% DZ); and 4.1% of discordant pairs. Of family members of concordant twin pairs, brothers were most often affected with speech and language disorders (47% MZ; 43% DZ). More mothers reported a history of speech and language disorders than fathers, with 22% of MZ mothers and 17% of DZ mothers, while 9% of MZ fathers and no DZ fathers reported speech-language problems. These findings were in close agreement with our previous family studies (Lewis, et al., 1989), which also found that brothers of the probands were most often affected.

Discussion This was a preliminary study to demonstrate the feasibility of employing a classical twin study design to explore the genetic basis of developmental speech and language disorders. As the data were based primarily on parental report

TABLE 6. Family history for speech, language, and learning disorders. Both twins

One twin

MZ (%)

DZ (%)

MZ (%)

DZ (%)

Speech & language Mothers Fathers Sisters Brothers All 1st-degree relatives

22 9 25 47 23

17 0 43 43 27'

14 0 12 11 11

10 10 17 27 162

Learning disorders Mothers Fathers Sisters Brothers

4.3 26.1 0 33.3

16.6 16.6 42.81 42.81

0 0 10 0

0 10 0 9.1

All 1st degree relatives

16.4

30.71

5.42

2.8 2

Combined total both twins: Speech-language = 25%; learning disorders = 23.5%. Combined total one twin: Speech-language = 13.5%; learning disorders = 4.1%. Downloaded From: http://jslhr.pubs.asha.org/ by a La Trobe Univ User on 10/25/2016 Terms of Use: http://pubs.asha.org/ss/rights_and_permissions.aspx

Lewis & Thompson: Speech-Language DisorderedTwins

rather than direct testing of twin pairs, care must be taken in interpreting the results. Direct testing would allow verification of speech and language disorders, as well as the phenotype presented. Concordance for the speech sounds in error, as well as the nature of the language or learning disorder, could be examined. With this limitation in mind, the following conclusions may be stated.

Concordance of Twin Sets MZ twins had higher concordance for speech and language disorders than DZ twins, thus providing support for a genetic basis for some developmental speech and language disorders. Previous family studies demonstrated that speech and language disorders tend to aggregate within families; however, genetic and environmental influences could not be distinguished. This study suggests a genetic component to some speech and language disorders. The concordance rates obtained in this study (86% MZ; and 48% DZ) are within the range of concordances reported in previous studies (Matheny & Bruggemann, 1973; Munsinger & Douglaus, 1976; Osborne et al., 1968) of speechlanguage skills in normal twin pairs without histories of speech-language treatment. However, this study relied on parental report rather than direct testing of twin pairs. Further investigation is warranted to confirm that concordances for normal speech-language skills are similar to concordances for speech and language disorders. The mechanism for the genetic influence suggested by this study is unknown. Two explanations proposed for normal speech-language skills are (a) similarity in structure and function of the articulatory mechanism (Locke & Mather, 1989) and (b) similar cognitive processing abilities (Matheny & Bruggemann, 1973). Locke (1983) proposed that phonological errors can be described within an anatomical and physiological framework. MZ twins are more similar in their physical features than DZ twins, including the morphology of the speech sound production system. This anatomical similarity of the speech production system may result in more shared speech sound errors in MZ than DZ twins. Thus, genetic control may account for the higher concordance in articulation disorders of MZ than DZ twins. Alternatively, MZ twins show greater similarity than DZ twins in specific cognitive abilities, with verbal abilities appearing to be the most genetically influenced (Thompson et al., 1991). Greater similarities in cognitive ability (Matheny & Bruggemann, 1973) may account for a higher concordance in MZ twins than DZ twins for speech and language disorders. Studies of normal twins have demonstrated greater similarity in MZ than DZ twins on semantic and syntactic measures, as well as articulation (Mather & Black, 1984; Munsinger & Douglaus, 1976). Greater similarity in MZ than DZ twins for speech and language disorders may be due to more shared difficulty in the processing and production of language. Future studies may design tasks that directly test these two hypotheses. A third possible explanation for the higher concordance of MZ than DZ twins for speech and language disorders is the Downloaded From: http://jslhr.pubs.asha.org/ by a La Trobe Univ User on 10/25/2016 Terms of Use: http://pubs.asha.org/ss/rights_and_permissions.aspx

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reported slightly higher incidence of idioglossia (autonomous languages) in MZ than DZ twins (Mittler, 1971). The articulation disorders observed in the school-age twins may be residual articulation errors that once comprised the autonomous language. While such a hypothesis may account for concordance for articulation disorders in some twin pairs, it is unlikely that it accounts for all speech and language disorders, as few of the parents of twins concordant for articulation disorders (N = 4; 2 sets MZ, and 2 sets DZ twins) reported "twin talk" in this study. None of the parents of twins discordant for speech and language disorders reported autonomous language. Further, it cannot account for the increased incidence of speech, language, and learning disorders among nuclear family members. Future studies may employ a longitudinal design to determine the effect of autonomous languages on later speech and language disorders. Types of Speech and Language Disorders MZ twins were more likely to demonstrate similar types of speech and language disorders than DZ twins. Again, this finding is viewed as preliminary because direct testing of twins was not performed. The categories of speech, language, and learning disorders employed in this study were broad, and accurate classification of the disorder depended on the reliability of parental reports. However, it suggests a genetic influence not only on whether or not the individual demonstrates a speech and language disorder, but also on the type of disorder that the individual presents. Locke and Mather (1989) found that in normal twin pairs, MZ twins had more shared articulatory errors than DZ twins. Future investigations of twins with speech and language disorders may examine whether or not this finding is applicable to speechand language-disordered populations as well. Family Histories for Speech and Language Disorders Both MZ and DZ twins reported positive family histories for speech, language, and learning disorders, with a larger percentage of first-degree relatives affected when twin sets were concordant for speech and language disorders than when twins were discordant. Family history reports of developmental speech, language, and learning disorders of first-degree relatives of the twins are in agreement with previous family studies of speech and language disorders (Lewis, et al., 1989; Tallal et al., 1989; Tomblin, 1989). The percentages of affected nuclear family members closely parallel our previous data. In both studies, brothers of probands were most often affected. The greater incidence in males than in females (2.8 males to 1 female) is consistent with studies of the incidence of speech and language disorders reported in the general population (Leske, 1981). Clinical Implications Parents often question the speech-language pathologist as to the possible cause of their child's disorder. Many times

1092 Journal of Speech and Hearing Research the therapist can offer no explanation for the child's problem. The results of this study suggest that one etiological factor that should be considered is a familial or genetic basis for some developmental speech and language disorders. Frequently, speech-language pathologists obtain this family history information through parental interviews during diagnostic evaluations. Although the study of the genetic basis of speech and language disorders is far from complete, and molecular genetic techniques (i.e., biochemical analysis to examine the molecular structure of DNA) have not yet been applied to the study of speech and language disorders, the speechlanguage pathologist may use the results of current research in clinical practice. Positive family histories for speech, language, or learning disorders may be useful in deciding whether or not to enroll a young child in speechlanguage treatment. Early screening tools may incorporate questions concerning family history for disorders. Prognostic statements may be formulated in the context of the child's family history for similar problems. Future studies of speech, language, and learning disorders should directly test twins and other family members to document a genetic basis for some speech and language disorders and to estimate the relative contributions of genetics and environment to those disorders.

Acknowledgments This work was supported in part by NICHD Grant HD-21947 and MH-46512. We are grateful to the Western Reserve Twin Project and Douglaus K. Detterman for generously sharing their subjects and data with us. We also wish to thank Lisa Freebaim-Tarr for her assistance in this project and the parents of the twins who took the time to respond to our questionnaire.

References Bakker, P. (1987). Autonomous languages of twins. Acta Geneticae Medicae et Gemellologiae, 36, 233-238. Bawkin, H. (1974). Reading disability in twins. Developmental Medicine and Child Neurology, 15, 184-187. Bentley, P. (1988). Speech and language delays in identical twins. Report No. EC 211 260. National College of Education. (ERIC Document Reproduction Service No. ED 300 965). Borges-Osorio, M. R. L., &Salzano, F. M. (1985). Language disabilities in three twin pairs and their relatives. Acta Geneticae Medicae et Gemellologiae, 34, 95-100. Conway, D., Lytton, H., &Pysh, F. (1980). Twin-singleton language differences. Canadian Journal of Behavioral Sciences, 12, 264271. DeFrles, J. C., & GlIlls, J. J. (1991). Etiology of reading deficits in learning disabilities: Quantitative genetic analysis. In Neuropsychological foundations of learning disabilities. New York: Academic Press. Dunn, L. M., & Dunn, L. M. (1981). Peabody Picture Vocabulary Test-Revised. Circle Pines, MN: American Guidance Service. Felsenfeld, S., McGue, M., & Broen, P.A. (1992). A 28-year follow-up of adults with a history of moderate phonological disorder: Linguistic and personality results. Journal of Speech and Hearing Research, 35, 1114-1125. Hallgren, B. (1950). Specific dyslexia: A clinical and genetic study. Acta Psychiatrica Neurological Scandanavian (Suppl), 65, 1-287.

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35 1086-1094 October 1992 Hay, D. A., & O'Brien, P.J. (1983). The La Trobe twin study: A genetic approach to structure and development of cognition in twin children. Child Development, 54, 317-330. Hay, D. A., Prior, M., Collett, S., & Williams, M. (1987). Speech and language development in twins. Acta Geneticae Medicae Gemelloliae, 36, 213-223. Hermann, K. (1959). Reading disability: A medical study of word blindness and related handicaps, Springfield, IL: Charles C. Thomas Publishing. Hollingshead, A. B. (1975). Four factor index of social class. Unpublished manuscript. Yale University, New Haven, CT. Howle, P. M. (1981a). Concordance for stuttering in monozygotic and dizygotic twin pairs. Journal of Speech and Hearing Research, 24, 317-321. Howle, P. M. (1981 b). Intra-pair similarity in frequency of dysfluency in MZ and DZ twin pairs containing stutterers. Behavior Genetics, 11, 227-238. Jastak, S., & Wilkinson, G. S. (1984). Wide Range Achievement Test--Revised. East Aurora, NY: Slosson Educational Publications, Inc. Kidd, K. K. (1983). Recent progress of the genetics of stuttering. In C. A. Ludlow & J. A. Cooper (Eds.), Genetic Aspects of Speech and Language Disorders. New York, NY: Academic Press. Koch, H. (1966). Twins and twin relations. Chicago, IL: Univ. of Chicago Press. Lenneberg, E. (1967). Biological foundations of language. New York: Wiley. Leaks, M. C. (1981, March). Prevalence estimate of communication disorders in the US: Speech disorders. Asha, pp. 217-225. Levi, G., &Bernabel, P. (1976). Specific language disorders in twins during childhood. Acta Geneticae Medicae Gemelloliae, 25, 366368. Lewis, B. A., Ekelman, B. L., &Aram, D. M. (1989). A familial study of severe phonological disorders. Journal of Speech and Hearing Research, 32, 713-724. Locke, J. E. (1983). Phonological acquisition and change. New York: Academic Press. Locke, J. L., & Mather, P. L. (1989). Genetic factors in the ontogeny of spoken language: Evidence from monozygotic and dizygotic twins. Child Language, 16, 553-559. Matheny, A. P., & Bruggemann, C. E. (1973). Children's speech: Heredity components and sex differences. Folia Phoniatrica, 25, 442-449. Mather, P. L., & Black, K. N. (1984). Heredity and environmental influences on preschool twins' language skills. Developmental Psychology, 20, 303-308. Mittler, P. (1970). Biological and social aspects of language development in twins. Developmental Medicine and Child Neurology, 12, 741-757. Mittler, P. (1971). The study of twins. London: Penguin Books. Mittler, P. (1976). Language development in young twins: Biological, genetic, and social aspects. Acta Geneticae Medicae Gemellologiae, 25, 359-365. Munsinger, H., & Douglaus, A. (1976). The syntactic abilities of identical twins, fraternal twins, and their siblings. Child Development, 47, 40-50. Nelson, S. F., Hunter, N., & Walter, M. (1945). Stuttering in twin types. Journal of Speech Disorders, 10, 335-343. Nichols, R.C., & Blibro, W.C. (1966). The diagnosis of twin zygosity. Acta Genetics et Statistics Medics, 16, 265-275. Osborne, R. T., Gregor, A. J., & Miele, F. (1968). Heritability of factor V: Verbal comprehension. Perceptual and Motor Skills, 26, 191-202. Plomin, R., DeFrles, J. C., & McCleam, G. E. (1990). Behavior genetics: A primer. (2nd Ed.). New York: H. W. Freeman. Seemann, M. (1959). Sprachstorungen bei kindren. Marhold, Germany: Halle/Saale. Tallal, P., Ross, R., & Curteis, S. (1989). Familial aggregation in specific language impairment. Journal of Speech and Hearing Disorders, 54, 167-173. Thompson, L. A., Detterman, D. K., & Plomin, R. (1991). Associations between cognitive abilities and scholastic achievement:

Lewis & Thompson: Speech-Language Disordered Twins Genetic overlap, but environmental differences. Psychological Science, 3, 158-165. Tomasello, M., Mannle, S., & Kruger, A. (1986). Linguistic environment of 1 to 2 year-old twins. Developmental Psychology, 22, 169-176. Tomblln, J. B. (1989). Familial concentration of developmental language impairment. Journal of Speech and Hearing Disorders, 54, 587-595. Vandeberg, S. (1967). Heredity and dyslexia. Bulletin of the Orton Society, 17, 54-56.

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1093

Weinechenk, C. (1965). Die erbliche Rechtschreibschwache und hire sozlalpsychietrischen Auswirkungen. Bern, Switzerland: Huber. Received August 8, 1991 Accepted January 24, 1992 Contact author: Barbara A. Lewis, PhD, Department of Pediatrics, Rainbow Babies and Children's Hospital, 2101 Adelbert Road, Cleveland, OH 44106.

1094

Journal of Speech and Hearing Research

35

1086-1094 October 1992

Appendix Date Caiedd: Sublect ID' Name:

TWIN STUDY QUESTIONNAIRE Name

FAMILY HISTORY

Family IDO

Addres:

SubjectID:

List all nuclear family members: (include brothers, siters, ons and daughters)

DOB' Dimnr

Twin's

7~u

Age.

r l.i .

.ygalmy.

erne,

NAME DOS SEX RELATION HANDEDNESS

LAST SPEECH READING GRADE THERAPY PROBLEM

Oate:

2.

SPEECH HISTORY Oates ofspeech therapy. ·"PIr "'·

3. 4.

rs Merv

nrlrne. rbereprBrs ranlePnam

Reasonfortherapy'

. Check all Iap h problem noted:

8.

Artculaton

Stuttering

Voice hoarse

Heanng

Mothers occupation: Father's occupation:

Latetalking

Grammar

Comprehension

Vocabulary

Readingproblems

Spellingproblems

Tonguethrust

Lstening problems

LISp

Unintelligible

Pleee check all that apply In present or pest: (Number Individual mnbers) PROBLEM

Listall sounds (letters)thathe/shehadtroublewith

a.

Stuttering

b.

Unintelligible speech

Mother Father Slter I

c. Articulation disorder

Didhe/sheever leavesoundsout of words?

d. Reading difficulties Didhe/sheever substituteonesoundfor another? Howwouldyou rate hilser speech problem? Mild

Profound

Severe

Moderate

Spelling

f.

Learning disability

g. Hearing h. Voice hoarse

HEARING HISTORY Hasyourchildever had anyhearingproblem? Explain.

i.

Late talking

j.

Grammar

k. Comprehension

Check all that apply: Ear Infections

a.

I. _

Howmany?

Hearingloss

Vocabulary

m. Tongue thrust n.

Listening problem

o.

Lisp

Wornhearingaid RingingIn ears Vergo or dizzyspells Tubesin ears ACADSMIC HISTORY

Send release of information form? Obtain records from'

Address:

Hasyour child everattendedapecal classes? Explain: Readingproblems?

ADDITIONAL INFORMATION: Spellingproblems? Expin

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Brother I

Ier

2 Brother 2