The Heritability of Language: A Review and Metaanalysis of Twin, Adoption, and Linkage Studies

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THE HERITABILITY OF LANGUAGE: A REVIEW AND METAANALYSIS OF TWIN, ADOPTION, AND LINKAGE STUDIES KARIN STROMSWOLD RutgersUniversity Some researchers argue that the ability to acquire and use language is largely the result of innatepredispositionsthatarespecifictolanguage(the INNATENESSHYPOTHESIS).Iftheinnateness hypothesis iscorrect, these predispositions mustbe encoded forin our DNA. Thisarticle reviews more than one hundred genetic studies of language. The results of these studies strongly suggest that genetic factors play a role in the variation in the rate of language acquisition and linguistic proficiency attained by children and adults. Genetic factors account for much of the variance in linguistic abilities among people with written or spoken language disorders and some of the varianceinlinguisticabilitiesamongnormalpeople.Inadditiontoheritablefactorsthatinfluence both nonverbal and verbal abilities, there appear to be genetic factors that specifically influence linguistic abilities. Furthermore, some studies suggest that different genetic factors are involved in different aspects of language (e.g. written language vs. spoken language; lexical vs. syntactic abilities).* This article reviews theresults of genetic studies that investigatethe extent to which heritable factors play a role in the acquisition and use of language. The key questions that will be addressed are 1. Do heritable factors affect people’s ability to acquire and use language? 2. Are heritable factors responsible for (some of) the variation in linguistic abilities observed among‘normal’ people, or do heritable factors only account for the variance observed for people diagnosed with language disorders? 3. Are there heritable factors that are specific to linguistic abilities? 4. If genetic factors that are specific to language do exist, do such factors play a role in all aspects of language or just some? 5. If genetic factors play a role in multiple aspects of language, are different factors involved in different aspects of language? 1. INNATENESS AND THE HERITABILITY OF LANGUAGE. To what extent is the ability to acquireanduselanguagetheresultofinnatepredispositionsthatarespecifictolanguage (henceforth, the INNATENESS HYPOTHESIS)? Many different types of evidence suggest that language is due (in part) to innate cognitive and neural processes. For example, supporters of the innateness hypothesis point out that human languages share certain universal properties (linguistic arguments), that, even in the absence of negative evidence,childrenacquirelanguageveryquicklyandwithrelativelyfewerrors(learnability arguments), that functional neuroimaging and lesion-deficit correlational studies implicate left perisylvian cortical brain for certain language tasks (neuroanatomical arguments), and that some acquired and developmental disorders preferentially impair orsparelanguage(modularityarguments).Althoughthesedataaregenerallyconsistent *This work was supported by grants from the National Science Foundation (BCS-9875168), the John Merck Foundation on the Biology of Developmental Disabilities and the Johnson & Johnson Foundation. I thankmembersoftheaudienceatthetwentiethannualBostonUniversityConferenceonLanguageDevelopment, a Princeton Linguistic Colloquium, a University of Connecticut Linguistics Colloquium, and a UCLA Psychology Colloquium for helpful comments and suggestions. I am also grateful to Dorothy Bishop, Philip Dale, Jennifer Ganger, Jeffrey Gilger, Robert Plomin, and J. Bruce Tomblin for helpful discussion and for providingcopies ofunpublished manuscriptsand data.I amindebted totwo anonymousreferees andto Jean Berko Gleason for their very detailed and helpful reviews of an early version of this paper. 647 LANGUAGE, VOLUME 77, NUMBER 4 (2001)648 with the innateness hypothesis, the data are not all as clean as is sometimes presented. Space does not permit a review of arguments for and against these types of data, but interested readers may want to consult Pinker 1994 and Stromswold 2000 for a proinnatistperspectiveofthesedataandMu¨eller1996andKarmiloff-Smithandcolleagues (Elman et al. 1996, Karmiloff-Smith 1991, Karmiloff-Smith & Karmiloff 2001) for a more skeptical view. Geneticstudiesoflanguageareanotherwayofinvestigatingtheinnatenesshypothesis.Ifthe innatenesshypothesisiscorrect, thecognitiveandneural predispositionsthat enableustoacquireanduselanguagemustbeencodedforinourDNA.Thus,ifstudies reveal that genetic factors play a role in people’s linguistic abilities, this supports the innateness hypothesis.But there areat least two reasonswhy genetic studiesmight fail to reveal evidence of the heritability of language. One possibility is that the innateness hypothesis is incorrect, and environmental factors determine how facile people are at acquiring and using language. The other possibility is that languageisthe result of our genetic endowment, but everyone has exactly the same genetic endowment when it comes to language. Here’s how the second possibility works: for a genetic study to reveal that a trait is heritable, genetic factors must account for some of the variation observed amongindividuals for that trait. If individuals do not differ genetically with respect to a given trait, then heritability estimates for that trait will be zero even if the...

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