- The Heritability of Language: A Review and Metaanalysis of Twin, Adoption, and Linkage Studies
Some researchers argue that the ability to acquire and use language is largely the result of innate predispositions that are specific to language (the innateness hypothesis). If the innateness hypothesis is correct, these predispositions must be encoded for in our DNA. This article 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 variance in linguistic abilities among normal people. In addition to heritable factors that influence 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 the results of genetic studies that investigate the 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 acquire and use language the result of innate predispositions that are specific to language (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, children acquire language very quickly and with relatively few errors (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 or spare language (modularity arguments). Although these data are generally consistent [End Page 647] 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 proinnatist perspective of these data and Müeller 1996 and Karmiloff-Smith and colleagues (Elman et al. 1996, Karmiloff-Smith 1991, Karmiloff-Smith & Karmiloff 2001) for a more skeptical view.
Genetic studies of language are another way of investigating the innateness hypothesis. If the innateness hypothesis is correct, the cognitive and neural predispositions that enable us to acquire and use language must be encoded for in our DNA. Thus, if studies reveal that genetic factors play a role in people’s linguistic abilities, this supports the innateness hypothesis. But there are at least two reasons why genetic studies might 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 language is the 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 among individuals 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...