Evolution & Genetics of Language 1 Genetics and the Evolution of Language : What genetic studies reveal about the evolution of language

In this paper I argue that genetic studies of language provide insights about the evolution of language. The finding that genetic factors affect all aspects of language is consistent with theories that argue that language is at least partially the result of innate factors, and that the factors have evolved. Genetic factors appear to play a greater role for syntax and phonology than for the lexicon, suggesting that the evolutionary history of syntax and phonology differs from that of the lexicon. Genetic factors also affect the linguistic abilities of languageimpaired children more than normal children, consistent with the evolutionary history of language in language-impaired and normal children being different. The high degree of genetic overlap for linguistic abilities and oral motor and fine motor abilities is consistent with evolutionary theories that argue that complex fine motor skills and non-linguistic oral motor skills were the precursors to language. Similarly, the high degree of genetic overlap for linguistic and social ability suggests that aspects of language could have evolved from social abilities. The greater genetic overlap for syntax and phonology than for either the lexicon and syntax or the lexicon and phonology suggests that the evolutionary history of syntax and phonology is more similar than that of the lexicon and either syntax or phonology. Taken as a whole, these genetic findings are most consistent with linguistic models and evolutionary theories of language that invoke both general and language-specific factors. Stromswold, Evolution & Genetics of Language 2 Genetics and the Evolution of Language: What genetic studies reveal about the evolution of language Karin Stromswold Rutgers University – New Brunswick In this paper, I will discuss how genetic studies of language can inform discussions about the evolution of language. Depending on the results of genetic studies, the answer could be that genetic studies have nothing to add to such discussions. I take a more optimistic view and argue that genetic studies can and do provide insights about the evolution of language. The organization of this paper is as follows. I begin with a brief discussion of the relationship between genetics and the evolution of language, outlining some of the assumptions and limitations that exist about their relationship. I then briefly summarize findings of published behavioral studies and our ongoing twin study of language. I end with a brief description of current theories about the evolution of language , discussing how results from genetic studies can inform and constrain theories about how human language evolved. 1 Portions of this work were supported by grants from NSF (BCS-9875168, BCS-0002010, BCS0042561, BCS-0124095, BCS-0446838), NIH (HD37818), the Charles & Johanna Busch Biomedical Research Fund and the Bamford-Lahey Children's Foundation. I am grateful to the participants of the Morris Symposium on the Evolution of Language for their insights and comments. I am indebted to the twins and their parents who have participated in the Perinatal Environment and Genetic Interactions Study. Stromswold, Evolution & Genetics of Language 3 XXXI then will proceed to discuss molecular genetic studies of language and their implications for theories of the evolution of language. XXXI then will proceed to discuss molecular genetic studies of language and their implications for theories of the evolution of language. 1. The relationship between genetics and the evolution of language 1.1 Linguistic variability and the role of genetic factors. If language is heritable, this suggests that genetic factors play a role in language acquisition and/or the linguistic proficiency that people attain. It is logically possible that genetic factors affect the rate at which children acquire language, but that all people eventually attain the same linguistic proficiency, albeit at different ages. If this is the case, we would expect substantial heritability for the linguistic abilities of children, but not for adults. If all people have the same linguistic abilities by the time they reach sexual maturity, the lack of phenotypic differences among adults means that linguistic ability could not be selected for, unless children’s linguistic abilities affect the probability they will survive to adulthood. Results of some studies indicate that language acquisition does vary among children. Although the course of acquisition is remarkably similar for all children acquiring a given language (for a summary, see Stromswold, 2000), some children learn language more rapidly than others. For example, some normal 24-month old English-speaking children say only a handful of words, whereas others say over 300 words (e.g. Fenson et al., 1994; Goldfield & Stromswold, Evolution & Genetics of Language 4 Reznick, 1990). Brown (1973) and Cazden (1968) investigated when 3 English-speaking children acquired 14 grammatical morphemes, and found that, although all 3 children acquired these morphemes in the same order, the children acquired them at different ages. For example, all 3 children mastered the third person singular verbal inflection -s after they had mastered the homophonous plural and possessive morphemes, but one child mastered the -s verbal inflection at 2 years, 3 months (2;3), whereas the other two children did not achieve mastery until ages 3;6 and 3;8. Similar differences in the rate of acquisition have been reported for questions (Stromswold, 1988, 1995), auxiliary verbs (Stromswold, 1989, 1990), and datives, verbal particles and related constructions (Snyder & Stromswold, 1997; Stromswold & Snyder, 1995). It is also logically possible that all children acquire language at the same rate, but that as adults, they differ in their linguistic abilities. It is possible, for example, that whereas all children acquire the same basic competence in their language, at a certain age the linguistic development of some children stagnates where the linguistic development of other children continues. This is most plausible (and indeed is likely) with respect to vocabulary acquisition. Clearly, adults differ in the size of their vocabularies, and this may reflect the fact that some people regularly acquire new vocabulary items as adults, whereas the vocabularies of other people fossilize. It is also possible that adults have different grammatical knowledge (competence) or performance. For example, all normal adults may acquire the basic morphosyntax of their language, but only some adults master the morphosyntax of rare linguistic constructions (e.g., the English subjunctive, if I were a plant, I would have chlorophyll rather than hemoglobin). It is also possible that all adults have the same grammatical competence, but performance factors (such as short term memory size) affect the ease with which adults produce and perceive language. Given that what could potentially be selected for is observable phenotypes (in this case linguistic Stromswold, Evolution & Genetics of Language 5 performance) rather than unobserved differences in underlying competence, what is important from an evolutionary standpoint is that adults differ in linguistic performance rather than in the underlying linguistic ability that linguists are typically interested in. Furthermore, even if adults’ linguistic performance differs, it is possible that these differences do not affect reproductive success, just as eye color presumably does not affect reproductive success. In other words, even if we find significant heritabilities for adults’ linguistic abilities, it might tell us nothing about the evolution of language. Some studies suggest that, although all normal adults have a basic level of linguistic competence in their native language, some adults are more linguistically adept than others. For example, individual differences have been reported in verbal fluency (Day, 1979), in the interpretation of novel compound nouns (Gleitman & Gleitman, 1970), in sentence processing (Bever, Carrithers, Cowart, & Townsend, 1989; Just & Carpenter, 1992), and in the ability to acquire a second language as an adult (Fillmore, 1979). Even among native adult speakers of the same dialect, there are individual differences in grammaticality judgments (Cowart, 1997; Nagata, 1992; Ross, 1979; Schutze, 1996). 1.2 The role of genetics in language-impaired and linguistically normal people. It is possible that genetic factors play a role in the acquisition of language and the linguistic proficiency of language-impaired people, but genetic factors play no role for people who are not language-impaired (henceforth, normal people). If this is the case, it suggests that different genetic factors affect the linguistic abilities of language-impaired and normal people. Even if the same genetic factors are involved, if genetic factors play a substantially greater role for language-impaired than normal people, this suggests that there the populations may be genetically distinct from one another. Stromswold, Evolution & Genetics of Language 6 1.3 Heritability Estimates and Linguistic Variability. Heritability (h) is an estimate of the extent to which genetic factors account for the observed (phenotypic) variance in a trait. If people do not vary in linguistic ability, then the heritability estimate for language will be zero, even if genetic factors are completely responsible for human language. If there is significant heritability for language, this means that the linguistic abilities of people differ today and genetic factors account for a non-negligible amount of the variance. As discussed above, if there is genetically-determined variance in people’s linguistic abilities, this means that something that contributes to linguistic ability could have been selected for. It does not, however, mean that any linguistically-related ability was selected for. Furthermore, even it some language-related ability was selected for, it