One of the central issues in cognition today is the degree to which cognitive abilities such as language are the result of specialized, genetically-encoded structures and predispositions (e.g., Pinker, 1994, Stromswold, 2000). In order to determine the extent to which genetic and environmental factors play a role in normal language and language disorders, we have conducted comprehensive reviews and meta-analyses of over 80 existing family aggregation studies, pedigree studies, sex-ratio studies, linkage studies, quantitative trait loci studies, twin studies and adoption studies which investigate the heritability of language. These meta-analyses reveal that heritable factors play a strong role in SLI and dyslexia, and in normal spoken and written language (Stromswold, 1996a, 1998, 2001, 2005a). These analyses also suggest some of these heritable factors are specific to linguistic tasks.
We have also collected and analyzed longitudinal data from of a set of monozygotic (MZ) and dizygotic (DZ) SLI twins (Stromswold & Rifkin, 1996). Even though all 4 children were diagnosed as being SLI, for all measures (age of acquisition, rate of errors, indices such as mean length of utterance and type/token ratios, performance on comprehension and production tests, etc.) and all areas of language assessed (phonology, lexicon, morphology, and syntax), the MZ twins were more similar to one another than the DZ twins. Furthermore, consistent with recent findings of Robert Plomin and Steven Reznick, whereas the MZ twins continued to exhibit very similar linguistic profiles over the course of the study, the DZ twins became linguistically more different as time passed. These results indicate that genetic factors play an important role in the acquisition of all aspects of language and that the role of genetic factors may increase as children get older.
In our on-going Perinatal Environment and Genetic Interactions (PEGI) twin study (Stromswold 2005a, 2005b, 2006a, 2006b, 2007; Stromswold et al., 2005), we are investigating how perinatal environment and genetic factors acting along and in concert with other factors affect linguistic and nonlinguistic development. Initial results of this study indicate that 1) genetic factors play a greater role in the linguistic abilities of language-impaired than those of normal twins; 2) genetic factors play a role in all aspects of language, 3) some of these factors are specific-to-language and some are not, 3) some genetic factors are specific to particular aspects of language (e.g., genetic factors that affect articulation and syntax but not vocabulary/lexical access), 4) genetic factors play a greater role for articulation and syntax than for vocabulary/lexical access), 5) adverse perinatal factors affect linguistic development more than cognitive development, and 6) adverse postnatal factors (e.g., low SES) affects cognitive development more than linguistic development. Lastly, we have explored the relationship between genetics and the structure, acquisition and evolution of language (Ganger & Stromswold, 1998; Stromswold, 2005b, 2007).
- Ganger, J. & Stromswold, K. 1998. The innateness, evolution and genetics of language. Human Biology 70, 199-213.
- Stromswold, K. 1996a. The genetic basis of language acquisition. Proceedings of the 20th Annual Boston University Conference on Language Development 2 , 736-747
- Stromswold, K. 1996b. Genes, specificity and the lexical/functional distinction in language acquisition. Behavioral and Brain Sciences 19, 648-9
- Stromswold, K. 1998. Genetics of spoken language disorders Human Biology, 70 , 297-324.
- Stromswold, K. 2000. The cognitive neuroscience of language acquisition. M. Gazzaniga (ed.), The new cognitive neurosciences, second edition, 909-932. Cambridge, MA: MIT Press.
- Stromswold, K. 2001. The heritability of language: A review and metaanalysis of twin, adoption and linkage studies. Language 77, 647-723.
- Stromswold, K. 2005a. Genetic specificity of linguistic heritability. A. Cutler (ed.), Twenty-first century psycholinguistics: Four cornerstones , 121-140. Mahwah, NJ: Lawrence Erlbaum Associates.
- Stromswold, K. 2005b. Genetics and the evolution of language: What genetic studies reveal about the evolution of language. Paper presented at the Alice V. and David H. International Symposium on Language and Communication: The Evolution of Language. SUNY Stony Brook, October 2005.
- Stromswold, K. 2006a. Why aren't identical twins identical?: Genetic, prenatal and postnatal factors, Cognition 101, 333-384.
- Stromswold, K. 2006b. Biological and psychosocial factors affect linguistic and cognitive development differently: A twin study. Proceedings of the Thirtieth Annual Boston University Conference on Language Development 2 , 595-606. Somerville MA: Cascillida Press.
- Stromswold, K. 2007. Genetics and the structure, acquisition and evolution of language. Paper presented at Biolinguistic Investigations, Santa Domingo, Dominican Republic, February 2007.
- Stromswold, K. & Rifkin, J. 1996. Language acquisition by identical versus fraternal SLI twins. Paper presented at the Symposium on Research in Child Language Disorders. June, 1996.
- Stromswold, Schramm, K., Molnar, D., Holodak, S. & Sheffield, E. 1996. The role of specific and non-specific genetic factors in language development. Society for Research in Child Development, April 2005.