Between 5 to 10% of apparently normal children have difficulty acquiring written or spoken language. Twin studies are the most common type of study used to tease apart the impact of environmental and genetic factors on language development. Because identical twins share 100% of their alleles and fraternal twins share only 50% of their alleles, if identical twin pairs' linguistic abilities are more similar than fraternal twin pairs', this suggests that genetic factors play a role in language. Furthermore, the rate of language impairments and delays is twice as high for twins as singletons, probably reflecting pre- and postnatal environmental hazards associated with twinning. Thus, twin studies can be used to investigate how specific environmental factors affect language development.
This project will use longitudinal data from 450 twin pairs to investigate how genetic and prenatal and postnatal environmental factors affect linguistic and nonlinguistic development. The project will address the following key questions: 1. How great a role do genetic factors play in the linguistic and nonlinguistic development of linguistically normal and language-impaired children? 2. How specific to language (or subcomponents of language) are these genetic factors? 3. To what extent do prenatal and early postnatal environmental factors affect linguistic and nonlinguistic development? 4. How developmentally stable are the effects of these genetic and environmental factors? 5. How do interactions among environmental and genetic factors affect normal and language-impaired children's linguistic and non-linguistic development? The direct and indirect costs of language-related learning disabilities are enormous to the impaired child, their family and society. This study may help uncover which environmental factors protect children from impairments and which increase their risk. This study may also provide needed insight into the ways in which genetic factors and environmental factors interact to raise or lower the risk of linguistic and nonlinguistic impairment. Armed with this knowledge, clinicians and teachers may be able to tailor therapeutic and educational practices so as to maximize each child's potential.
