ABSTRACT
The most common method used to study the role of genetic factors in development is to determine whether monozygotic (MZ) cotwins are linguistically more similar to one another than dizygotic (DZ) cotwins. Because MZ and DZ cotwins share essentially the same pre- and postnatal environment, whereas MZ cotwins share 100% of their DNA and DZ cotwins share only 50% of their DNA, if MZ cotwins are linguistically more similar than DZ cotwins, this suggests that genetic factors play a role in language. If, on the other hand, MZ cotwins are no more similar to one another than DZ cotwins, this suggests that genetic factors play a negligible role for language. Putting aside the possibility of interactions and correlations between genetic and environmental factors, the variation in linguistic abilities in a population (the phenotypic variance) is due to genetic variance plus environmental variance. Heritability is a measure of the proportion of the phenotypic variance that is due to genetic variance. In twin studies, environmental factors that may contribute to phenotypic variance are divided into those environmental factors that cotwins do and do not share. Shared environmental factors include the linguistic input children receive (assuming parents of twins speak the same way to both cotwins), and nonshared environmental factors include illnesses or accidents that only occur to one cotwin.
