questions as whether the strength of genetic influences (heritability) changes with age, whether same or different genes influence the trait at different stages of development, and whether the rate of developmental changes is influenced by genetic factors (McArdle, 2006; Posthuma et al., 2003). Analysis of twin data can also reveal sex differences in genetic and environmental influences, i.e. whether heritability is different in males and females and whether the trait is influenced by the same or distinct genetic factor in the two sexes. Finally, data obtained from twins reared apart allow researchers to control for shared familial environment. For a more detailed overview of the twin method and its various extensions, the reader is referred to methodological reviews (Boomsma, 2002; Rijsdijk, 2002; Dongen, 2012).
