Twin Studies of General Mental Ability

Abstract

Twin studies are a vital source of information about genetic and environmental influences on general mental ability. The classic twin design—comparison of the relative similarity between monozygotic (MZ) and dizygotic (DZ) twins— is a simple and elegant approach to estimating the effects of genes and experience on developmental traits. However, while this method was considered state of the art in behavioral genetics in the 1960s and 1970s, it is now only one of many more sensitive and sophisticated twin designs. Twin research on behavioral and medical traits, in general, and on intelligence, in particular, has advanced at an impressive rate. The focus of the present chapter is on what twin studies have thus far contributed to our understanding of individual differences in intelligence. The chapter begins by briefly summarizing key events and controversies that have marked the ontogeny of twin research on intellectual development. Subsequent sections examine new twin research designs, analytic methods, findings, and their implications. Topics include recent evidence from studies of twin-singleton differences, twins reared apart and together, virtual twins, longitudinal analyses, prenatal environments, parenting practices, shared environments, epigenetic processes (e.g., DNA methylation), the heritability of relevant endophenotypes, associations between genetic variance and socioeconomic status (SES), and the search for specific genes underlying intelligence. Links between twin studies and other research areas, both within and outside behavioral genetics, are explored.