Twin studies of complex traits and diseases

This chapter will first present an overview of the genetic and environmental mechanisms that cause differences in human complex traits and diseases. We focus on findings that demonstrate that all traits are heritable, whether and how heritability of traits and diseases differ between men and women, and the same genetic factors account for why two traits or two diseases correlate. We then present findings on how environmental factors, both measured and unmeasured, augment, moderate, and correlate with genotype to maximize (and minimize) genetic expression of traits and diseases. Here, we give the three most common examples in the behavior genetics literature: cognitive ability, personality, and psychopathology. Finally, we cover the ways in which behavior genetics will be important in future research for clarifying the role of genotype and environment in understanding the etiology of traits and diseases.