Absent from this review thus far is consideration of the environment. Indeed, the environment is important not just in its own right, but also as a contextual moderator of heritability estimates. That is, although our genetic code is static, estimates of heritability will vary across environments that increase or decrease the role of genetically influenced individual differences. For example, a heritability estimate of 50% may reflect a scenario where a trait is entirely due to the environment in half the population and entirely due to genetics in the other half of the population (Purcell, 2002). Due to this gene-environment interplay, genetic estimates are always best considered in light of the environment they are measured in. The term environment is used here to include both aspects of our external surroundings and our internal milieu at different developmental stages. Consideration of the environment is specifically relevant to cross-disorder efforts as the pattern of overlap across disorders may also shift across contexts, including developmental stages and socioeconomic backgrounds. Recent work has specifically demonstrated the moderating role of biological sex for genetic correlations across
