Potentially relevant differences among samples include ethnicity, age and culture. Some evidence suggests that heritability increases in the context of permissive environments, such as increased acceptance of smoking among women in younger populations (Kendler et al. 2000), although this is not always the case (Kendler et al. 2004). We conducted post-hoc analyses to assess the extent to which heritability differed by cohort by dividing our sample into two using a median split by birth year. The resulting heritability estimates were quite similar in both groups (data not shown), though this does not preclude the possibility that cohort differences are contributing to the discrepant results across samples. Overall, our estimates of the heritabilities of these traits are comparable with previous reports.
