(i.e., parents and their offspring), this increased similarity could be attributed to greater environmental similarity among sibling as compared to parent-offspring pairs (i.e., to shared environmental effects). The pattern of familial correlations is consistent with heritable factors having a similar phenotypic effect across the two generations. Nonetheless, whether SNP effects for behavioral phenotypes vary by age is a distinct possibility supported by the observation that the heritability of many behavioral traits increases with age, at least between adolescence and early adulthood (Bergen, Gardner, & Kendler, 2007). Indeed, in the previously mentioned study where we looked at the association of smoking with the aggregate effect of 92 SNPs, we found that the resulting SNP score was significantly associated with smoking at ages 20 and 24 but not at ages 14 and 17. The developmentally contingent nature of genetic effects for behavioral phenotypes is a major research question that will undoubtedly be a focus of future research in this area.
