Although bivariate genetic analyses may indicate that (for example) elements of the family environment are genetically correlated with both antisocial behaviour and depression (Pike et al. 1996), we cannot assume that including both of these behavioural phenotypes in the same analysis will explain more of the heritability of parental negativity than does either one alone. This is because antisocial behaviour and depression are also genetically correlated (Rowe et al. 2008). This means that their contributions to the heritability of parental negativity are not independent of one another. In other words, their joint contribution may be less than the sum of their individual contributions. As such, studies are needed that incorporate multiple behavioural phenotypes into multivariate genetic analyses, allowing researchers to assess the extent to which behavioural measures together account for the heritability of environmental measures.
