These results need to be considered within the context of a number of methodological limitations. First, the cross sectional nature of this sample makes it difficult to evaluate the possible causal relationship between PPD and DV. Although it is possible to test causal relationships between 2 variables, it is often difficult to distinguish the correct model among alternative causal models using a cross-sectional dataset. Moreover, for the purposes of this analysis, where both causation and correlation are likely to both play a role, such causal analyses would not prove valuable. Furthermore, in this analysis, we only considered the role of PPD as a moderator and DV as the phenotypic outcome. This was the logical order, as affiliating with delinquent peers can provide an environmental background within which genetic and environmental risk might vary, whereas one’s own behavior (i.e. DV) seems to be the logical outcome. However, this may not be the only scenario; it is also be possible that levels of DV moderate the magnitude of genetic, shared environmental, and non-shared environmental variance of PPD.
