Central to the field of psychiatric genetics is the search for ‘vulnerability genes'.1 Particularly important in this search is uncovering the mechanisms whereby such genes influence disease risk, and determining whether they are directly associated with more general psychological and behavioral disturbance (that is, act through ‘main effects') or whether they are principally related to dysfunction only under specific environmental conditions or in response to particular developmental experiences (that is, act through ‘gene- × -environment (G × E) interaction'). Findings of studies linking candidate vulnerability genes (or ‘risk alleles') directly to specific psychopathological conditions have proven notoriously difficult to replicate.1
