Despite the strength of these results, there are several limitations that should be considered. First, although our sample is only moderately sized by current twin study samples, previous power analyses (Purcell, 2002) suggest that it is more than adequate for the G × E models used here. Nevertheless, analyses incorporating sex would probably be unwieldy and underpowered in this sample. It thus remains unclear whether the G × E identified here varies across sex (although it is worth noting that CP heritability estimates in general do not vary significantly across sex; Burt, 2009a, b). We also did not directly examine the effects of age. As such, the current results should not be applied to other developmental periods. Next, although shared environmental influences on CP were moderated in these analyses, examinations of other environmental risk and protective factors in these data have revealed evidence of genetic moderation (Humbad et al. 2012). The current results are thus specific to the association between parent–child conflict and child CP, and do not imply that all G × E underlying childhood CP are bioecological in nature.
