Evaluating the relative importance, or frequency of existence, of each type of interaction is complicated by the fact that there is far more power to detect crossover interactions than fan-shaped interactions. Knowing that most of our genetic studies are likely underpowered, we would expect a preponderance of crossover effects to be detected as compared to fan-shaped effects purely as a statistical artifact. Further, even when a crossover effect is observed, power considerations can make it difficult to determine if it is “real.” For example, an interaction observed in our data between the gene CHRM2, parental monitoring, and adolescent externalizing behavior yielded consistent evidence for a gene-environment interaction, with a crossing of the observed regression lines (as in Figure 1b). However, the mean differences by genotype were not significant at either end of the environmental continuum, so it is unclear whether the crossover reflected true differential susceptibility or simply overfitting of the data across the environmental levels containing the majority of the observations, which contributed to a crossing over of the regression lines at one environmental extreme (Dick et al. 2011).
