each genotype, which indicate how much the environment is modifying genetic effects. In the crossover interaction shown in Figure 1b, the lines cross at environment 0 (i.e., in the middle). But crossover interactions can vary in the location of the crossover. It is possible that crossing over only occurs at the environmental extreme. As previously noted, the crossing over of the genotypic groups in the Caspi et al. publications of the interactions between the 5-HTT gene, life events, and depression (Caspi et al. 2003) and between MAOA, maltreatment, and antisocial behavior (Caspi et al. 2002) occurred at the extreme low ends of the environmental measures, and the degree of crossing over was quite modest. Rather, the shape of the interactions (and the way the interactions were conceptualized in the papers) was largely fan-shaped, whereby certain genotypic groups showed stronger associations with outcome as a function of the environmental stressor. Also, in both cases, the genetic variance was far greater under one environmental extreme than the other, rather than being approximately equivalent at both ends of the distribution (but with genotypic effects in opposite directions) as is the case in the crossover effect illustrated in Figure 1b. In general, it is
