In addition to generalizability, there are several key assumptions of the classical twin design that need to be kept in mind when reviewing findings from these studies. These include (1) the equal environments assumption, (2) random mating, and (3) lack of correlation or interaction between genetic and environmental influences. We briefly review each of these assumptions here—both in general and as they pertain to aggressive behavior in particular—and consider their possible effects on the results summarized across studies.
