In the classical twin design, genetic and environmental variance components for aggressive behavior can be estimated using data from same-sex MZ and DZ twins. Apart from estimating genetic and environmental effects on aggression, it is also possible to investigate whether sex-specific genetic or environmental influences are important. Such effects are referred to as sex-limitation or sex-limited effects. There are two primary questions about sex limitation in genetic research, one being whether there are qualitative differences between males and females, such that different genes and/or environmental influences operate in the two sexes, and whether quantitative differences exist in the relative magnitude of influences across sexes. To assess whether the magnitude of genetic and environmental effects in aggressive behavior differ between males and females (i.e., quantitative sex differences), only data from same-sex twin pairs are required. However, to determine whether or not it is the same set of genes or shared environmental experiences that influences aggressive behavior in males and females (i.e., qualitative sex differences), data from opposite-sex twin pairs are also needed. If qualitatively different genetic influences are important for aggressive behavior in males and females, then the opposite-sex twins will be less genetically similar for the trait than DZ twins.
