Different types of aggressive behavior have been investigated across twin and adoption studies, with notable distinctions between reactive and proactive forms of aggression, as well as direct/physical and indirect/relational aggression (Table 8.3). It is likely that there are different etiologies for different forms of aggression; for example, defensive reactions to threatening stimuli may be more environmentally influenced, while more planned, proactive forms may be more genetically influenced (Tuvblad et al., 2009). Comparing heritability estimates collectively across the various measures employed is a reasonable way to address this question about whether some kinds of aggression are more heritable than others. When multiple forms of aggressive behavior are measured within the same study, it is also possible to investigate the extent to which the same genes and/or environmental factors are important to different manifestations of aggressive tendencies.
