Table 2.

Directions for future research.

cross-species comparisons

There have been few efforts to systematically evaluate whether key individuals exert influence through similar processes across species. For example, are the mechanisms through which impact patrollers in chimpanzees exert influence on their group similar to those in other primates?

developmental trajectory of key individuals

There has been little work on the developmental factors that lead some individuals to become key individuals. Are specific developmental experiences or gene-environment interactions necessary for the emergence of key individuals? Is there a common or shared developmental trajectory for key individuals within or across species? Most human violence is produced by young males. Does this pattern hold for impact individuals?

evolutionary origins of key individuals

To what extent is being a key individual the result of selective processes? What mechanisms would be responsible for maintaining this phenotype, i.e. frequency-dependent selection?

why can personality be so effective at mobilizing others?

In humans, being a key individual appears to require the ability to recruit followers. Doing so effectively may be due to experience, knowledge or skill. Among humans, however, charisma also appears to be important but does not reduce to knowledge, skill or experience [103]. What explains the effects of charisma in catalysing followers for intergroup violence?

variation in types of aggression between individuals

Key individuals catalyse coalitional aggression but are the same individuals involved in different types of aggression, including intergroup, intragroup, offensive and defensive aggression? Are the mechanisms by which they motivate others to participate similar across types of aggression? For example, among chimpanzees, impact patrollers do not appear to be the same individuals as impact hunters, suggesting that key individuals differ between types of aggression.