 Lack of clear definitions of behavior. Behavioral traits need to be clearly defined. For example, it is postulated that males are naturally more aggressive than females. What counts as aggression is unclear. Is it going to war, or punching another individual, or having very active adrenal function? The appropriate grain of analysis is not clear. If one lives in a culture in which males are considered more aggressive than females, then one may notice the higher proportion of men rather than women who commit violent crimes and not notice cases of females fighting to defend their offspring or competing with each other for resources. The issue of defining traits is not only a matter of clarity. Ontology is at stake because for evolutionary hypotheses to explain what exists in the world, our definitions must divide up the world into traits that are passed from generation to generation and on which natural selection can act.