ISU professor Jill Pruetz collaborated with 29 other primatologists on a study. They found that lethal aggression was not a result of human impact but rather an adaptive behavior.

Variable	Human Impact	Adaptive Strategy
Chimps kill more than bonobos	None	Study found only 1 suspected bonobo event compared to 151 chimp event
Sex bias: attackers	None	Study found 92% attackers were male
Sex bias: victims	None	Found 73% were males
Number asymmetries	None	Attackers outnumbered victims 8:1
Eastern vs. Western	None	Found more lethal events in eastern Africa

Mean number of males	None	Communities were there were more males, there was more aggression
Mean population density	None	More dense communities also saw more lethal events

In the variables they tested, they discovered evidence to support the idea that lethal aggression is an adaptive strategy.