Are chimpanzees naturally violent to one another, or has the intrusion of humans into their environment made them aggressive?

A study published Wednesday in Nature is setting off a new round of debate on the issue.

The study’s authors argue that a review of all known cases of when chimpanzees or bonobos in Africa killed members of their own species shows that violence is a natural part of chimpanzee behavior and not a result of actions by humans that push chimpanzee aggression to lethal attacks. The researchers say their analysis supports the idea that warlike violence in chimpanzees is a natural behavior that evolved because it could provide more resources or territory to the killers, at little risk.

But critics say the data shows no such thing, largely because the measures of human impact on chimpanzees are inadequate.