The urge to band together against strong aggressors is a key to humanity's success as a species, biomathematician Sergey Gavrilets at the University of Tennessee said.
He used mathematical models to investigate why humans exhibit strong egalitarian, or socially equal, behaviors.
Gavrilets used the models to compare the prosperity of groups that allowed stronger members to consume the best resources, and those in which "helpers" aided the weak individual by standing up to, or fighting off, the stronger bully.
Extrapolated over thousands of generations, the models showed groups with helpers prospered.
The findings suggest people evolved a genetic drive to help weak individuals fight back, ultimately leading to widespread cooperation among humans as well as empathy and compassion.
"Based on the results, helping the victim then is the evolutionary 'right' thing to do, not only from a victim's point of view or a societal point of view, but also the helper's point of view," Gavrilets told the Los Angeles Times.
"I'd speculate that this is also a psychologically rewarding thing to do in spite of the risks potentially involved."