What Makes People Go to War? Defensive Intentions Motivate Retaliatory and Preemptive Intergroup Aggression

Although humans qualify as one of the most cooperative animal species, the scale of violent intergroup conflict among them is unparalleled. Explanations of the underlying motivation to participate in an intergroup conflict, however, remain unsatisfactory. While previous research shows that intergroup conflict increases ‘in-group love’, it fails to identify robust triggers of ‘out-group hate’. Here, we present a controlled laboratory experiment, which demonstrates that ‘out-group hate’ can be provoked systematically. We find direct and causal evidence that the intention to protect the in-group is not only a crucial motivator of ‘out-group hate’ in defensive reactions, but also promotes preemptive offensive actions against out-group threat. Hence, the strength of ‘out-group hate’ depends on whether the own group is perceived to be on the offensive or the defensive side of the conflict. This finding improves our understanding of the escalation of intergroup conflicts and may have important implications for their prevention, as we find in our experiment that removing out-group threat substantially reduces intergroup aggression, leading to full peace.