Men may wage wars in part to spread their seed. In a recent report in Personality and Social Psychology Bulletin, Chinese researchers argue that alongside revenge and resource acquisition, mating is a key motivator for taking up arms. Expos­ing heterosexual men to images of attractive women increased their professed support for international aggression. Pictures of flags did not have the same effect, and men did not associate attractive women with aggression against males in general or with peaceful resolution to trade conflicts.

An evolved war-mating association makes sense: the authors’ analysis of 20th-century records suggests warriors gain an estimated 10 times as many sexual op­portunities as civilians, wheth­er through raping the enemy or showing off their battle scars.