Emergence of characteristically human traits, including extensive collaboration, prosociality, and cooperative communication, has presumably necessitated evolution toward lower aggression. While empirical studies have provided evidence for evolutionary reduction in human aggression, such as canine teeth reduction in hominins and craniofacial feminization inHomo sapiens, the underlying mechanisms for the reduction in human aggression are virtually unknown. A promising hypothesis is social selection by partner choice; that is, individuals' choice of collaborative partners may have induced selection against exceedingly aggressive individuals. However, it is thus far unclear whether partner choice can give unaggressive individuals a fitness benefit that more than compensates for the lost advantage of being aggressive in interindividual conflict. This study explores the plausibility of the social selection hypothesis, using a game-theoretic model of coalition formation. Analysis of the game shows that partner choice can induce selection favoring reduced fighting ability if individuals are ecologically demanded to collaborate, they can be viewed as rational fitness maximizers, and they have communicative skills to bring a synergy in collaboration. The model also suggests a possible feedback loop between reduction of fighting ability and a correlated enhancement of communicative skills. Significance statement Domesticated animals often share a suite of morphological, physiological, and cognitive characteristics, called the domestication syndrome, that are distinct from their wild ancestors. Experimental studies have shown that the domestication syndrome can arise as a by-product of selection against aggressiveness. A similar process may occur in wild animals; in particular, it has been suggested that "self-domestication" through selection against aggression may have played a major role in human evolution. Despite the proposed role of reduced aggression, however, it is thus far unclear in what ecological context less aggressive individuals can be selectively favored over more aggressive ones. This study provides a theoretical foundation for the social selection hypothesis for the evolution of reduced aggression, using a game-theoretic model of coalition formation, and specifies the conditions under which individuals' choice of collaborative partners can induce selection favoring reduced aggression.