Abstract
The Male Warrior Hypothesis (MWH) establishes that men’s psychology has been shaped by inter-group competition to acquire and protect reproductive resources. In this context, sex-specific selective pressures would have favored cooperation with the members of one’s group in combination with hostility towards outsiders. We investigate the role of developmental testosterone, as measured indirectly through static markers of prenatal testosterone (2D:4D digit ratio) and pubertal testosterone (body musculature and facial masculinity), on both cooperation and aggressive behavior in the context of intergroup conflict among men. Supporting the MWH, our results show that the intergroup conflict scenario promotes cooperation within group members and aggression toward outgroup members. Regarding the hormonal underpinnings of this phenomenon, we find that body musculature is positively associated with aggression and cooperation, but only for cooperation when context (inter-group competition) is taken into account. Finally, we did not find evidence that the formidability of the group affected individual rates of aggression or cooperation, controlling for individual characteristics.
Highlights
The Male Warrior Hypothesis (MWH) establishes that men’s psychology has been shaped by intergroup competition to acquire and protect reproductive resources
We found that the sum of skeletal muscle mass (SMM) in the group was not related to contributions in the Public Good Game (PGG), either as a main effect (B = 1.865, t = 0.270, p = 0.787) or in interaction with the context (B = −14.448, t = −1.041, p = 0.299)
We tested several predictions derived from the male warrior hypothesis[8,14]
Summary
The Male Warrior Hypothesis (MWH) establishes that men’s psychology has been shaped by intergroup competition to acquire and protect reproductive resources. The male warrior hypothesis argues that “humans, men, may possess psychological mechanisms enabling them to form coalitions capable of planning, initiating and executing acts of aggression on members of outgroups, with the ultimate goal of acquiring or protecting reproductive resources”[8] In this context, testosterone has been shown to play an important role in cooperative and aggressive behaviors, functioning as a status-seeking hormone[9]. The male warrior hypothesis is a sex-specific proposal primarily supported theoretically by the greater degree of variance among men than among women in terms of reproductive success[10,11] and the lower level of obligatory parental investment by men[12] These two factors have enhanced intrasexual competition in men, favoring sexual dimorphism in size and strength, accompanied by a significant sex-based difference in physical aggressiveness[6]. In the specific context of intergroup conflict, cooperation and aggression are inevitable interdependent behaviors as men cooperate and form coalitions to outcompete other groups, or in other words, they cooperate to aggress
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