Sex differences in in-group cooperation vary dynamically with competitive conditions and outcomes

From the book : Evolutionary Psychology 10 issue 1 : 102-119.
Men’s but not women’s investment in a public goods game varied dynamically with the presence or absence of a perceived out-group.
Three hundred fifty-four (167 male) young adults participated in multiple iterations of a public goods game under intergroup and individual competition conditions.
Participants received feedback about whether their investments in the group were sufficient to earn a bonus to be shared among all in-group members.
Results for the first trial confirm previous research in which men’s but not women’s investments were higher when there was a competing out-group.
We extended these findings by showing that men’s investment in the in-group varied dynamically by condition depending on the outcome of the previous trial: In the group condition, men, but not women, decreased spending following a win (i.e., earning an in-group bonus).
In the individual condition, men, but not women, increased spending following a win.
We hypothesize that these patterns reflect a male bias to calibrate their level of in-group investment such that they sacrifice only what is necessary for their group to successfully compete against a rival group.