Pilot study of relationship between fertility risk and bargaining.

Competition for resources to support a healthy pregnancy and later offspring was likely very important for ancestral women. Therefore, it was predicted that women evolved a propensity for intrasexual competition over resources during times of their highest conception risk. To investigate this hypothesis, women played a series of ultimatum games, bargaining games that test participants' willingness to share a monetary stake. During periods of high conception risk, intrasexual competition increased as evidenced by lower offers to share a stake with others as well as higher rates of rejection of offers.