Men exhibit a stronger tendency to favor the in-group over the out-group than women. We examined if this male-specific "coalitional psychology" represents in-group love or out-group hatred. One hundred thirty-three college freshmen played a Prisoner's Dilemma Game with a member of their own group and a member of another group. Both groups consisted of same sex participants. An in-group bias -- cooperation at a higher level with the in-group than the out-group -- based on expectations of cooperation from the in-group was observed for both men and women. When such expectations were experimentally eliminated, women did not show any in-group bias whereas men still exhibited an in-group bias. The male-specific in-group bias in this condition was found to be a product of intra-group cooperation rather than inter-group competition. These findings suggest that the male-specific coalitional psychology catered more toward within-group solidarity than promotion of aggression against the out-group.