Cooperative behavior provides a benefit to the recipient and can only be selected for if it also provides direct or indirect benefits to the actor that accepted the costs. Understanding the evolution of cooperation in large groups and between non related individuals remains a challenging problem for economists and evolutionary biologists. I am studying a potential effect of sexual selection on the evolution of cooperative behaviors in two social species: in humans and in mound-building mice Mus spicilegus. In humans, economics games will be used to measure quantitatively these behaviors, and to explore their inter-individual variability. In the mound-building mice, I will measure quantitatively their cooperative behavior during the building of their mound (accumulation of seeds covered with earth used as food storage for winter). I will also assess the potential influence of testosterone levels (male sex hormone) on variations in cooperativeness in these two species. Study the biological basis of cooperative behaviors has the potential to add substantially to our understanding of the evolution of cooperation.