I have a little modification of your experiment. * Divide the big group into smaller groups (where they are giving each other inside the group only). Now indi

I have a little modification of your experiment. * Divide the big group into smaller groups (where they are giving each other inside the group only). Now individuals in the group where many give to each other get an advantage over individuals in other groups. Individuals that keep for themselves while staying in a group that gives also keep the better advantage on cost of their group.