When carried out between members of a shared social group (e.g., a village, a tribe, a nation, humanity)[REF] people offer "fair" (i.e., 50:50) splits, and offers of less than 30% are often rejected [REF].
One limited study on monozygotic and dizygotic twins claims that genetic variation can have an effect on reactions to unfair offers, though the study failed to employ actual controls for environmental differences [REF].