Human altruism is a widespread phenomenon that puzzled evolutionary biologists since Darwin. Economic games illustrate human altruism by showing that behavior deviates from economic predictions of profit maximization. A game that most plainly shows this altruistic tendency is the Dictator Game. We hypothesized that human altruistic behavior is to some extent hardwired and that a likely candidate that may contribute to individual differences in altruistic behavior is the arginine vasopressin 1a (AVPR1a) receptor that in some mammals such as the vole has a profound impact on affiliative behaviors. In the current investigation, 203 male and female university students played an online version of the Dictator Game, for real money payoffs. All subjects and their parents were genotyped for AVPR1a RS1 and RS3 promoter-region repeat polymorphisms. Parents did not participate in online game playing. As variation in the length of a repetitive element in the vole AVPR1a promoter region is associated with differences in social behavior, we examined the relationship between RS1 and RS3 repeat length (base pairs) and allocation sums. Participants with short versions (308-325 bp) of the AVPR1a RS3 repeat allocated significantly (likelihood ratio = 14.75, P = 0.001, df = 2) fewer shekels to the 'other' than participants with long versions (327-343 bp). We also implemented a family-based association test, UNPHASED, to confirm and validate the correlation between the AVPR1a RS3 repeat and monetary allocations in the dictator game. Dictator game allocations were significantly associated with the RS3 repeat (global P value: likelihood ratio chi(2) = 11.73, df = 4, P = 0.019). The association between the AVPR1a RS3 repeat and altruism was also confirmed using two self-report scales (the Bardi-Schwartz Universalism and Benevolence Value-expressive Behavior scales). RS3 long alleles were associated with higher scores on both measures. Finally, long AVPR1a RS3 repeats were associated with higher AVPR1a human post-mortem hippocampal messenger RNA levels than short RS3 repeats (one-way analysis of variance (ANOVA): F = 15.04, P = 0.001, df = 14) suggesting a functional molecular genetic basis for the observation that participants with the long RS3 repeats allocate more money than participants with the short repeats. This is the first investigation showing that a common human polymorphism, with antecedents in lower mammals, contributes to decision making in an economic game. The finding that the same gene contributing to social bonding in lower animals also appears to operate similarly in human behavior suggests a common evolutionary mechanism.