| Abstract: |
Human altruism is a widespread phenomenon that puzzled evolutionary biologists
since Darwin. Economic games illustrate human altruism by demonstrating that
behavior deviates from economic predictions of profit maximization. A game
that most plainly demonstrates 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 & RS3 promoter-region repeat
polymorphisms. Parents did not participate in online game playing. Since
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-sq = 11.73, DF= 4,
p-value = 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 postmortem
hippocampal mRNA levels than short RS3 repeats (One way-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. |