De Drue et. al. published an article entitled "The Neuropeptide Oxytocin Regulates Parochial Altruism in Intergroup Conflicts Among Humans" in the 11 June 2010 issue of Science. Parochial altruism is defined as self-sacrifice to benefit our own group ("in-group love") and to hurt or sabotage out-groups ("out-group aggression"). In-group love supports the power of the in-group and is an indirect way of competing with the out-group. Out-group aggression is a way of reducing the our-group's power and so an indirect way of supporting the in-group. This is an important explanation for human social evolution and the authors explore whether oxytocin is a part of the biological basis. Oxytocin, a nonapeptide made by the magnocellular neurons and released from the posterior pituitary, acts as a neurotransmitter and hormone, and influences the amygdala, hippocampus, brainstem and the autonomic nervous system through its influence on the spinal cord. The authors quote prior work that demonstrated that affiliating with close kin associates with increases in blood plasma oxytocin. Greater empathy, generosity and other-regarding preferences associate with increased oxytocin receptors (OXTR). They also note that oxytocin administered by nasal spray promotes trust and cooperation and reduces the tendency to take advantage of others.
Three experiments were designed to evaluate the in-group-out-group issues. All were placebo-controlled, double-blinded and involved male participants receiving nasal spray oxytocin or placebo. After thirty minutes, the participants were assigned to two three-person groups. The assigned group was the in-group and the other the out-group. They were introduced to prisoners dilemma game in which they made confidential decisions that had consequences for the fellow in-group and the our-group.
Analysis of the experiments goes beyond what I will discuss here due to time and complexity factors, however, what they suggest follows. It has been shown that oxytocin promotes social bonding in humans and animals. In this study,male participants made three confidential decisions between cooperation and noncooperation. The researchers postulated since that noncooperation with the out-group promotes in-group love they would expect it to be stronger under the influence of oxytocin. They also expected protectionism to be higher when out-group fear was high. What they found was that noncooperation was higher in those given oxytocin compared to placebo when out-group fear was high rather than low. They also showed that in-group protectionism and trust correlated with participant noncooperation toward the out-group. The authors state "these findings reflect a 'tend and defend' pattern in which oxytocin stimulates humans to aggress against out-group threat in order to protect their in-group."
So this work is another in a string of findings that link oxytocin to bonding pathways found in humans, voles, and zebra finches. It also illuminates a rational basis for considering altruism and aggression as close cousins. Finally, this study encompassed only one half of humanity - males. How big a role this plays in our integration into groups and their success remains a question.