Oxytocin: out-group aggression, social cues, and amygdalar action

Three recent papers are a reflection of the recent outpouring of work on oxytocin (a peptide hormone containing the 9 amino acids shown in the figure), which (from Miller's review):

...promotes social bonding in a wide range of animals, including humans. Sold on the Internet in a formulation called "Liquid Trust," the peptide hormone is marketed as a romance enhancer and sure ticket to business success. Australian therapists are trying it alongside counseling for couples with ailing marriages. And police and military forces reportedly are interested in its potential to elicit cooperation from crime suspects or enemy agents.

The hormone is now being found to have a prickly side, and is coming to be regarded as much more than just a touchy-feely "trust hormone." De Dreu et al. have designed experiments to demonstrate that oxytocin drives a "tend and defend" response in that it promotes in-group trust and cooperation, and defensive, but not offensive, aggression toward competing out-groups.

In another study on oxytocin, Gamer et al. add to studies that have shown that oxytocin decreases aversive reactions to negative social stimuli, and find that subjects given oxytocin, relative to subjects given placebo, are more likely to make eye movements toward the eye region when viewing images of human faces. They find that subregions of the amygdala are important in mediating this effect. Oxytocin:

...attenuated activation in lateral and dorsal regions of the anterior amygdala for fearful faces but enhanced activity for happy expressions, thus indicating a shift of the processing focus toward positive social stimuli. On the other hand, oxytocin increased the likelihood of reflexive gaze shifts toward the eye region irrespective of the depicted emotional expression. This gazing pattern was related to an increase of activity in the posterior amygdala and an enhanced functional coupling of this region to the superior colliculi. Thus, different behavioral effects of oxytocin seem to be closely related its specific modulatory influence on subregions within the human amygdala.

These finding have implications for understanding the role of oxytocin in normal social behavior as well as the possible therapeutic impact of oxytocin in brain disorders characterized by social dysfunction.