The neural signaling molecule oxytocin, which influences how animals interact, is sometimes portrayed as a "love hormone," but it's not as simple as that. This should not be surprising, given that it is an evolutionarily ancient neuromodulator, one that impacts a wide range of emotion and social interactions—these are hardly simple things.

A more current, nuanced view of its effects holds that, rather than simply enhancing bonding, love, and trust, it serves to amplify people's social awareness. This can have a downside—like alcohol, it can amplify whatever feelings one already has toward someone else, whether they're positive or negative. It can also strengthen bonds only within a clique, to the exclusion of everyone at the other lunch tables.

While it's clear that oxytocin definitely has potent effects on emotion and social cognition, it's less clear how any of this happens. Individuals express varying levels of oxytocin, but these different levels have never been conclusively linked to any social abilities or disabilities—and that is not for lack of trying.

Like all hormones, oxytocin can't work alone; it exerts its effects by binding to a receptor. As with oxytocin itself, individuals express varying levels of the oxytocin receptor. The oxytocin receptor gene is subject to methylation, an epigenetic modification that chemically modifies the gene's DNA, which regulates how much of the receptor gets made. More methylation means less receptor gets made, so less oxytocin signaling reaches cells.

More methylation at the receptor gene has been found in people with autism, anorexia, and those with "callous emotional traits," suggesting that it may act as a biomarker for these behavioral tendencies. Researchers at UVA wondered whether varying levels of methylation might be associated with differences in neural activity during a particular social perception task: emotional face processing. Lo and behold, they were.

The researchers determined the methylation levels of ninety-eight Caucasian adults, put them in a fMRI scanner, and showed them pictures of angry or fearful faces. Upon seeing the faces, study participants with higher levels of methylation—so less oxytocin activity—had increased levels of blood flow to the left hemispheres of their amygdalas. That region of the brain is known to be responsive to negative stimuli, like angry faces.

Other areas of the brain that are important for face and social perception and emotion processing were also hyper-activated in these people, but functional connectivity between the amygdala and these areas was diminished. The researchers suggest that oxytocin is involved in attenuating the fear response generated by the amygdala and note: "These results provide compelling evidence to suggest that social perceptual and emotional processes thought to involve the endogenous oxytocin system may be governed by epigenetic processes."

It has been proposed that oxytocin may promote social behavior not only by enhancing social sensitivity, but also by alleviating social anxiety. These results bolster the latter idea. People with lower levels of methylation on the oxytocin receptor gene have more oxytocin signaling, and therefore they do not get as worked up when they see angry or fearful faces, as evidenced by the relatively sluggish response of their amygdalas. These people also show increased coupling between their amygdalas and other brain regions, which could desensitize them to the negative and threatening stimuli represented by the faces.

The authors point out that there is a marked "lack of behavioral evidence to reveal how these epigenetic and neural markers impact the overt social phenotype." In other words, outside of signals in an MRI scan, it's not clear what the significance of these changes are.

This study thus does not provide an excuse for anyone to go out and start sniffing oxytocin (yes, unfortunately, it is available as a nasal spray). But it does demonstrate that epigenetic mechanisms could affect very complex interpersonal behaviors. And since these mechanisms are both heritable and can be altered by the environment, these results could have far-reaching and fascinating implications.

PNAS, 2014. DOI: 10.1073/pnas.1422096112 (About DOIs).