We intuitively know that our personalities and temperaments — whether we're introverts or extroverts, how we respond to novelty or adversity, whether we're hard-driving or laid back — are the result of a complex interaction of nature and nurture.
We likely start with some general social tendencies established by genes inherited from our parents. But it seems equally evident that experience matters. Did you grow up beloved or neglected? Have your surroundings and people close to you encouraged confident exploration or grim self-protection? What has happened when you stumbled? The answers, we suspect, play a role in sculpting the social selves we come to be from the raw material nature provides.
If only we understood the machinery by which nature and nurture interact to produce the social creatures we are and will become. We might gain new appreciation for our individual differences. We might know better how to prevent the emergence of despair, anxiety and hate. In adulthood, we might make choices — in diet and exercise, in friends, in pastimes — that promote the development of our better social selves.
A new study, published Monday in the Proceedings of the National Academy of Sciences, takes a small first step on that long, winding road. It explores how "epigenetics" — the ever-changing instructions that turn genes off and on — interact with a key gene to influence our social behavior.
The gene in question is one which codes for the production of oxytocin, a hormone that's been associated with trust, sociability and nurturing behaviors. And the epigenetic mechanism studied is methylation, a chemical signaling tool that can result from environmental wear and tear.
DNA methylation is just one way that the epigenome acts to turn certain genes off. It's a normal and common epigenetic mechanism, but it's not always internally driven, and it's not always benign. Increasingly, research suggests that environmental factors such as toxins, stress, malnutrition and social adversity can cause the methylation of genes, switching them to the "off" position.
In 121 study participants, the new research found that the level of methylation seen on the oxytocin gene is a good predictor of a person's comfort in forming social bonds and ability to judge others' emotional states. The oxytocin gene's methylation level was also linked to the strength of activity seen in regions of the brain that are crucial to social functioning. And in the case of one brain structure that plays a key role in our ability to read others' intentions — the right fusiform gyrus — the oxytocin gene's methylation level was also linked to size.
Harvesting the oxytocin gene from participants' saliva, researchers from the University of Georgia, Emory University and Stanford University found that among those who carried a more methylated version, average sociability levels were lower, and activity levels and volume in brain regions linked to sociability were lower as well. Among those whose oxytocin genes were least methylated, they found "attachment styles" that were more secure, greater skill in reading others' states of mind, and brains that appeared better built for positive social interaction.
Oxytocin, a hormone produced by the brain's hypothalamus, is sometimes called the "love hormone." Research has shown it wells up when a mother nurses her infant, when men in hunter-gatherer societies return home from the hunt, and when humans gaze into the soulful eyes of their pet dogs. Research has linked autism and its social deficits with specific variations in the gene that codes for the oxytocin receptor. And inhaled oxytocin is considered a promising aid to social-skills training in those with autism.
Along with recent studies that have tied changes in the oxytocin gene with a range of social behaviors, "this is a very exciting area of investigation and represents a new frontier of social psychology," said neuroscientist Sarina Saturn of the University of Portland.
"Many of our personality traits are dictated by our genes — inherited," added Saturn, who wasn't involved in the new study. "But this area of research implies that our DNA can also learn from powerful emotional and social experiences" and influence how our brains and bodies respond to social stimuli, she added.
The new research is far from definitive in its findings. University of Georgia psychology professor Brian W. Haas, who led the study team, acknowledged that the authors did not actually measure oxytocin levels in study participants, just methylation of the oxytocin genes found in their saliva. An increased level of DNA methylation is normally associated with that gene's decreased expression. In the case of the oxytocin production gene, more methylation should have resulted in less oxytocin in a participant's brain and bloodstream.
"It's a leap" to assume that those with methylated oxytocin genes had less oxytocin, Haas said. "We don't know that empirically."
Harvard University geneticist Steven McCarroll, who studies the genetics of psychiatric disease and was not involved in the new research, added that since epigenetic changes act differently on genes throughout the body, there's no certainty that the methylation of oxytocin genes in saliva reflects the methylation of the same genes in the brain.
"There's a lot of good evidence that oxytocin levels fluctuate in response to experience," McCarroll said. "But whether that relates to methylation is not known, and we wouldn't want to accept it — or dismiss it — casually." At the very least, he added, "you'd want to measure that in the cells that actually make the oxytocin and that shape your mood and actions."
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