It often starts off innocently — a brief exchange of glances on the daily train commute, messaging back-and-forth with an old friend on Facebook, or lively banter with a work colleague during coffee breaks. Such interactions with other people, no matter how desirable they may be, certainly don't lie outside the boundaries of a healthy relationship.

But for some individuals, what begins as a mild flirtation eventually turns into an out-and-out affair. Others are tempted, but never cross the line into infidelity. Why?

Among other factors, recent research is finding evidence that your likelihood of straying from a relationship is may be partly programmed into your DNA.

“Probably many genes subtly influence hormone levels that in turn act on the brain to orient us towards or away from attraction to people who are not our partners,” said Brendan Zietsch, a cognitive psychologist at the University of Queensland in Australia, in an e-mail. “But the details are totally unknown... Certainly we know that there will be many genes involved, not just a few, and each gene will have a very small effect.”

In an attempt to find potential genetic effects on cheating, Zietsch analyzed data from 7,378 Finnish twins and siblings who had been in a monogamous relationship for at least one year. The study was published earlier this year in the journal Evolution and Human Behavior.

Twin studies are commonly used by researchers to estimate how heritable a specific trait is in humans. Because identical twins have the same genome, any differences between them can be chalked up to environmental factors. On the other hand, fraternal twins and ordinary siblings only share share half of their genes on average. Data from both identical and fraternal twins, along with non-twin siblings, are carefully compared to tease out the impact of nature versus nurture.

When surveyed, 9.8 percent of men and 6.4 percent of women admitted to having more than one sexual partner in the last year. For both sexes, identical twins correlated strongly with one other in terms of unfaithfulness while fraternal twins and siblings did not. The results suggest that there is indeed a genetic component to one's likelihood of committing infidelity — as in, nature accounts for roughly 63 percent of relationship-straying for males and 40 percent for females, with environment responsible for the rest.

“The clear finding is that an individual's genetic makeup in general influences how likely he or she is to cheat,” said Zietsch. “We could tell that because genetically identical twins were similar in their fidelity or infidelity to partners, whereas nonidentical twins and normal siblings were not similar in this respect.”

The leading theory is that genetic variations are involved in tuning our hormones levels up or down, which affect how strongly bonded to our partners we feel. Notable studies with the monogamous prairie vole, a small rodent that mates for life, have found that vasopressin plays a key role in pair-bonding. Prairie voles possess many more of this hormone's receptors in the brain as compared to promiscuous montane voles — a free-wheeling species with no interest in settling down. However, increasing the number of receptors in the brain of a male montane vole quickly turns him into a one-female-only type of partner.

Based on these findings from animal studies, Zietsch and his colleagues tested a subset of subjects for links between cheating and the arginine vasopressin receptor 1A gene (AVPR1A), which encodes the protein that acts as a receptor for vasopressin. Significant associations with specific AVPR1A variants were found for women, but not for men.

“With regard to individual genes, we provided some evidence that the AVPR1A gene relates to likelihood of cheating, but it is a small effect and would need to be replicated before we can be confident the effect is real,” he said. “This is the case with any effect of an individual gene — such effects have proved not very robust in the past, and this is with regard to all traits, not just infidelity.”

An earlier study, published by the journal PLOS ONE in 2010, pointed to a different place within the human genome responsible for modulating the neurotransmitter dopamine, which helps control the brain's reward and pleasure centers. People with a specific genotype of the dopamine D4 receptor gene (DRD4) reported a higher rate of promiscuous sex and more instances of sexual infidelity than controls.

This DRD4 genetic variant causes reduced binding for the neurotransmitter dopamine in the brain's reward circuit, implying that these people need more novelty and stimulation to achieve a “normal” level of excitement. It has previously been linked to sensation-seeking activities like alcoholism, gambling addiction, and even a love of horror films.

But as Zietsch notes, while there may be a clear genetic influence on our tendency to cheat, there is no such thing as a single “infidelity gene.” Other, more nuanced factors may play a role — for instance, the motivation to have an affair isn't always boredom. Among mental health professionals, the belief that infidelity always arises from an intrinsic flaw within the transgressor — such as a failure to commit or abnormally high sensation-seeking — is largely outdated.

Christian Jordal, a licensed Marriage and Family Therapist and Clinical Assistant Professor in the Department of Couple & Family Therapy at Drexel University, is hesitant to believe that one's propensity to cheat and genetic makeup are entwined.

“I'm not a geneticist or medical researcher, but there can be a tremendous amount of ambiguity around why people cheat,” said Jordal. “It's the same sort of mystery of the human heart: How is it that we choose to be attracted to someone? Why do we think or feel the way we do around love and romance? In the same way we don't always have these answers, we don't have the answers around infidelity issues.”