Why He Cheated on You

If you're why wondering why he cheated on you -- blame his dad. And his grandfather and his great-grandfather.

It turns out that some of us are genetically programmed to be unfaithful. At least, that's the finding of researchers from Binghamton University, State University of New York.

While everyone has the dopamine receptor D4 polymorphism, or the DRD4 gene that promotes thrill-seeking behavior, about half of us have a variation of it that makes us more vulnerable to sexual promiscuity.

The gene, which can influence the brain's chemistry and, thus, an individual's behavior, is also responsible for alcohol and gambling addictions.

Lead study author Justin Garcia told ABC News that those who have the DRD4 variant are "more likely to have a history of uncommitted sex, including one-night stands and acts of infidelity." He says the desire to cheat or have sex with many different people appears to originate in the brain's pleasure and reward center, where the "rush" of dopamine motivates those who are vulnerable.

The study: More than 180 student volunteers took an anonymous survey on their previous sexual behavior, including answering such questions as the number of sex partners they had and if they had ever been unfaithful. Each student's DNA was tested after orally rinsing with a special mouthwash, so researchers were able to identify who had the variation of the DRD4 gene and who did not.

The results: The students who had the variation of the DRD4 gene had a 50 percent increase in the instances of sexual cheating. "Just as height varies, the amount of information in the gene varies. In those who have more...they are more prone to thrill-seeking," Garcia told ABC News. "It's inheritable, too. If your parents have it, you have it."

How does it work? Basically, when you drink alcohol, jump from a plane or have sex with a stranger, your brain is stimulated and that causes the release of dopamine, a pleasure-response hormone. "It's rewarding and makes us excited and gives us pleasure," Garcia explained to ABC News. "But the people with the DRD4 gene need more stimuli to feel satiated. Some of us say 'wow,' that was a rush after jumping out of a plane. Others ask, 'When is the plane going back up?'"

Important caveat: Just because someone has the cheating gene doesn't necessarily mean he'll be unfaithful. Just beware, though, because the study results also show that sex drive and thrill can function independently of love.