John Coleman, a 22-year-old from Syracuse, N.Y., has been engaged for the last two years and cannot fathom having sex with anyone other than his girlfriend.
"I find cheating appalling," said Coleman. "There's got to be something going on in your head to cheat."
It turns out Coleman is right.
In what is being called a first of its kind study, researchers at Binghamton University, State University of New York (SUNY) have discovered that about half of all people have a gene that makes them more vulnerable to promiscuity and cheating.
Those with a certain variant of the dopamine receptor D4 polymorphism -- or DRD4 gene -- "were more likely to have a history of uncommitted sex, including one-night stands and acts of infidelity," according to lead investigator Justin Garcia.
The desire to cheat or sleep around seems to originate in the brain's pleasure and reward center, where the "rush" of dopamine motivates those who are vulnerable, the researchers say.
In the study, Garcia instructed 181 student volunteers at SUNY to take an anonymous survey on their previous sexual behavior, asking them questions like how many sex partners they had and if they had ever been unfaithful.
He then tested their DNA by oral rinsing with a special mouthwash -- a buccal wash -- and genotyped the DRD4.
His team discovered that there is a variation in the thrill-seeking gene and those with much longer alleles are more prone to, well, getting prone. (An allele is part of the gene's DNA sequence responsible for different traits such as eye color or curly hair.)
Those with at least one 7-repeat allele reported a higher rate of promiscuity -- that is admitting to a "one-night stand." The same group had a 50 percent increase in instances of sexual cheating.
"It turns out everyone has got the gene," said Garcia, who is a doctoral fellow in the laboratory of evolutionary anthropology and health at SUNY Binghamton. "Just as height varies, the amount of information in the gene varies. In those who have more, their alleles are longer and they are more prone to thrill-seeking."
"It's inheritable, too," he said. "If your parents have it, you have it."
When the brain is stimulated -- drinking alcohol, jumping from planes, having sex -- it releases dopamine, the pleasure response hormone.
"It's rewarding and makes us excited and gives us pleasure," said Garcia. "But the people with the DRD4 gene need more stimuli to feel satiated. Some of say 'wow,' that was a rush after jumping out of a plane. Others ask, 'When is the plane going back up?'"
But not everyone is convinced a roving eye is rooted in DNA.
"Certain people are vulnerable to affairs, but in the end, it's about personal choice," said Jenn Berman, a psychotherapist and host of "The Love and Sex Show" on Cosmo Radio. "And it depends on how well-developed their impulse control is."
Still, the study could have some interesting implications.
Armed with that kind of data, John Coleman said he might be inclined to test his fiance and himself as well.
"It's like getting tested for STDs," he said. "It's the ultimate form of honesty, really," he said.
But Garcia said the gene for risk also might have an evolutionary advantage, beyond producing more children.
The gene evolved about 30,000 to 50,000 years ago when humans were moving out of Africa.
"Having some individuals who have wanderlust and want to see what's on the other side mountain. It's important for new places to live. But it's also risk-taking. Sometimes, going to the other side of the mountain means that something eats you. There is a cost and a benefit."
Some of the implications of this study might be "huge," and not just in the bedroom. "The big question is what happens in drug rehab if you have a long allele and others don't? They might have different treatments."
Not All Who Have Gene Will Be Sluts or Cheaters
The study also strongly suggests that sex drive and thrill can function independently of love.
That might be the case with Emma, a 20-year-old student from University of Southern Florida, who just broken up with her boyfriend after a two-year monogamous relationship.
She wanted to try something different, so she slept with three men in one month. Two were encounters with guys she had been friends with and another was a fling that transformed into a longer relationship.
"I'd never done anything like that before," said Emma, who did not want to reveal her last name. "It was something so new to me."
She said it's not in her personality to take risks. Defying college stereotypes, Emma's never touched alcohol and has only smoked marijuana once.
And now that she is in a committed relationship, Emma is certain she won't be unfaithful.
"We are learning more and more about genes implicated in behaviors," she said. "Every time a genetic study comes out, responsible scientists also stress that we have choice -- nature and nurture," she said.
"Not everyone with the gene is promiscuous and not everyone who is promiscuous will have that gene."
And can't risk-taking be a good thing?
"Sometimes that overlaps with creativity, with entrepreneurship and wanting to push the boundaries," she said. "In relationships that can be exciting and fulfilling and help the whole couple move into new areas."
So should a woman have her boyfriend tested before accepting his marriage proposal?
"By the time she meets him, unless he is very young, his track record will prove whether he has acted on his infidelity gene or not," said Quilliam. "If he has been unfaithful in the past, he is likely to do it in the future."
Maureen Finn, a 19-year-old television, film and radio major at Syracuse University, agrees.
"I mean if you meet a guy at a party and he's making out with three other girls, that's a hint," she said. "If you're disrespecting me, something tells me you're not going to respect me enough to be faithful."
ABC's On Campus reporters Sierra Jiminez of Syracuse University and Meg Wagner of University of Florida contributed to this story.