It turns out, the age-old adage 'once a cheater always a cheater' may actually be true.

The statement implies if someone cheats on a partner in a relationship once, they are bound to do it again and again, with serial cheaters constantly seeking out new sexual partners in a chronic pattern of infidelity.

Now, researchers believe there is scientific data to back up that belief. 

About 40 percent of unmarried couples and 25 percent of married couples report infidelity in their relationships, and 2018 study found people who have cheated in the past were three times more likely to do so again. 

While not a label formally recognized in psychology, researchers have identified certain genetics-driven traits chronic cheaters have in common, suggesting repeated cheating could be written into someone's DNA. 

Scientists believe people with the ‘thrill-seeking’ gene – called DRD4 VNTR – have a specific variant that is associated with reduced sensitivity to dopamine, which may mean they need more stimuli to feel satisfied.

The gene variation could be the one responsible for alcohol and gambling addictions, promiscuity and the inclination to cheat.  

In one study, habitual cheaters were asked if they could spot three differences in two similar photos, even though there were only one or two. But the cheaters still claimed to find three. 

Brain scans performed during the exercise showed when cheaters were dishonest, regions of their brains linked to reward-seeking behavior lit up. 

The study found when participants thought about whether to be honest or dishonest, certain brain areas related to self-reflection became more active, especially in cheaters facing moral dilemmas and weighing the consequences of their actions.

While this isn't a surefire test to determine if a person will cheat, the results show cheaters may be less likely to reflect on how their actions impact others and how regions of their brains responsible for self-reflection don't function the same way as those in honest people.

But researchers stress that having a certain genetic inclination toward a certain behavior does not guarantee that behavior will manifest. Many people with the DRD4 VNTR gene variation remain faithful in their relationships, and many without it do not.  

Genes are not destiny, experts say. 

The DRD4 VNTR gene is inherited from one’s parents and influences how the brain processes dopamine, known as the 'feel-good' hormone, which is involved in pleasure, motivation, reward- and risk-taking behavior.

People with this variant are less sensitive to dopamine than those without it, which means they don't experience the same amount of pleasure from a typical dopamine-producing activity, driving them to seek out more stimulating or risky experiences. 

Often, those with this genetic mutation gamble excessively, act quickly on their desires without thinking about consequences and engage in illicit affairs or risky sexual behavior.

A 2010 study by researchers at Binghamton University in New York, Brown University in Rhode Island, and the University of Georgia, studied people with this genetic mutation, as well as their sexual preferences and behaviors.

They found people with the DRD4 VNTR gene ‘were significantly more likely to report having ever engaged in promiscuous sex (like a one-night stand).’ 

And among those who admitted to cheating, those with this specific mutation did so more frequently.

Jessica Alderson, a relationship expert and founder of the dating app So Syncd, which matches people based on personality type, told DailyMail.com: ‘Some people are naturally more prone to cheating no matter what state their relationship is in.'

Hannah Reeves, a licensed marriage and family therapist, added: 'This is a common nature vs nurture question and surprisingly there is some evidence to suggest that it's the former. 

'Personality traits like impulsivity and an affinity for risk-taking do have genetic components. Some argue that people with these traits are more likely to cheat.' 

Additionally, at the root of serial cheating often lies narcissism or narcissistic personality disorder, a clinical psychiatric diagnosis that affects an estimated 0.5 to five percent of Americans.

Renee Zavislak, a licensed marriage and family therapist based in California, told DailyMail.com serial cheaters are often individuals with narcissistic traits, which include a lack of empathy, an inflated sense of self-importance, entitlement, grandiosity, and manipulative behaviors focused on their own benefit.

She said: ‘These personalities are the result of a biological predisposition combined with profound emotional rejection, abuse, or neglect by primary caregivers in childhood.’

Dr Cammy Froude, a licensed trauma therapist, added that narcissists ‘might see cheating as their “right” and feel little to no guilt about it because they struggle with empathy.

‘Traits like narcissism or deep-seated entitlement can make someone feel like the rules don’t apply to them.’

Impulsivity is also a driving factor. Serial cheaters often don’t think about the long-term consequences of their actions in their pursuit of side partners.

Some people are naturally more impulsive, Dr Froude said, either due to genetics or how they were raised, ‘and that can make it harder for them to resist temptation or think through the long-term impact of their choices.’

It isn't all bad news, though. Not everyone who cheats is guaranteed to cheat again, according to clinical psychologist and researcher specializing in intimate relationships Dr Kayla Knopp.

Dr Knopp, who co-founded the therapy clinic and training institute Enamory, regularly works with individuals and couples, emphasizing non-monogamy, sexual expansion, and psychedelic-assisted therapy. 

For many people, she said, cheating is a slippery slope. The person takes one step and then another step, and then another step, and then, before they know it, they're entangled in something that perhaps wasn't their original intention.

Once you give in to something — such as having a slightly inappropriate flirtation with a coworker — it might make it a bit easier to take things further the next time.

She told DailyMail.com: 'We learn about what we're capable of from our own past experiences. And sort of, once you cross that line, it might not be as hard to cross it again.' 

She and a team of psychologists wanted to know if past infidelity meant a higher likelihood of committing infidelity in the future, and if people who had previous partners who had cheated on them were more likely to experience it again in future relationships.

If someone engaged in infidelity in a past relationship, they were three times more likely to do so again in a future relationship.

But Dr Knopp cautioned this is not to say that 'once a cheater, always a cheater' is always accurate. 

The likelihood of cheating again increases for those with a history of infidelity but doesn’t mean everyone who cheats once will always cheat. 

The 'three times more likely' statistic reflects a higher risk, not a guarantee. 

Her research indicated more than half of individuals who had cheated in the past don’t repeat the behavior in future relationships. 

Most of the nearly 500 participants did not report repeated infidelity experiences.

She also maintained that no one is actually 'born a cheater' and cheating is not a fixed trait. It's a behavior shaped by societal expectations for relationships, particularly, monogamy.  

Chronic infidelity can often be seen as a mismatch between the expectations set by societal or relationship norms and an individual's personality traits, preferences, or natural inclinations. 

She said: This is not the case for everybody who cheats in relationships.

'But for at least some of them, it ends up being the case that they're just not a very good fit for a monogamous partnership, and if they can make peace with that, then they can live much more honestly and authentically and openly in a way that makes them much happier and makes their partners much happier.'