In a 2017 Gallup poll, only 9% of Americans said infidelity was morally acceptable. If you've never cheated, then you might have a hard time understanding what drives a person to break the bonds of trust in a relationship. What inspires someone to jeopardize their existing partnership for the sake of one night with a stranger? How does someone live with themselves while secretly dating multiple people at once?

As it turns out, there's actually a specific personality trait that tends to crop up in the psychological profiles of people who repeatedly cheat on their partners.

Experts seem to agree: cheating and narcissism go hand in hand

True narcissism is more than simply liking what you see in the mirror.

The American Psychological Association considers a narcissist someone who is convinced that they warrant special treatment and repeatedly takes advantage of others to satisfy their own desires.

The narcissist feels that the normal rules shouldn't apply to them, and that what they want should supersede anything else. They also perceive their own attractiveness, sexual prowess, and intelligence to be above average.

Being in a relationship with someone who operates this way is a perfect setup for infidelity.

"People who have been conditioned to always think about what makes them happy and content - and believing their needs being met is the most important thing - are more likely to be serial cheaters," Sydney-based clinical psychologist Jacqui Manning told Mamamia.

Adding weight to this theory is a 2014 study that examined the relationship between infidelity and "sexual narcissism," i.e. how good someone thinks they are in bed and how entitled they feel to sex.

After following 135 newlywed couples over a period of four years, the researchers found that spouses who exhibited sexually narcissistic traits were more likely to be unfaithful.

When someone is narcissistic, they don't see why they should sacrifice their own happiness for someone else's.

Board-certified behavior analyst Carmen McGuinness says that narcissists are more likely to have a greater desire for casual sex and to engage in sex without emotional attachment. In a piece for Mind Body Green, she explains that narcissists are also more likely to have poor communication skills and have a harder time coping with problems in their lives.

Being unable to constructively discuss and solve problems can certainly put pressure on a relationship.

Similarly, if one partner tends to separate emotions and physical intimacy, it may be easier for that person to rationalize sex outside of the relationship.

There's actually a recognized psychological condition called Narcissistic Personality Disorder, or NPD.

The hallmarks of NPD include a lack of empathy for other people, a need for admiration, and a manner that can be described as self-centered, manipulative, and demanding.

Of course, this isn't to say that every cheater has NPD. However, the evidence is there to suggest that people who display narcissistic personality traits are more at risk of straying.

"Not all unfaithful partners are sexual narcissists, but for people high on the facets of this trait, the risk is greater that they, or their partners, will cheat," Dr. Susan Krauss Whitbourne, professor of psychological and brain sciences at University of Massachusetts Amherst, told Psychology Today.

That's because, for a narcissist, staying faithful isn't simply a matter of having a good relationship - the rush of being admired and desired by other potential sexual or romantic partners is often enough to sweep away concerns about their primary partner's feelings.

And being in a relationship with a narcissist can have a real impact on your own psychological state. "When a partner is unfaithful to you, it can hurt your mental health as well as the health of your relationship," says Dr. Whitbourne.

There could also be a deeper, more disturbing psychological reason that a cheater feels it's OK to stray.

According to McGuinness, infidelity and narcissism have a strong correlation to psychopathy. Psychopathy is a condition in which an individual is unable to feel empathy for other people.

She said that there is a 95% likelihood that a psychopath or sociopath will seek other sexual partners outside a relationship.

McGuinness also stated in her piece for Mind Body Green that "a disregard for social mores and the rights and feelings of others, and a failure to feel remorse or guilt" are some of the traits that make psychopaths more likely to cheat.

Regardless of whether someone is actually psychopathic or simply narcissistic, shattering that bond of trust in a relationship by cheating is strong evidence of problems in the partnership.

Maintaining open lines of communication, discussing each other's needs honestly, and staying alert to potentially tempting situations are all actions couples can take to prevent infidelity.

Additionally, evaluating your partner or yourself for narcissistic tendencies can be a great first step to strengthening your relationship.

"Ask whether you sense these tendencies in your partner," said Dr. Whitbourne. "Your relationship will be more likely to endure if you are able to address, and possibly fix, problems before they begin. Personality isn't necessarily that amenable to change, but change is possible."

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