When we enter into a long-term sexual relationship, we expect our partner to be completely committed to us. We want to believe they’ll be faithful to us, but is there any way to predict if they’re likely to stray? This is the question that University of Quebec psychologist Valerie Guilbault and her colleagues explored in a recent article in the Journal of Social and Personal Relationships.

Previous research has identified a number of factors that can predict the likelihood of a partner straying. One factor is , in that many studies have shown men to be more likely than women to have sex outside of the relationship. However, as Guilbault and colleagues point out, this gap has been narrowing in recent years. Furthermore, in the studies we’re about to look at, the researchers found that both men and women cheated on their partners with equal frequency (about 13% over a ten-month period, consistent with findings from other studies).

A second factor that predicts is avoidance. People who are securely attached find effective ways of dealing with the inevitable conflicts that arise in any relationship. However, those with avoidant attachment withdraw at the first sign of trouble—husbands withdrawing to their man cave, wives giving their spouses the silent treatment. Some even seek out illicit sex partners to exact or assuage their feelings of or low .

A third factor is a person’s attitudes toward casual sex. People who strongly believe in sex as an expression of love within a committed relationship are less likely to stray compared with those who have a past of multiple sex partners. That former playboy is unlikely to be good husband material. Nevertheless, there are still plenty of people who sow their wild oats in young adulthood only to settle into as they mature.

Finally, sex drive is also an important factor. People vary widely in the strength of their sex drive. It seems obvious that those with low desire for sex have little incentive to cheat. In contrast, those with a high sex drive may be tempted to look elsewhere, especially if their needs repeatedly go unmet in their relationship.

Guilbault and colleagues believe there is a single trait underlying the above factors, one that can more accurately predict whether a partner will be unfaithful than any of the others alone. They call this trait “sexual passion,” and it encompasses both high sex drive and open attitudes toward . People with strong sexual passion view sex as a defining feature of their lives. They think about sex a lot, and they engage in sexual acts frequently—with a partner if possible, but they’re happy to go solo as well.

Among people with strong sexual passion, according to the researchers, there are two types—harmonious and obsessive sexual passion. This distinction is based on an influential personality theory proposed by Julian Rotter in the 1950s known as “ .”

According to this theory, people with an internal locus of control believe they have mastery over the events in their lives, which leads to higher self-esteem and as well as lower levels of and . In contrast, people with an external locus of control believe the events of their lives are driven by outside forces—whether those be fate, luck, or other people—and, as a result, they suffer from self-esteem issues and tendencies.

In the model proposed by Guilbault and colleagues, people with harmonious sexual passion believe they are in control of their sexuality. They choose when and whether to engage in sex acts, so they avoid conflicts between their sexual passion and other aspects of their lives. Conversely, people with obsessive sexual passion feel compelled by sexual thoughts and urges that they give in to even when they know that doing so will land them in trouble.

The researchers proposed that those with obsessive sexual passion would be more likely to cheat on their partners, and they tested this hypothesis with two studies. In the first, over 600 young adults responded to questions intended to assess harmonious and obsessive sexual passion. They also reported any past instances in which they’d been unfaithful to a partner. As expected, those with obsessive sexual passion had engaged in far more acts of infidelity than had those with harmonious sexual passion.

While the results from the first study support the idea that obsessive sexual passion drives infidelity, the researchers also wanted to test whether this personality trait would predict future acts of unfaithfulness. To this end, they followed 84 young adults over a 10-month period.

At the outset of the study, the participants responded to questions that assessed their levels of harmonious and obsessive sexual passion. Ten months later, they reported whether they’d had sex with anyone besides their partner. Again as predicted, those with obsessive sexual passion reported far more instances of infidelity than those with harmonious sexual passion.

The researchers also examined the reasons the participants gave for their infidelities. A common explanation that both types gave was a strong physical to the other person. Relationships in young adulthood are fluid, and many of the respondents weren’t in the same committed relationship for the entire 10 months of the study. It’s quite possible these acts of unfaithfulness occurred at the transition from one relationship to another, although the researchers didn’t ask about this.

More interesting were the other reasons that those with obsessive sexual passion gave for being unfaithful. These included a desire to exact vengeance against their partner, to boost their self-esteem, or to live up to perceived social expectations, such as the notion that “real men” have many sex partners. Those with harmonious sexual passion rarely gave such reasons for their infidelity.

To conclude, this research suggests that persons who view themselves as controlled by external forces are more likely to cheat, especially when coupled with strong sexual passion. It’s best to steer clear of people who always see themselves as victims since a relationship with them is bound to be unhappy. Infidelity is only one of many heartbreaks such people have in store for you.

In contrast, people who believe they are in control of their fate are likely to make better partners. When they also are high in harmonious sexual passion, they'll likely make better lovers and life partners. Furthermore, their secure sense of self also makes them less likely to stray from a committed relationship.

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