In most industrialized nations, people are marrying later and later and having increasingly significant, romantic relationships prior to marriage. People who are seriously involved with another tend to expect sexual faithfulness even though they are not married. But how many people in a serious romantic relationship have sex with someone besides their partner? And what are the characteristics of the people who do versus those who do not have sex with someone else?

Loads of studies look at infidelity in marriage, but few have looked at cheating in unmarried serious relationships—which is surprising, given that these relationships have massive impacts on peoples’ lives. To fill in this gap, my colleagues and I examined infidelity in unmarried relationships with a national sample of 1,300 younger adults. We recruited young adults who, at the study’s outset, were aged 18 to 34 and unmarried but in serious romantic relationships that had lasted at least two months, with the average present relationship having lasted for two years.

Overall, 14% of young adults reported having sex outside of their relationship during the year and a half following recruitment into the study. I refer to this as “cheating” in the title of this post because that is what most of it would entail. (We are not able to tease out the small number of people who would have agreed with their partners to be in open relationships where it would not be cheating but it would merely be extradyadic sex. In addition, analyses of which people cheated excluded those who had already cheated when the study began.)

14% of unmarried young adults in a recent 18-month study reported having sex outside of their relationship.

Our research team, headed by Amanda Maddox Shaw, then examined who was most likely to have sex with someone outside of their relationship during the same time period, and made some surprising discoveries. In no particular order, here are the individual and relationship variables that we found to be associated with having extradyadic sex over the first 1.5 years of the study.

Individual variables associated with extradyadic sex:

  • Having more sexual partners prior to the present relationship
  • Greater use of alcohol
  • Having parents who never married

Individual variables not associated with extradyadic sex:

  • Gender (males were not more likely than females to cheat)
  • Age
  • Education
  • Religiousness
  • Having children (with partner or from prior relationships)
  • Parental divorce

Relationship variables associated with extradyadic sex:

  • Lower relationship satisfaction
  • Lower levels of dedication (commitment) to the partner
  • Higher levels of negative communication
  • A history of physical aggression in the relationship
  • Not having mutual plans for marriage
  • Suspicion of partner having sex with other(s)
  • Partner has had sex with another

Relationship variables not associated with extradyadic sex:

  • Frequency of sex in present relationship
  • Satisfaction with sex in present relationship
  • Living together

As you can see from the lists, while there are a few individual characteristics associated with having extradyadic sex, many more individual characteristics were not associated with it. The big story from the study is that the characteristics of the relationship, not the individuals, best predict who is likely to cheat. Those who were less satisfied in their relationships, less committed to their partner, and who had reported more negative patterns of communicating were the ones most likely to have sex with someone besides their partner. Contrary to what you might have guessed, sexual frequency and sexual satisfaction with the partner were not associated with cheating. Rather, the overall quality of the relationship, apart from sex, is what mattered most—for both men and women.

The characteristics of the relationship, not the individuals, best predict who is likely to cheat.

In addition, suspecting one’s partner of cheating or knowing one’s partner has cheated in the past was strongly associated with a person’s likelihood of cheating on their partner in the future. In other words, people who think or know that their partner has had sex with someone else are more likely, over time, to do the same. Some of that is due to the relationships having lower commitment and some is doubtless revenge, but I’d bet more on low commitment being the primary factor there.

The finding that I think many people would consider most surprising is that living together was not significantly associated with whether a person reported having had sex with someone other than their partner.  That is, cohabitation was not associated with greater odds of cheating nor was it associated with lower odds of cheating. Living together just didn’t provide information about sexual exclusivity.

This news could come as a surprise because many people believe that moving in together equals strong commitment, or at least an increase in commitment. Closely related, many people assume that when two people live together, they are “off the market,” so to speak. I know of no evidence that backs up this belief. There are some places and conditions where living together does signal a higher level of commitment—but for most people in most communities, that’s not the case. So cohabitation is not like engagement or a mutual plan to get married, both of which are clear signals of higher commitment.

Living together just doesn’t provide information about sexual exclusivity or commitment.

There is some practical advice in this. If you want to know whether your partner is really committed to you, don’t look at whether you’re living together; look for something that actually reflects commitment. Similarly, if you are trying to figure out if a new relationship is exclusive and has a future, the starting point isn’t going to be moving in together—nor would it be the willingness of someone to have sex with you. It’s much more informative to talk openly (when it is the right time to start doing so) about whether the two of you share a mutual commitment and understanding about the relationship. Too many people find out the hard way that moving in together didn’t mean what they thought or hoped it meant.