The average duration of the first relationship was 38

8 months, while the average duration of the second was 29.6 months. Thus, the relationships studied were mostly serious and of substantial duration. No one was married at the start of the project, but some would have married that first partner or the second during the time frame of the study. For the most part, however, it is best to think about these findings in the context their website of the stage of life in which people are often seriously involved, but not yet married-a stage of life that has grown substantially in the past few decades.

At each time point (which tended to be every four-to-six months), participants were asked, “Have you had sexual relations with someone other than your partner since you began seriously dating?” Participants were also asked if they had either known or suspected their present partner of having sex with someone else. Obviously, there are biases when people self-report such behavior, but that’s a problem for the entire literature. Further, the specific questions used in this study may exclude emotional affairs, as well as some online affairs in which there is some sexual aspect, but the respondents tell themselves they are not actually having sex. (Also, in such a sample there would be some small percentage of people who would have been in some sort of consensual non-monogamous arrangement, in which having sex with someone outside the relationship would not be the same as cheating, because there was some agreement about this. )

Forty-four percent of this sample reported having had sex with someone other than their present partner in one or both of the relationships studied

Knopp and colleagues controlled for some of the variables known to be associated with a greater and lower risk of being unfaithful, net of other factors like relationship quality and commitment to one’s partner. That is, the study controlled for age, gender, socioeconomic status, and race.

Knopp and colleagues note that there is no way to isolate such relationships within this data set, but there are strong reasons to believe that such open relationships are a very small percentage of the overall sample

Further, 30 percent reported that they knew that at least one of their partners in the two relationships had cheated on them. That seems to me like quite a bit of infidelity. Nevertheless, keep in mind that this is not a good estimate of the odds that someone will be unfaithful in an unple, a person would have had to have broken up in at least one serious relationship and entered another. Thus, this result does not mean that 44 percent of those under 40 in the U.S. have been unfaithful to a partner, and it certainly does not mean that such a high percentage of people who get married in a similar age range have been or will be unfaithful. Getting that percentage measured correctly would require a different type of sample and method. Closely related to that question, Galena Rhoades and I found in a previous study that 16 percent of those followed into marriage in the study’s parent project reported that they had cheated on their eventual spouse sometime before their marriage.[viii]

In this new study, 45 percent of individuals who reported cheating on their partner in the first relationship reported also doing so in the second. Among those who had not cheated in the first, far fewer (18 percent) cheated in the second. While the odds of cheating on a partner were far greater if one had done so in the past, a person cheating in one relationship was not destined to do so in the next. In fact, slightly more people who had cheated in the first relationship studied did not report cheating in the second.