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What Exactly Is Infidelity?

Infidelity is, partly, in the eye of the beholder.

Eric Ward / Unsplash
Source: Eric Ward / Unsplash

In 1992, a now-classic article in the behavioral sciences was published in Psychological Science. David Buss and his colleagues provided evidence for the existence of substantial sex differences in reactions to infidelity. These researchers presented men and women with two kinds of hypothetical examples of infidelity. One kind of example focused on sexual infidelity, in which one's partner was cheating in a physical and sexual manner. Another kind of example focused on emotional infidelity, in which one's partner was essentially falling in love with someone else.

The brief version of the results is as follows: Males were more likely than were females to have a negative reaction to sexual infidelity, while females were more likely than were males to have a strong negative reaction to emotional infidelity. And while follow-up research has pointed out all kinds of variables that relate to these reactions, generally, these patterns have stood up to the test of time (see Buss, 2005).

Why Would Men and Women Differ in Reactions to Infidelity?

As an evolutionary behavioral scientist, Buss's explanation as to why males and females differ from one another in reactions to infidelity is reasonably straightforward. This explanation focuses on differentiated male and female reproductive anatomy and physiology, especially the fact that, due to internal fertilization, it is possible for a man to be cuckolded: A woman can give birth to a baby that is not really the guy's kid. From an evolutionary perspective, this situation poses an adaptive hurdle that is specific to men. So a partner cheating sexually is a problem in a relationship, but it is, from an evolutionary perspective, a bigger problem for males, as sexual cheating could lead to cuckoldry. And evolutionary forces would favor adaptations (such as a negative response to sexual infidelity) that would lower the likelihood of cuckoldry.

Females have a different adaptive hurdle that surrounds their reproductive systems. For females, the costs of having offspring are high. In fact, these costs are much higher than are comparable costs for males, and they include pregnancy, childbirth, and nursing — all major investments in time and energy. Childrearing in humans is costly, and it benefits from multiple adult caregivers. A partner who engages in emotional infidelity, by falling in love with someone else, is a liability in this domain, as such infidelity might signal possible desertion. And while desertion is never a great thing in a relationship, it can be particularly devastating for women, as they would be disproportionately affected by being abandoned by a partner while raising young children. So women, relative to men, would have evolved psychological adaptations to be particularly wary of signals of emotional infidelity.

Defining Sexual and Emotional Infidelity

In a recent publication from our research team (the New Paltz Evolutionary Psychology Lab), we explored a question surrounding research on reactions to infidelity. One of the issues in this research has pertained to the definitions that have been used for the concepts of sexual and emotional infidelity. The lion's share of research on this topic has used researcher-based definitions of sexual and emotional infidelity. In our study (Guitar, Geher, Kruger, Garcia, Fisher, & Fitzgerald, 2016), we asked hundreds of laypeople (in this case, college students) to offer their definitions of sexual and emotional infidelity. Then in a second study, we had hundreds of other college students read the statements that were generated by participants in the first study and then rate each of these statements in terms of how prototypical it was. This methodology allowed us to hone in on the ways that real young adults think about and define sexual and emotional infidelity.

Defining Features of Sexual Infidelity

Hundreds of examples of infidelity definitions that were generated by participants were evaluated for basic themes. The four themes that emerged as being most defining of sexual infidelity were as follows:

  • Sexual activity with an individual other than one’s partner (found in 51 percent of participant-generated examples)
  • Physical sexual behaviors other than full sexual intercourse (found in 20 percent of participant-generated examples)
  • Sexual intentions (found in 7 percent of participant-generated examples)
  • Lack of partner’s consent/permission (found in 7 percent of participant-generated examples)

As you can see, sexual infidelity is somewhat straightforward. A majority of participants included something about sexual infidelity with someone other than one's partner in their sample definitions. Things got a bit murky, however, when we examined the defining features of emotional infidelity.

Defining Features of Emotional Infidelity

The same participants who were asked to provide definitions of sexual infidelity were asked to provide definitions of emotional infidelity. As you can see below, the top four themes here were less cut-and-dried compared with the themes found for sexual infidelity.

  • Attending important events with someone else (found in 16 percent of participant-generated examples)
  • Deceiving one’s partner about feelings toward them (found in 15 percent of participant-generated examples)
  • Attached/dedicated emotionally to someone else (found in 6 percent of participant-generated examples)
  • One partner is not emotionally satisfying the other (found in 6 percent of participant-generated examples)

Here, not a single theme emerged in the majority of participants. In fact, the most common themes here, attending important events with someone else and deceiving one's partner about one's feelings for someone else, were only found in 16 percent and 15 percent of participant-generated definitions respectively. When it comes to how real people understand emotional infidelity, responses are complicated and are, essentially, all over the place.

Sex Differences in Rating Infidelity Definitions

A related finding in this research was that when participants rated how prototypical the different definitions of sexual and emotional infidelity were, men and women generally showed more agreement in what constituted a clear example of sexual infidelity compared to what they thought was a strong example of emotional infidelity. For instance, when it came to rating sample definitions of sexual infidelity, both men and women rated the following as the most prototypical:

Sexual infidelity is when you are in a relationship or a marriage and engage in sexual activity with another individual that is not your girlfriend/boyfriend, husband or wife. Having an affair, or cheating in a sexual manner.

However, when it came to rating sample definitions of emotional infidelity, the item that was rated as most prototypical by men differed from the item that was rated as most prototypical by women. For men, the most prototypical definition of emotional infidelity was:

Emotional infidelity is when a person in a relationship creates an emotional distance by spending an excessive amount of time with, or thinks about, another person outside of the relationship, to the point that the other partner becomes ignored or rejected emotionally.

The most prototypical emotional infidelity definition as rated by women was:

Emotional infidelity is being "in love" or more dedicated emotionally to someone other than the partner, or family, someone with romantic potential.

Bottom Line

Since the classic paper by David Buss and colleagues was published on sex differences in reactions to infidelity, many insights into the factors associated with relationship dissolution from an evolutionary perspective have been uncovered. Our research (Guitar et al., 2016) delved into the nuances regarding how sexual and emotional infidelity are actually conceptualized by real people in an effort to hopefully inform future research in this area.

In short, we found that sexual infidelity is relatively clearly defined. People typically know it when they see it; people typically agree on what all constitutes sexual infidelity.

Emotional infidelity, on the other hand, seems like a different beast altogether in many ways. Definitions of the elements that comprise emotional infidelity varied wildly among participants in our first study. And in our second study, men and women varied very much from one another in terms of what all constituted a solid example of emotional infidelity.

Iakov Filimonov/Shutterstock
Source: Iakov Filimonov/Shutterstock

I agree with David Buss and his colleagues that there are two important classes of infidelity: sexual infidelity and emotional infidelity. This said, based on our research, it seems that emotional infidelity is, and perhaps always will be, a more amorphous, hard-to-define class of behavior. With sexual infidelity, people know it when they see it. When it comes to emotional infidelity, however, it seems like we may all have our own unique take on what it actually is. And this fact probably adds to all kinds of misunderstandings in modern relationships.


Buss, D. M., Larsen, R. J., Westen, D., & Semmelroth, J. (1992). Sex differences in jealousy: evolution, physiology, and psychology. Psychological Science, 3(4), 251–255.

Buss, D. M., & Haselton, M. (2005). The evolution of jealousy. Trends in Cognitive Sciences, 9(11), 506–507.

Guitar, A. E., Geher, G., Kruger, D., Garcia, J. R., Fisher, M., & Fitzgerald, C. J. (2016). Defining and interpreting definitions of emotional and sexual infidelity. Current Psychology.

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