Blow and Hartnett's 2005 review examines self-reported infidelity in an attempt to get estimate the actual prevalence of the phenomenon. One major confound is the reporting differences between men and women, which are very likely to be profound (see 8/13/07): women are less likely to admit to having extra-marital affairs. However, as previously discussed, there is a strong sense that men are more likely to desire multiple partners than women (see related, below). For a specific figure, they cite Laumann et al's 1994 book: The Social Organization of Sexuality: Sexual Practices in the United States, which says that 25% of married men and 15% of married women report having had extramarital sex.
Blow and Hartnett conclude that (US-only) extramarital sex "occurs in less than 25% of committed relationships, and more men than women appear to be engaging in infidelity." However, they also note that gender differences in infidelity appear to vary widely between cultures.
Cheating is the topic of my weekly poll (a feature on my personal blog), and while I've covered differences in sexual appetites several times, I didn't think I'd ever presented an estimate of infidelity prevalence. Honestly, that's a hell of a lot lower than I would have guessed, and I think it's a lower bound. I don't see any likelihood that someone is going to report infidelity that has not occured, but I see a number of reasons why they would not report infidelity that had. I know I have a great deal of observer bias on the subject, as well as a non-normative sample, but it's always seemed to me like more than half of relationships have some cheating.
Adultery Gap, 10/28/08
Marital and Extra-marital sex, 9/18/08
Evolutionary Psychology 101, 5/30/07
The Jealous Type, 10/25/06
Find out the day's topic before you read: follow diffblog on Twitter! Diffblog also available on LiveJournal.