1. Most committed couples are monogamous at the outset, but between 20-40% of heterosexual married men and 20-25% of heterosexual married women have an extramarital affair at some point in their life time (Tafoya & Spitzberg, 2007, Laumann et al., 1994).
2. Traditionally, men have reported engaging in sexual inﬁdelity signiﬁcantly more than women (Smith, 2006, Wiederman, 1997), but this trend is changing. Male and female rates of infidelity are becoming increasingly similar, particularly in younger cohorts in developed countries (Atkins et al., 2001, Choiet al., 1994, Feldman and Cauffman, 1999)
3. When the parameters of infidelity are expanded to encompass "any form of romantic and/or sexual involvement, short or long-term, including kissing, while the individual is in a relationship with another person", 31.4% of female university students report having been unfaithful at some point, compared to 24% of males (Brand, Markey, Mills and Hodges, 2007).
4. Because sexual infidelity is a secret behavior and people are reluctant to admit to it, even in anonymous surveys, it is very difficult to get exact estimates for prevalence.
5. In fact, Tom Smith, who directs the General Social Survey at the National Opinion Research Center (NORC) at the University of Chicago suggests that "There are probably more scientifically worthless facts on extramarital relations than on any other facet of human behavior."
6. Smith says "the best estimates are that about 3-4% of currently married people have a sexual partner besides their spouse in a given year, and about 15-18% of ever-married people have had a sexual partner other than their spouse while married."
7. Extramarital sex matters because it is a recognised predictor of divorce. In a meta-analysis of nine studies of divorce, extramarital sex was a prominent feature in eight (Kitson, Babri, and Roach, 1985). And South and Lloyd (1995) found that in at least one third of divorce cases, one or both spouses had been involved with another person prior to the marriage ending.
8. Interestingly, the more common infidelity seems to become, the more we seem to disapprove of it. Figures show that in the UK, male condemnation of non-exclusivity in marriage increased from 45% in 2000, to 63% in 2013. Similarly, female disapproval of non-exclusivity in marriage increased from 53% to 70% in the same time period (Natsal, 2013).
9. In the US, a 2013 Gallup survey of 1,535 American adults found that 91% considered extramarital infidelity to be morally wrong. This was higher than the figure for people who objected to human cloning, suicide, and polygamy.
10. Given its apparent prevalence, our increasingly strident attitude towards towards infidelity seems a bit puritanical. However, the political scientist John Sides argues that far from being regressive, hardened attitudes to infidelity simply reflect a growing impatience with people who wreak emotional havoc because they are too cowardly to end one relationship, before starting another. As he says "If you're in an unhappy marriage, don't cheat. Just get divorced."