After the Kinsey Reports came out in the early 1950s, findings suggested that historically and cross-culturally, extra-marital sex has been a matter of regulation more than sex before marriage (Christensen, 1962). The Kinsey Reports asserted that about one half of men and a quarter of women had committed adultery (Greely, 1991). The Janus Report on Sexual Behavior in America also reported that one third of married men and a quarter of women have had an extramarital affair (Greeley, 1991).
According to The New York Times, the most consistent data on infidelity comes from the University of Chicago's General Social Survey (GSS). Large-scale interviews conducted since 1972 by the GSS of people in monogamous relationships reveals that the number of men admitting to extramarital affairs is 12 percent and for women, 7 percent. Results, however, can be variable depending on the year data is gathered, and also based on the age groups surveyed. For example, one study conducted by the University of Washington, Seattle found slightly, or significantly higher rates of infidelity for populations under 35, or older than 60. In that study which involved 19,065 people during a 15 year period, rates of infidelity among men were found to have risen from 20 to 28%, and rates for women, 5% to 15%. In more recent nationwide surveys, several researchers found that about twice as many men as women reported having an extramarital affair (Wiederman, 1997) A survey conducted by Choi, Catania, and Dolcini in 1990 found 2.2% of married participants reported having more than one partner during the past year. In general, national surveys conducted in the early 1990s reported that between 15-25% of married Americans reported having extramarital affairs. Treas and Giesen (2000) found that the likelihood of sexual infidelity was higher for those who had stronger sexual interests, more permissive sexual values, lower subjective satisfaction with their partner, weaker network ties to their partner, and greater sexual opportunities Studies suggest around 30–40% of unmarried relationships and 18–20% of marriages see at least one incident of sexual infidelity. Men are more likely than women to have a sexual affair, regardless of whether or not they are in a married or unmarried relationship.
Rates for females are thought to increase with age. In Blow's data set, men were found to be only “somewhat” more likely than women to engage in infidelity, with rates for both sexes becoming increasingly similar
(Blow & Harnett, 2005). A study done by Liu (2000) found that the likelihood for women to be involved in some type of infidelity reached a peak in the seventh year of their marriage and then declined afterwards; whereas for married men, the longer they are in relationships the less likely they are to engage in infidelity, except during a critical point in the eighteenth year of marriage where at that point the chance that men will engage in infidelity increases