From There

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...Conservative infidelity statistics estimate that "60 percent of men and 40 percent of women will have an extramarital affair. These figures are even more significant when we consider the total number of marriages involved, since it's unlikely that all the men and women having affairs happen to be married to each other. If even half of the women having affairs (or 20 percent) are married to men not included in the 60 percent having affairs, then at least one partner will have an affair in approximately 80 percent of all marriages. With this many marriages affected, it's unreasonable to think affairs are due only to the failures and shortcomings of individual husbands or wives."

Note that the above adultry statistics of the prevalence of affairs were made more than a decade ago; so based on changes in society during the intervening years, the current percentage of the population who have had affairs is probably somewhat HIGHER. For instance, the continuing increase of women in the workplace and the increase of women having affairs on the Internet means that the numbers for women having affairs is probably similar to those for men—about 60%.
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No further comment needed.