Infidelity Statistics

These are obviously more recent, however I think they still have relevance. They come from Dear Peggy.com

According to Annette Lawson, author of Adultery first published in 1989 by Basic Books:
"The various researchers arrive at a general consensus…suggesting that above one-quarter to about one-half of married women have at least one lover after they are married in any given marriage. Married men probably still stray more often than married women—perhaps from 50 percent to 65 percent by the age of forty."

According to Maggie Scarf, author of Intimate Partnersfirst published in 1987 by Random House, re-issued in 1996 by Ballentine:
"Most experts do consider the 'educated guess' that at the present time some 50 to 65 percent of husbands and 45 to 55 percent of wives become extramaritally involved by the age of 40 to be a relatively sound and reasonable one."

According to Peggy Vaughan, author of The Monogamy Myth first published in 1989 by Newmarket Press (third edition published 2003):
"Conservative estimates are 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."