There are a number of studies that are done on cheating. Statistics show that between 20 to 80 percent of men cheat on their partners during their lifetime, depending on the population studies and the type of study done. About 10 to 60 percent of women cheat on their partners, again depending on the population during their lifetime.
If you average these statistics, one half of men and about one third of women cheat on their partners. These statistics may be under inflated, because a lot of people are not honest about their infidelity, or they do not define some of their encounters as infidelity per say.
I think the statistic is actually a little bit higher. I think about close to 80 percent of men and 60 percent of women engage in some sort of adulterous or cheating behavior during their lifetime. Usually, infidelity statistics are higher for younger people than older people.
Much of that is driven by our hormones; testosterone for men and testosterone, estrogen, progesterone for women. There are other critical times during people's lifetime when they are more likely to engage in infidelity, such as mid-life crisis, when children leave home, and other important times in their lives, retirement and so forth.
However, it seems like younger people are more predisposed to cheating than older people are.