Several books I’ve been reading recently have discussed this, and it’s pretty interesting. To put it briefly, infidelity has been pretty frequent throughout human evolution, as evidenced by the size of human testicles.
Gorillas live in small isolated bands with one dominant male and many females. The male monopolizes the females, and has no reproductive competition from other males. Gorillas, therefore, have no need to produce a lot of sperm; it’s a waste of energy. Gorillas therefore have relatively small testicles.
Chimpanzees, on the other hand, live in what are essentially hippie communes. There are dominant males, but they can’t prevent the other males from mating with the females too, and the eventual result is that during mating season, every male chimpanzee in a given troop mates with every female chimpanzee of the troop, several times. Chimpanzees thus have a HUGE amount of reproductive competition going on, and it’s to the males’ advantage to produce as many sperm cells as possible in the hopes of drowning out the competition, so to speak. Chimpanzees are thus endowed with gigantic testicles. Bonobos, who mate even more frequently than regular chimps, have even larger testicles.
So where do humans fit in? Well, human testicle size is in the middle. On average, our testicles are about four times larger than gorilla testicles, but only about one-third the size of chimpanzee testicles and a quarter of the size of bonobo testicles. This indicates adaptation somewhere in between the chimpanzees and the gorillas as well – human males don’t generally have exclusive harems of females, nor do human males generally mate with every female they come across. As one researcher put it, human testicular size is indicative of a species “characterized by monogamy, with a significant degree of infidelity.”
Actual rates of adultery in America, according to a study quoted in book I recently read (I think the book was Skipping Towards Gomorrah, but I’m not positive) are around 60% for males and 40% for females. And since those percentages don’t always overlap, the study concluded that nearly 80% of marriages would at some point be “touched by adultery.”