I am a psychiatrist, a medical doctor who was specially trained in brain chemistry and medications. In 1975, I met a young professor at the University of Washington, David P. Barash, who was an expert in animal behavior and evolution. I had always wanted to be Dr. Doolittle or a vet, so I was very intrigued with David. We started dating in 1976, married in 1977, and have been together ever since. All told, we have 4 children and 3 grandchildren with more on the way. That being said, many years ago David started studying "adultery" in seagulls, the response of males to apparent female adultery. At the same time, I saw many infidelity crises in my practice of psychiatry. In 2000, we wrote a book together, called the Myth of Monogamy, exploring the evidence from DNA fingerprinting that monogamy (life long sexual exclusivity) is extremely rare. Even birds are rarely monogamous. They may be socially monogamous (keep house together exclusively) but both sexes take opportunities to have sex with partners that are not sitting on the nest. Even swans. Even penguins. Penguins are monogamous for a season, and then drift on to other partners. And so it is with people. Exclusive sexual fidelity hardly ever happens, and there is little in our biology to suggest that monogamy was ever natural. Instead, people seem evolved to be moderate sexual opportunists, with male infidelity somewhat more common than female but not a lot. The natural default is for human beings to crave a number of different sexual partners. Males trade resources for sex, and females trade sex for resources. In most countries, serial monogamy (one partner at a time) with departures (adultery) and dating is the most common pattern, followed by polygamy (one male, several or more females). Thus the European ethos of a single partner for life has no basis in biology.