1. Why is long-term sexual monogamy so difficult for many couples?
Several factors conspire to make long-term sexual monogamy difficult for people. As a species, we’ve evolved to be sexually responsive to novelty. From a genetic point of view, the lure of new partners (known to scientists as the Coolidge effect) combined with less responsiveness to the familiar (the Westermarck effect) motivated our ancestors to risk leaving their small hunter/gatherer societies to join other groups, thus avoiding incest and bringing crucial genetic vigor to future generations.
Another problem is that most people in the West marry because they’re “in love,” which is a temporary, blissfully delusional state we should enjoy, but not expect to last forever. As the German poet, Goethe put it, “Love is an ideal thing, marriage a real thing. A confusion of the real with the ideal never goes unpunished.”
2. Why do you specify “sexual monogamy?” Isn’t all monogamy sexual?
Biologists distinguish sexual monogamy from social monogamy. As DNA testing has grown cheaper in recent years, we’ve learned that most species formerly classified as “monogamous” (primarily birds) are socially monogamous, but not sexually so. In other words, they form pairs that cooperatively care for that season’s brood of young, but the male may well not be the biological father. Applied to humans, we argue that a more flexible approach to sexual fidelity can increase marital stability and thus lead to greater social and family stability.