Evolutionary biologists believe that sexual variety is adaptive, and that it evolved to prevent incest and inbreeding in ancestral environments. The idea is that when our spouse becomes as familiar to us as a sibling — when we’ve become family — we cease to be sexually attracted to each other.

It doesn’t take a scientist to observe that because the sex in a long-term committed monogamous relationship involves the same partner day after day after day, no one who is truly human (or mammalian) can maintain the same level of lust and ardor that he or she experienced when that love was uncharted and new.

We may love our partners deeply, idolize them, and even be willing to die for them, but these feelings rarely translate into long-term passion. And studies show that in long-term relationships, women are more likely than men to lose interest in sex, and to lose it sooner. Why? Because women’s idea of passionate sex depends far more centrally on novelty than does men’s.

When married couples reach the two-year mark, many mistake the natural shift from passionate love to companionate love for incompatibility and unhappiness. For many, the possibility that things might be different — more exciting, more satisfying — with someone else proves difficult to resist. Injecting variety and surprise into even the most stable, seasoned relationship is a good hedge against such temptation. Key parties — remember “The Ice Storm”? — aren’t necessarily what the doctor ordered; simpler changes in routine, departures from the expected, go a long way.

In a classic experiment conducted by Arthur Aron and his colleagues, researchers gave upper-middle-class middle-aged couples a list of activities that both parties agreed were “pleasant” (like creative cooking, visiting friends or seeing a movie) or “exciting” (skiing, dancing or attending concerts) but that they had enjoyed only infrequently. Researchers instructed each couple to select one of these activities each week and spend 90 minutes doing it together. At the end of 10 weeks, the couples who engaged in the “exciting” activities reported greater satisfaction in their marriage than those who engaged in “pleasant” or enjoyable activities together.

Although variety and surprise seem similar, they are in fact quite distinct. It’s easy to vary a sequence of events — like choosing a restaurant for a weekly date night — without offering a lot of surprise. In the beginning, relationships are endlessly surprising: Does he like to cook? What is his family like? What embarrasses or delights him? As we come to know our partners better and better, they surprise us less.

Surprise is a potent force. When something novel occurs, we tend to pay attention, to appreciate the experience or circumstance, and to remember it. We are less likely to take our marriage for granted when it continues to deliver strong emotional reactions in us. Also, uncertainty sometimes enhances the pleasure of positive events. For example, a series of studies at the University of Virginia and at Harvard showed that people experienced longer bursts of happiness when they were at the receiving end of an unexpected act of kindness and remained uncertain about where and why it had originated.