By SONJA LYUBOMIRSKY

Published: December 1, 2012 208 Comments

IN fairy tales, marriages last happily ever after. Science, however, tells us that wedded bliss has but a limited shelf life.

American and European researchers tracked 1,761 people who got married and stayed married over the course of 15 years. The findings were clear: newlyweds enjoy a big happiness boost that lasts, on average, for just two years. Then the special joy wears off and they are back where they started, at least in terms of happiness. The findings, from a 2003 study, have been confirmed by several recent studies.

The good news for the holiday season when families gather in various configurations is that if couples get past that two-year slump and hang on — for another couple of decades — they may well recover the excitement of the honeymoon period 18 to 20 years later, when children are gone. Then, in the freedom of the so-called empty nest, partners are left to discover one another — and often their early bliss — once again.

When love is new, we have the rare capacity to experience great happiness while being stuck in traffic or getting our teeth cleaned. We are in the throes of what researchers call passionate love, a state of intense longing, desire and attraction. In time, this love generally morphs into companionate love, a less impassioned blend of deep affection and connection. The reason is that human beings are, as more than a hundred studies show, prone to hedonic adaptation, a measurable and innate capacity to become habituated or inured to most life changes.

With all due respect to poets and pop radio songwriters, new love seems nearly as vulnerable to hedonic adaptation as a new job, a new home, a new coat and other novel sources of pleasure and well-being. (Though the thrill of a new material acquisition generally fades faster.)

Hedonic adaptation is most likely when positive experiences are involved. It’s cruel but true: We’re inclined — psychologically and physiologically — to take positive experiences for granted. We move into a beautiful loft. Marry a wonderful partner. Earn our way to the top of our profession. How thrilling! For a time. Then, as if propelled by autonomic forces, our expectations change, multiply or expand and, as they do, we begin to take the new, improved circumstances for granted.

Sexual passion and arousal are particularly prone to hedonic adaptation. Laboratory studies in places as far-flung as Melbourne, Australia, and Stony Brook, N.Y., are persuasive: both men and women are less aroused after they have repeatedly viewed the same erotic pictures or engaged in similar sexual fantasies. Familiarity may or may not breed contempt; but research suggests that it breeds indifference. Or, as Raymond Chandler wrote: “The first kiss is magic. The second is intimate. The third is routine.”

There are evolutionary, physiological and practical reasons passionate love is unlikely to endure for long. If we obsessed, endlessly, about our partners and had sex with them multiple times a day — every day — we would not be very productive at work or attentive to our children, our friends or our health. (To quote a line from the 2004 film “Before Sunset,” about two former lovers who chance to meet again after a decade, if passion did not fade, “we would end up doing nothing at all with our lives.” ) Indeed, the condition of being in love has a lot in common with the state of addiction and narcissism; if unabated, it will eventually exact a toll.

WHY, then, is the natural shift from passionate to companionate love often such a letdown? Because, although we may not realize it, we are biologically hard-wired to crave variety. Variety and novelty affect the brain in much the same way that drugs do — that is, they trigger activity that involves the neurotransmitter dopamine, as do pharmacological highs.

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.

Such reactions may have neuroscientific origins. In one experiment, scientists offered drinks to thirsty subjects; those who were not told what kind of drink they would get (i.e., water or a more appealing beverage) showed more activity in the portion of the brain that registers positive emotions. Surprise is apparently more satisfying than stability.

The realization that your marriage no longer supplies the charge it formerly did is then an invitation: eschew predictability in favor of discovery, novelty and opportunities for unpredictable pleasure. “A relationship,” Woody Allen proclaimed in his film “Annie Hall,” “is like a shark. It has to constantly move forward or it dies.” A marriage is likely to change shape multiple times over the course of its lifetime; it must be continually rebuilt if it is to thrive.

The good news is that taking the long view on marriage and putting in the hard work has calculable benefits. Research shows that marital happiness reaches one of its highest peaks during the period after offspring have moved out of the family home.

The nest may be empty, but it’s also full of possibility for partners to rediscover — and surprise — each other again. In other words, an empty nest offers the possibility of novelty and unpredictability. Whether this phase of belated marital joy lasts, like the initial period of connubial bliss, for longer than two years is anybody’s guess.

Sonja Lyubomirsky is a professor of psychology at the University of California, Riverside, and the author of the forthcoming book “The Myths of Happiness: What Should Make You Happy, but Doesn’t, What Shouldn’t Make You Happy, but Does.”