Read: What you lose when you gain a spouse

But suppose you’ve managed to survive your courtship without any problems. (This may take more imagination.) You’ve just graduated from dating to blissful matrimony. Your soul soars, your heart sings, and your brain is awash in oxytocin, dopamine, and other neurochemicals associated with love. You are probably in no mood to participate in a scientific study, but some other newlyweds were persuaded to do so for a long‑term project called PAIR. (The full, unromantic name is Processes of Adaptation in Intimate Relationships.) These couples, in central Pennsylvania, were interviewed during their first two years of marriage by psychologists who cataloged both the positive and negative aspects of the relationships.

Some of the people were already ambivalent or hostile toward their partners—and tended to get divorced quickly—but most couples showed lots of mutual affection and went on to celebrate several anniversaries. Over the long haul, though, those tender early feelings were not a reliable harbinger. More than a decade later, a disproportionate number of the couples who had been “almost giddily affectionate” were no longer together. As a group, those who divorced had been a third more affectionate during the early years than the ones who went on to have long, happy marriages. Over the short term, their passion had enabled them to surmount their misgivings and their fights, but those positive feelings couldn’t keep the marriage going forever. It was how they dealt with the negative stuff—their doubts, their frustrations, their problems—that predicted whether the marriage would survive. Negativity hits young people especially hard, which is one reason that people who marry earlier in life are more likely to divorce than ones who delay marriage. (Another reason is that younger people tend to have less money, which means more stress.)

Some couples, of course, are better off splitting up, but far too many of them sabotage a relationship that could have worked. Researchers who track couples have repeatedly been puzzled to see relationships destroyed even when there are no obvious causes. To test a theory, the psychologists Sandra Murray and John Holmes brought couples into a lab and gave them questionnaires to be filled out at tables arranged so that the partners sat with their backs to each other. They’d both be answering the same questions, the experimenter explained, and it was important that they not communicate in any way as they filled out the forms.

In fact, though, the questionnaires were different. One form asked people what they didn’t like about their partners. They could list as many traits as they wanted, but were told it was fine to name just one. These people, who’d been dating on average for a year and a half, had a few complaints but were mostly pretty satisfied. They typically wrote down one or two things about their partners that were less than ideal, and then they put down their pens. The other partners were given a much different task: listing all the things in their home. Instructed to name at least 25 items, they’d start writing—cataloging pieces of furniture, kitchenware, gadgets, books, artwork, whatever—and were often still working away at it five minutes later.