The psychologist John Gottman was one of those researchers. For the past four decades, he has studied thousands of couples in a quest to figure out what makes relationships work. I recently had the chance to interview Gottman and his wife, Julie, also a psychologist, in New York City. Together, the renowned experts on marital stability run the Gottman Institute, which is devoted to helping couples build and maintain loving, healthy relationships based on scientific studies.

John Gottman began gathering his most crucial findings in 1986, when he set up the “Love Lab” with his colleague Robert Levenson at the University of Washington. Gottman and Levenson brought newlyweds into the lab and watched them interact with each other. With a team of researchers, they hooked the couples up to electrodes and asked the couples to speak about their relationship, like how they met, a major conflict they were facing together, and a positive memory they had. As they spoke, the electrodes measured the subjects’ blood flow, heart rates, and how much sweat they produced. Then the researchers sent the couples home and followed up with them six years later to see if they were still together.

From the data they gathered, Gottman separated the couples into two major groups: the masters and the disasters. The masters were still happily together after six years. The disasters had either broken up or were chronically unhappy in their marriages. When the researchers analyzed the data they gathered on the couples, they saw clear differences between the masters and disasters. The disasters looked calm during the interviews, but their physiology, measured by the electrodes, told a different story. Their heart rates were quick, their sweat glands were active, and their blood flow was fast. Following thousands of couples longitudinally, Gottman found that the more physiologically active the couples were in the lab, the quicker their relationships deteriorated over time.

But what does physiology have to do with anything? The problem was that the disasters showed all the signs of arousal—of being in fight-or-flight mode—in their relationships. Having a conversation sitting next to their spouse was, to their bodies, like facing off with a saber-toothed tiger. Even when they were talking about pleasant or mundane facets of their relationships, they were prepared to attack and be attacked. This sent their heart rates soaring and made them more aggressive toward each other. For example, each member of a couple could be talking about how their days had gone, and a highly aroused husband might say to his wife, “Why don’t you start talking about your day. It won’t take you very long.”

The masters, by contrast, showed low physiological arousal. They felt calm and connected together, which translated into warm and affectionate behavior, even when they fought. It’s not that the masters had, by default, a better physiological makeup than the disasters; it’s that masters had created a climate of trust and intimacy that made both of them more emotionally and thus physically comfortable.