By observing these types of interactions, Gottman can predict with up to 94 percent certainty whether couples—straight or gay, rich or poor, childless or not—will be broken up, together and unhappy, or together and happy several years later. Much of it comes down to the spirit couples bring to the relationship. Do they bring kindness and generosity; or contempt, criticism, and hostility?

“There’s a habit of mind that the masters have,” Gottman explained in an interview, “which is this: they are scanning social environment for things they can appreciate and say thank you for. They are building this culture of respect and appreciation very purposefully. Disasters are scanning the social environment for partners’ mistakes.”

“It’s not just scanning environment,” chimed in Julie Gottman. “It’s scanning the partner for what the partner is doing right or scanning him for what he’s doing wrong and criticizing versus respecting him and expressing appreciation.”

Contempt, they have found, is the number one factor that tears couples apart. People who are focused on criticizing their partners miss a whopping 50 percent of positive things their partners are doing and they see negativity when it’s not there. People who give their partner the cold shoulder—deliberately ignoring the partner or responding minimally—damage the relationship by making their partner feel worthless and invisible, as if they’re not there, not valued. And people who treat their partners with contempt and criticize them not only kill the love in the relationship, but they also kill their partner’s ability to fight off viruses and cancers. Being mean is the death knell of relationships.

Kindness, on the other hand, glues couples together. Research independent from theirs has shown that kindness (along with emotional stability) is the most important predictor of satisfaction and stability in a marriage. Kindness makes each partner feel cared for, understood, and validated—feel loved. “My bounty is as boundless as the sea,” says Shakespeare’s Juliet. “My love as deep; the more I give to thee, / The more I have, for both are infinite.” That’s how kindness works too: there’s a great deal of evidence showing the more someone receives or witnesses kindness, the more they will be kind themselves, which leads to upward spirals of love and generosity in a relationship.

There are two ways to think about kindness. You can think about it as a fixed trait: either you have it or you don’t. Or you could think of kindness as a muscle. In some people, that muscle is naturally stronger than in others, but it can grow stronger in everyone with exercise. Masters tend to think about kindness as a muscle. They know that they have to exercise it to keep it in shape. They know, in other words, that a good relationship requires sustained hard work.