Scientists determined the two most important traits in a healthy long-lasting relationship: kindness and generosity.

Only three out of every 10 marriages remain happy, and this research looked at what make the lucky few successful. The Atlantic interviewed psychologist John Gottman and his wife Julia, who have run The Gottman Institute, which works to help couples maintain healthy relationships.

To make their findings the researchers hooked couples up to electrodes and asked them to talk about details of their relationship such as how they met and happy memories they had together. The electrodes measured the subject's blood flow, heart rate, and how heavily they were sweating. The couples were then followed up for a period of six years.

After the follow-up period the couples were split into two major groups: "masters" and "disasters." The masters were still in happy relationships six years later while the disasters had broken up or were having serious relationship problems. They observed the disasters looked calm during interviews, but had elevated heart rates and more active sweat glands than the masters and were constantly in a state of "fight or flight" mode; the masters on the other hand exhibited low psychological arousal.

The researchers also observed 130 newlyweds going about their daily business, and found the happier couples made "bids" for each other's attention throughout the day and showed interest their partner when their own attention was requested. By observing these interactions Gottman says he can predict whether couples will break up with 94 percent accuracy. 

"There's a habit of mind that the masters have," Gottman said, the Atlantic reported. "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."

The findings show the more someone witnesses kindness the more likely they will be to practice it themselves, leading to a loving and generous relationship. 

"Kindness doesn't mean that we don't express our anger," Julie Gottman explained, "but the kindness informs how we choose to express the anger. You can throw spears at your partner. Or you can explain why you're hurt and angry, and that's the kinder path."