What’s remarkable about the default network, according to Lieberman’s research, is that it looks almost identical to another brain configuration—the one used for social thinking or “making sense of other people and ourselves,” as he writes: “The default network directs us to think about other people’s minds—their thoughts, feelings, and goals.” Whenever it has a free moment, the human brain has an automatic reflex to go social. Why would the brain, which forms only 2 percent of our body weight but consumes 20 percent of its energy, use its limited resources on social thinking, rather than conserving its energy by relaxing?

“Evolution has made a bet,” Lieberman tells me, “that the best thing for our brain to do in any spare moment is to get ready for what comes next in social terms.”

Evolution only makes bets if there are payoffs—and when it comes to being social, there are many benefits. Having strong social bonds is as good for you as quitting smoking. Connecting with other people, even in the most basic ways, also makes you happier—especially when you know they need your help.

One study of adults found that the brain’s reward center, which turns on when people feel pleasure, was more active when people gave $10 to charity than when they received $10. In another study, comforting someone in distress activated the reward center in a powerful way. Couples were brought into the lab and the girlfriend was placed inside a brain scanner while the boyfriend sat in a chair right next to her. In some cases, the boyfriend would receive a painful electrical shock.

The girlfriend, who knew when her boyfriend was being shocked, was instructed to either hold her boyfriend’s hand or to hold onto a small ball. When the scientists looked at the girlfriend’s brain activity, they found that her reward system was active when she was holding the hand of her boyfriend both when he was being shocked and when he wasn't in pain—but it was most active when she held his hand as he was being shocked. Holding your boyfriend’s hand feels nice, but it’s especially meaningful when you know that he needs your love and affection.

***

When economists put a price tag on our relationships, we get a concrete sense of just how valuable our social connections are—and how devastating it is when they are broken. If you volunteer at least once a week, the increase to your happiness is like moving from a yearly income of $20,000 to $75,000. If you have a friend that you see on most days, it’s like earning $100,000 more each year. Simply seeing your neighbors on a regular basis gets you $60,000 a year more. On the other hand, when you break a critical social tie—here, in the case of getting divorced—it’s like suffering a $90,000 per year decrease in your income.

You don’t have to be a social scientist to know how badly a breakup hurts. One of Lieberman’s most provocative studies, done in collaboration with his wife Naomi Eisenberger, shows that social loss and rejection are more painful than we might realize. The researchers put people in a brain scanner and then had them play an Internet video game called Cyberball where three people toss a ball around to each other. The research subjects were led to believe that the other people in the game were also part of the study when in fact they were just two pre-programmed avatars.