Matthew Lieberman, a distinguished social psychologist and neuroscientist, basically won the lottery. This past summer, he was offered three million dollars for an academic position—one million in raw income and two to do lab research. That’s a king’s ransom for a psychology professor. On average, psychology professors make less than six figures and rely on a patchwork of modest grants to sustain their research. All Lieberman had to do was spend four months this year and next year in Moscow, a nice enough city, doing some research—which he would have done anyway at home at UCLA.

But there was a catch. He would have to be away from his wife Naomi and seven-year-old son Ian for those eight months. They could not join him in Moscow. He had a basic trade-off problem, one that kept him up for many nights: Should I take the money and give up those eight months with my family or should I stay home and give up the money and research opportunities? In one form or another, we've all faced this dilemma, if on a more modest scale. Do you work late tonight or join your family for dinner? Do you go to the conference or to your friend’s wedding? Do you prioritize your career or your relationships?

Lieberman’s new book Social: Why Our Brains Are Wired to Connect hits the shelves this month. It’s a book about relationships and why relationships are a central—though increasingly absent—part of a flourishing life. Lieberman draws on psychology and neuroscience research to confirm what Aristotle asserted long ago in his Politics: “Man is by nature a social animal … Anyone who either cannot lead the common life or is so self-sufficient as not to need to, and therefore does not partake of society, is either a beast or a god.”

Just as human beings have a basic need for food and shelter, we also have a basic need to belong to a group and form relationships. The desire to be in a loving relationship, to fit in at school, to join a fraternity or sorority, to avoid rejection and loss, to see your friends do well and be cared for, to share good news with your family, to cheer on your sports team, and to check in on Facebook—these things motivate an incredibly impressive array of our thoughts, actions, and feelings.

Lieberman sees the brain as the center of the social self. Its primary purpose is social thinking. One of the great mysteries of evolutionary science is how and why the human brain got to be so large. Brain size generally increases with body size across the animal kingdom. Elephants have huge brains while mice have tiny ones. But humans are the great exception to this rule. Given the size of our bodies, our brains should be much smaller—but they are by far the largest in the animal kingdom relative to our body size. The question is why.

Scientists have debated this question for a long time, but the research of anthropologist Robin Dunbar is fairly conclusive on this point. Dunbar has found that the strongest predictor of a species’ brain size—specifically, the size of its neocortex, the outermost layer—is the size of its social group. We have big brains in order to socialize. Scientists think the first hominids with brains as large as ours appeared about 600,000-700,000 years ago in Africa. Known as Homo heidelbergensis, they are believed to be the ancestors of Homo sapiens and the Neanderthals. Revealingly, they appear to be the first hominids to have had division of labor (they worked together to hunt), central campsites, and they may have been the first to bury their dead.

One of the most exciting findings to emerge from neuroscience in recent years underlines the brain’s inherently social nature. When neuroscientists monitor what’s going on in someone’s brain, they are typically interested in what happens in it when people are involved in an active task, like doing a math problem or reaching for a ball. But neuroscientists have looked more closely at what the brain does during non-active moments, when we’re chilling out and the brain is at rest. Every time we are not engaged in an active task—like when we take a break between two math problems—the brain falls into a neural configuration called the “default network.” When you have down time, even if it’s just for a second, this brain system comes on automatically.

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.