What is it about friendships that is so powerful?

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If you had asked me a few years ago what we should focus on to expand economic opportunity in America, I would have trotted out the usual suspects: early childhood education, improved schools, stable families, neighborhood jobs. Friendships would not have been on the top of my list.

But as you may have seen, a giant new study led by Raj Chetty of Harvard and three others found that poor children who grew up in places where people have more friendships that cut across class lines earn a lot more as adults than children who don't. One of the most powerful predictors of whether you rise out of poverty is how many of the people you know are well off.

The size of the effect is astounding. Cross-class friendships are a better predictor of upward mobility than school quality, job availability, community cohesion or family structure. If these results are true, then we have largely ignored a powerful way to help people realize the American dream.

When I spoke with Chetty last week about the study, I asked him: What is it exactly about these friendships that is so powerful?

He said the data doesn't enable us to answer that question. But we can easily speculate that some of it must be informational. Kids whose parents have already been to college can tell their poorer friends how to play the college admissions game, where to sign up for the SATs and so on. A lot of it, too, must be connections. Affluent people can connect you to the right people to help you get a plum job or into the best schools.

I would point to the transformational power of friendship itself. That's because your friends are not just by your side, they get inside you. If you want to help people change, help them change their friendships.

We already know from the work by Yale's Nicholas Christakis and others that behavior happens in friend networks. If people in your friend network quit smoking, then you're more likely to quit smoking. If your friend gains weight, you are more likely to gain weight. Heck, if one of your friend's friends — who lives far away and whom you have never met — gains weight, then you're more likely to gain weight, too.

Our friends shape what we see as normal. If our friends decide that being 15 pounds heavier is normal and acceptable, then we'll probably regard being 15 pounds heavier as normal, too.

This is the key point. Your friends strongly influence how you perceive reality. First, they strongly influence how you see yourself. It's very hard to measure your own worth, your own competence, unless people you admire and respect see you as worthy, see you as competent. Plus, if your friends say, "We're all smart, talented people," you'll begin to see yourself that way, too.

Second, your friends shape how you see the world. A few decades ago, a theorist named James J. Gibson pioneered the theory of "affordances." The basic idea is that what you see in a situation is shaped by what you are capable of doing in a situation. Dennis Proffitt of the University of Virginia has demonstrated this theory in a bunch of ways: People who are less physically fit perceive hills to be steeper than people who are fit, because they find it harder to walk up them. People carrying heavy backpacks perceive steeper hills than people without them.

Third, our friends alter our desires. Desire is notoriously mimetic. We want what other people want and tell us is worth wanting. If you grow up around friends who naturally aspire to be doctors and accountants and engineers, you are probably going to aspire to such things, too.

Entering into a friendship can be a life-altering act, and entering into a friendship with someone different from yourself can be life-transforming. Philosopher Alexander Nehamas argues that when we enter into a friendship, we're surrendering our future selves to that relationship, in part because the friend may call forth parts of ourselves that don't yet exist.

Writing in Comment magazine, David Henreckson observes that when you venture into a new friendship you may wind up taking on new interests, new pursuits and even new enemies. It's daunting: "If in the early days of a relationship we knew all the ways that a particularly intimate friendship would change us, how it might transfigure some of our core values, we would be excused for being a little bit reluctant to jump in."

But the upside is that friends see us and believe in us in ways we'd like to see and believe in ourselves. Friends provide the comradeship that is the crown of a good life.

It's interesting to me that one of the most powerful tools for improving a person's life prospects is something that is not intentionally about improving life prospects. We don't enter friendships with the goal of getting more successful (at least if it's a true friendship). We don't enter into friendships to improve each other. We enter friendships because we delight in each other's company. We enjoy the same activities and interests.

Camp wasn't the rich kids tutoring the less-rich kids on how to live. Camp was a place where we lived, for a month or two each summer, intensely intermingled lives. We learned about one another's worlds and created the joint world out of our own friendships. We learned a capacity that I wouldn't have been able to name until decades later — social range.

My camp friends have gone off to become teachers, electricians, homebuilders, social workers, doctors, a fireman. I would say they are extremely comfortable in social settings different from their own. I would say they have realized how fun it is to resist the natural temptation to hang out with people like themselves. All of us in our band have enjoyed the pleasure of unexpected friendship, something our segmenting society makes it harder and harder to create.