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.

DAVID BROOKS became a New York Times Op-Ed columnist in September 2003. He has been a senior editor at The Weekly Standard, a contributing editor at Newsweek and the Atlantic Monthly, and he is currently a commentator on “The PBS Newshour.”