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 realise the American dream.

Social capital, as he presents it, is a form of wealth that can compound and throw off real dividends when it’s cultivated, or wither and die when it’s drawn down or neglected. Having access to social capital is a privilege. In the business world, social capital is earned over decades, and doled out carefully by those who have it: you can’t just... See more