5. Family Structure. Forget race, forget jobs, forget schools, forget churches, forget neighborhoods, and forget the top 1—or maybe 10—percent. Nothing matters more for moving up than who raises you. Or, in econospeak, nothing correlates with upward mobility more than the number of single parents, divorcees, and married couples. The cliché is true: Kids do best in stable, two-parent homes.

It’s not clear what, if any, policy lessons we should take from this truism. As my colleague Jordan Weissmann points out, we don’t really have any idea how to promote marriage. We can try telling people how great it is to get hitched. We can even get rid of the marriage penalties some low-income couples face. But these won’t, and haven’t, been making more people exchange till-death-do-us-parts. And should we even want to? Steve Waldman points out that poor women know better than upper-middle-class people yelling at them to get married whether they should or not. They know whether their boyfriend would make a good husband, a good father, a good teacher. And they know that marriage is important. That they’re not getting married tells us something. Sometimes no match is better than a bad match.