Reuters

Let's go ahead and call social mobility in America what it really is: social immobility.

Even in the parts of the country where students have the best shot at moving up the income ladder, the vast majority of them fail to make it past one rung. About 8 percent of kids born in the early 1980s who grew up in families in the poorest fifth managed to reach the top quintile of earners today. For those born smack dab in the middle of America's income distribution, there was just a 20 percent chance of making it to the top.

So for most children, where they're born on the economic ladder determines where they wind up. That's the conclusion of a tremendous social mobility study out today. Its bottom line is pretty simple. Your parents matter. A lot.

First, the income of your parents matters—not just as a strong predictor for your own income (given how weak social mobility is), but also as a nudge for your life path. The kids of rich parents are 80 percent more likely to attend college than those of low-income parents. Teenage daughters of the poorest parents are 37 percent more likely to have a child than girls born in the richest decile.

Second, your parents' marriage (or living arrangement) matters. The single strongest predictor of a child's economic fortunes is the fraction of single parents in the area where she grew up. Children of married parents have a much better shot of getting ahead even if they're in areas where single parents are the norm. "The fraction of children living in single-parent households is the strongest correlate of upward income mobility among all the variables we explored," the researchers said.