Reuters

I know, I know, you'd rather be born smart and rich (and charming, and with a lustrous head of hair, and a voice like Michael Bolton's). But if you had to choose? Chances are, your answer depends on whether you think the U.S. economy is a meritocracy—that intelligence and ambition are more important to lifelong success than the circumstances of your birth.

A recent Brookings paper gives reasons for optimism. Over the long term, it finds, smart kids earn more than rich kids. But sadly, there's a big catch.

The Brookings paper looked at the relationship between brains, motivation, and economic mobility among a group of youth the government began tracking in 1979. Here's the executive summary: If they were bright and driven, poor kids stood a decent chance of becoming upper-middle-class, or better. Of low-income teens who scored in the top third of test-takers on the Armed Forces Qualification Test (on the far left in green), more than 40 percent made it to the top two income quintiles by adulthood. Meanwhile, dimwitted children of affluence generally fell down the economic ladder. Among high-income teens who scored in the bottom third of AFQT takers (on the far right in orange), more than half ended up in the bottom two income quintiles.

Brains weren't everything, of course. As the researchers put it, "in terms of mobility, it’s better to be smart, motivated, and rich, than smart, motivated, and poor." And, as the authors note, there also seemed to be a "glass floor" that kept a great many wealthy kids with "mediocre skills" from sliding into (relative) poverty.