First, there are the health care costs. Twins are more likely to be born prematurely, which can lead to all sorts of expensive medical problems.

Birth weight matters, too: Rosenzweig did a study based on hundreds of female twins in Minnesota that looked at the effect of birth weight on lifetime earnings.

“The birth weights of twins are on average about 28 ounces lower,” he says. “So the earnings result was 16 percent lower, related to the fact that they had lower birth weight.”

That’s right: on average female twins make 16 percent less money over their lifetimes than non-twins — just because they’re born less chubby. And lest you think it’s only the girls who are in trouble, multiple studies have also found low birth weight in boys correlates with less educational success, which also means earning less money.