There's a huge amount of  literature showing that early childhood education yields major dividends. High-quality preschools and quality kindergarten have been shown to have large effects on income years into the future. But a new study suggests that kids have to start out healthy to get the most out of those programs — and that a little parental favoritism doesn't hurt, either.

Anna Aizer of Brown and Flavio Cunha of the University of Pennsylvania studied the initial crop of Head Start students in the late 1960s to see who benefited the most from the program. They found that students who were already doing well — those who had high birth weights, whose mothers didn't smoke during pregnancy, etc. — benefited the most.

There were two reasons for this. One is that the authors found that students who were already healthy got the most out of preschool. They were better able to learn and so saw higher IQ increases as a result of Head Start than did toddlers with low birth weights, smoking mothers, etc.

But they also found that parents gave more attention and expended more effort educating children who were healthy to start out. This was indicated by the fact that bigger families saw a tighter relationship between health at birth and benefits from Head Start, with bigger benefits to initial health. If parents weren't picking favorites, you'd expect the relationship to be the same regardless of family size. But this suggests that bigger families, where parenting resources are scarce, see those resources concentrated on already healthy children.

The study shouldn't change our perception of whether pre-school helps kids. We know it does, and we know that better programs than Head Start do yet more for kids, as Nobel laureate James Heckman has argued. But Aizer and Cunha's findings suggest that pairing prenatal health measures with preschool could make the effects stronger still.