I.

A lot of people pushed back against my post on preschool, so it looks like we need to discuss this in more depth.

A quick refresher: good randomized controlled trials have shown that preschools do not improve test scores in a lasting way. Sometimes test scores go up a little bit, but these effects disappear after a year or two of regular schooling. However, early RCTs of intensive "wrap-around" preschools like the Perry Preschool Program and the Abecedarians found that graduates of those programs went on to have markedly better adult outcomes, including higher school graduation rates, more college attendance, less crime, and better jobs. But these studies were done in the 60s, before people invented being responsible, and had kind of haphazard randomization and followup. They were also small sample sizes, and from programs that were more intense than any of the scaled-up versions that replaced them. Modern scaled-up preschools like Head Start would love to be able to claim their mantle and boast similar results. But the only good RCT of Head Start, the HSIS study, is still in its first few years. It's confirmed that Head Start test score gains fade out. But it hasn't been long enough to study whether there are later effects on life outcomes. We can expect those results in ten years or so. For now, all we have is speculation based on a few quasi-experiments.

Deming 2009 is my favorite of these. He looks at the National Longitudinal Survey of Youth, a big nationwide survey that gets used for a lot of social science research, and picks out children who went to Head Start. These children are mostly disadvantaged because Head Start is aimed at the poor, so it would be unfair to compare them to the average child. He's also too smart to just "control for income", because he knows that's not good enough. Instead, he finds children who went to Head Start but who have siblings who didn't, and uses the sibling as a matched control for the Head Starter.

This ensures the controls will come from the same socioeconomic stratum, but he acknowledges it raises problems of its own. Why would a parent send one child to Head Start but not another? It might be that one child is very stupid and so the parents think they need the extra help preschool can provide; if this were true, it would mean Head Starters are systematically dumber than controls, and would underestimate the effect of Head Start. Or it might be that one child is very smart and the so the parents want to give them education so they can develop their full potential; if this were true, it would mean Head Starters are systematically smarter than controls, and would inflate the effect of Head Start. Or it might be that parents love one of their children more and put more effort into supporting them; if this meant these children got other advantages, it would again inflate the effect of Head Start. Or it might mean that parents send the child they love more to a fancy private preschool, and the child they love less gets stuck in Head Start, ie the government program for the disadvantaged. Or it might be that parents start out poor, send their child to Head Start, and then get richer and send their next child to a fancy private preschool, while that child also benefits from their new wealth in other ways. There are a lot of possible problems here.