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One of Democrats' favorite policy proposals is for more preschool, either universal or for disadvantaged children. It polls well: Everybody likes the idea of helping little kids, especially those who start off with the odds against them. The problem is that it just doesn't do much good, as Bloomberg News's Megan McArdle argues convincingly.

She cites a recent study in Tennessee which showed that the initial positive effects of preschool wore off within about a year, and that by third grade preschool kids were actually a bit worse off than a control group which did not attend preschool.

As McArdle notes, it's difficult to conduct a randomized control trial of preschool, and like her I'm skeptical of the finding of the Tennessee study which showed (small) negative effects. But the fact that preschool advocates frequently find themselves citing a 50-year-old study from Ypsilanti, Michigan, of a program put on by dedicated researchers and highly trained staff is proof of the weakness of the case for preschool. As McArdle points out, it's very hard — probably impossible — to scale such a program, that is, to make it available to millions of children.

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