A few of you asked for follow-ups, given my discussion with Ezra Klein. Here is one paper that makes me skeptical:
That is sometimes called the Bologna preschool paper, and it is by Fort, Ichina, and Zanella. More generally, there is the question of whether a society will produce more top performers with the state as nanny, or with the parents as nanny. Maybe we don't know, but my intuition suggests with the parents. Here are some other reservations, and here are more yet. It is difficult to show lasting benefits from preschool. Here is the most extensive study I have seen, and it shows negative results for preschool. Or consider this RCT. Symbolically I am not crazy about the idea of building systems that imply human life is about "schooling" almost from the get go. I don't think the literature to date is conclusive, but I do think the case for preschool still remains to be made.