A few of you asked for follow-ups, given my discussion with Ezra Klein. Here is one paper that makes me skeptical:
Exploiting admission thresholds in a Regression Discontinuity Design, we study the causal effects of daycare at age 0–2 on cognitive and non-cognitive outcomes at age 8–14. One additional month in daycare reduces IQ by 0.5% (4.5% of a standard deviation). Effects for conscientiousness are small and imprecisely estimated. Psychologists suggest that children in daycare experience fewer one-to-one interactions with adults, which should be particularly relevant for girls who are more capable than boys of exploiting cognitive stimuli at an early age. In line with this interpretation, losses for girls are larger and more significant, especially in affluent families.
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