This is a pretty good summary of the research on daycare effectiveness. It is a Much More Than You Wanted To Know-style post from a science blogger criticalscience who has some interesting posts on Medium.
TL;DR:
- Adverse effects on children younger than three years old.
- The effect is worse the younger the children are.
- The effect is stronger for longer hours.
- The effect is
weakerCORRECTION: worse for children from socio-economically better-off families. - Nannies are OK.
This is great, thank you for the link! My 16-month-old niece recently started daycare and that's seemed like a shame to me since she was thriving on tons of one-on-one attention (being with a rotation of loving relatives from 7am to 5pm on weekdays, and parents the rest of the time). I think her parents made the switch because they want her to be socialized but I now see that that's not a thing; maybe I can get them to pull her back out and wait until she's older! :D
I think socialization may play a positive role later on in school, but for toddlers in daycare, the results are indeed very clearly negative.