How do we not know the effects? People have been plunking their kids down in front of the TV for hours a day for fifty years at this point.

edit: https://www.nytimes.com/2018/10/26/style/phones-children-sil...

Part of the reason is that showing a child a TV screen isn't the same as injecting heroin directly into their brainstem; the effects aren't super drastic, and researchers are reluctant to come out and say that X absolutely causes Y, when there's only some correlation, and it's not obvious.

We _know_ that eating a bunch of cyanide will kill you. We don't know exactly what sort of effects watching TV will have on you, because it's not quite so directly causative.

Anyway, we do know and suspect a lot of stuff:

https://www.npr.org/sections/ed/2014/08/28/343735856/kids-an...

https://www.psychologytoday.com/us/blog/behind-online-behavi...

https://www.forbes.com/sites/alicegwalton/2018/04/16/how-too...

Well, the obvious answer is that the effects are small enough to be very difficult to reveal with simple studies