This article contains a variety of citations about screen time[1]. And here's some stuff about the benefits of reading[2], from the reading-to angle. There's tons of stuff like this if you care to look in to it in more detail.
Nothing in here is definitive. I am not saying that negative effects of screen time are proven beyond the shadow of a doubt. Personally, what I care about is what's most likely to be true, so I can act on that for the sake of my son. I think it is more likely than not that >2 hours of screen time per day is a negative factor. I think it is more likely than not that reading is a positive factor. I'm not going to wait for something to be 99.99% proven before I act on the information.