The optimal amount of screen time for kids

At the start of the year, Cyril Morong (The Dangerous Economist) had a post titled "Are kids getting "too much" screen time?":

Kids might be spending alot [sic] of time on their phones, computers tablets, etc. watching videos, texting, etc. But is it too much?

A person can do too much (or too little) of anything. The optimal amount is found where marginal cost (MC) equals marginal benefit (MB). It is a good idea to keep doing something if the MB of the next unit is greater than the MC. In fact, you keep doing it right up to where they are equal (there is a graph below to illustrate this).

I want to pick up where Cyril left off, because there's some key points missing from his explanation. Anyway, the relevant diagram is laid out below. Marginal benefit (MB) is the additional benefit of one additional hour of screen time. In the diagram, the marginal benefit of screen time is downward sloping - the more screen time a kid gets, the less additional benefit another hour provides. The first hours of screen time each day are cool and exciting (maybe?), but once you've watched some the cool factor starts to wear off. Even kids get satiated (they call it 'bored'). Marginal cost (MC) is the additional cost of one additional hour of screen time. The marginal cost of screen time is upward sloping - the more screen time the kid engages in, the higher the opportunity costs (they have to give up more valuable alternative activities they could be engaging in, like homework, or sleep). The 'optimal quantity' of screen time (from the perspective of the kid) occurs where MB meets MC, at Q* hours of screen time. If the kid watches more than Q* hours (e.g. at Q2), then the extra benefit (MB) is less than the extra cost (MC), making them worse off. If the kid watches less than Q* hours (e.g. at Q1), then the extra benefit (MB) is more than the extra cost (MC), so one more hour of screen time would make them better off.

So there is an optimal amount of screen time for kids and, unless the benefit of the very first hour is less than the cost, the optimal amount of screen time is not zero. But it's probably not all-day-every-day either.