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The difference is that the science will have tightly defined terms for "cognitive development" and "well being" that is different than what a parent is measuring when they're looking at agitation, misbehavior, impulsiveness... etc.

My wife and I have seen the same thing with our child. More agitation, more self-satisfying behavior ("forgetting" to do as told and playing instead), more temper tantrums. Kiddo still gets screen time, but it's limited.

There's a much bigger correlation with bad behavior, mental "fuzziness," and impulsiveness to diet, than to screen time. Too much processed food, food with preservatives, and food with refined wheat flour... yikes. Night and day difference in some cases. (Turns out a lot of kids have a "wheat allergy" but it isn't just that.)