You can't do it with humans, for ethical reasons, but why not take some other social animal, say mice, put one population in a box with zero pollution, and one hooked up to an old car exhaust with leaded gasoline?
I'd wager the ones with the pollution are going to eat each other's faces at a much higher rate than those without.
For giggles and to test some fall of Rome hypotheses, run a 3rd cage where the water comes through lead pipes.
In any case, I agree you can't do this properly with humans. And lead is definitely a factor in many deceases including mental ones so it could well be one of the causes of increased violence.
It's still widely available (and in use) for small piston aircraft as avgas. And it seems like tetraethyl lead is still available for people to mix their own leaded gas to put in very old cars.
I think it would be very hard to isolate these factors into a "clean" study.