The introduction of tetraethyl lead fuel to an area was followed by an increase of youth violent crime with a twenty year lag, the reduction of leaded fuel was followed in twenty years by a reduction of violent crime.
This pattern was found in several diffrent times and places where the leaded fuel was introduced and removed on diffrent schedules.
This isn't proof, but it is a strong indicator and plausable so this deserves further investigation.
I think it plausable that less children per family and greater controll of the process of reproduction might improve the standard of living for a lot of people and allow greater recorces to be devoted to each child, but how did the onset of commonly availible birth controll coincide with increasing crime rates?