I'm personally convinced that a strong case is made that connects leaded gasoline with high crime rates. Certainly larger police forces and legalized abortion and various public policies, etc., are also in the picture. But can it be said that lead abatement from gasoline, if not the top cause, is nonetheless among the most likely reasons for the reduction in crime rates over the past few decades?
I looked into this a few years ago out of curiosity, most European countries (and Australia, Canada) hover around 190-200 police per 100,000, the US is ~300 which isn't that exceptional given Scotland has more as do some other countries.
I don't think it's necessarily about numbers so much as quality via recruitment, training and policy.