The theory that the fall in crime over the last generation has been due to the phasing out of lead additives in petrol has received another airing recently.
The last time I mentioned the idea on this blog it was to ridicule the idea, but over the last few years it seems to have stood up fairly well- to the point where it is perhaps the biggest underlying global cause of long term fluctuations in crime trends. (When I'm admitting I was partly wrong I tend to waffle).
However there is a danger of neglecting cultural and policy impacts on crime levels in affecting crime levels even if they aren't the only factors. For example there has been a major difference in the homicide rate of the northern and souther United States going back to before the country was actually formed- and long predating the use of lead in petrol. Even today the much vaunted fall in crime experienced by New York cannot really be explained by lead alone, because the rate in New York fell in relative terms compared to other cities (to the point where the much small city of Chicago has more murders in total than New York). So there still is a significant role for effective policing and sentencing in cutting crime.
If the theory continues to hold water then it is a significant victory for environmentalists- at least the saner ones- because it seems that pollution caused side effects far more severe than could have been predicted based upon the best evidence available at the time. The precautionary principle has in this case at least been vindicated.