The connection between lead exposure and crime is very robust. When we are talking about reducing spending on the Corrections budget, or reducing local property taxes, higher state spending on lead abatement needs to be part of the conversation. Spending more on lead abatement now will reduce the need to spend more on prisons and policing and the criminal justice system later on.
Correlation or causation? The curves that make the case for legalized abortion being the cause for the drop in crime rates look even more robust a connection.
I think the case for causation is fairly persuasive. Mark Kleiman gets into it even more in When Brute Force Fails.
Or both- one of the cited papers apparently also credits the abortion angle in equal parts. Family planning services and a clean environment. Go figure.