Does Lead Poisoning Cause Lead Poisoning?

An old theory is getting new new attention courtesy of Mother Jones magazine, that the epidemic of violence in the 1980s was linked to the rise in lead in the environment in the 60's due to leaded gasoline, and that the current ongoing decline in violence is due to the reduction of lead in the environment following the Clean Air Act of 1970.  The heart of the argument can be expressed in two graphs, a plot of violent crime versus lead used in gasoline (lagged by 23 years):

and a plot of intelligence in children versus blood lead content:

Leading to the hypothesis that lead (more specifically the tetraethyl lead used in gasoline) in the 60, absorbed by children, affected their brains, producing both lower intelligence and more violent behavior as young adults.

There is no doubt that lead, and tetraethyl lead, are powerful neurotoxins. However, repeat after me the magic chant that removes the emperor's clothes on many cases of bad science:

"Correlation is not causation!"

However much we want to believe in such a relationship, it remains very difficult to prove.  Take the graph showing violent crime vs lead use.  Literally hundreds if not thousands of industrial compounds have show similar usage trajectories, and if allowed some convenient lag period (why 23 years, are violent people most violent at 23? I would have guessed 19) could be fit to violent crime and show a similar result.  The combined effects of the Clean Air Act of 1970 and the Clean Water Act of 1972 was such that a large number of important pollutant compounds (e.g. organics such as DDT, PCB and dioxin and trace metals including Hg and Cd) show similar temporal patterns, and if you allow yourself to lag the data by similar suitable amounts, would produce similar graphs.

As for the lead in children graph, the analysis of lead in environmental media (tissue, water and sediments) underwent a revolution between 1971 and 1991.  I know; I was there doing lead analysis in the the environment for most of that time.  There was an enormous volume of erroneous trace element analyses made in the early days; it wasn't until the early and mid 80's that the role of clean techniques, and newer analytical methods made lead analysis reliable (mostly).

So, did lead in gasoline cause the '80s violence epidemic, and the removal of lead from gasoline stop it 23 years later?  Maybe, but the actual causal links are lacking, and we may never know.