Here is a quick overview about how the reduction of lead exposure in society could have contributed to the recent decline in crime rates:

One of the most fascinating questions in American sociology, political science, and public policy is the substantial decline in violent crime in America has been falling for two decades after a near-peak in 1991; the homicide rate hit a 50-year low last year despite the recession. There are a lot of interesting theories, none that (as far as I’ve read) is considered dominant. In November, Llewellyn Hinks-Jones wrote a compelling piece for the Atlantic about crime rates and the declining price of cocaine; there’s the evergreen broken-windows theory; Steven Levitt’s abortion theory (PDF); increasing incarceration rates; the destruction of massive public-housing complexes; improved trauma care; and many more. I was reminded of another one when reading up on public housing yesterday, thanks to a brief aside in Beryl Satter’s masterful Family Properties (emphasis mine):

The “peril to life and safety of the inhabitants” of slum buildings was often of a gruesome sort. Residents were injured on poorly lit stairways or ones with broken banisters. They were knocked out by falling plaster. They were scalded by the escaping steam of malfunctioning radiators. They perished in fires in buildings where fire escapes had collapsed from neglect. Their infants’ limbs were gnawed by rats. Each year approximately twenty-five children died from eating lead-filled paint chips. Others survived lead poisoning but were left mentally disabled.

Satter’s numbers come from a Chicago Daily News report from 1963. To put that in context, between 16 and 46 young Chicagoans died from accidents each year between 2002 and 2006, the leading cause of death in the 1-14 age group. In a 1962 Trib report, a board of health poison control pilot study found 35 deaths from 1959 to 1961: “Most of the victims were from 1 to 5 years old and came from rundown slum area buildings….” 465 cases were treated at County Hospital in those two years, and another 65 suffered severe brain damage…

That’s correlation, but what about cause? The great young science writer Jonah Lehrer explains the believed chain of causation from lead poisoning to violence (emphasis mine):

Here, for instance, is a recent PLOS study from the Cincinnati Lead Study, in which the blood lead level of babies born in poor areas of Cincinnati were repeatedly measured between 1979 and 1984. Twenty years later, the researchers tracked down these subjects and put them in MRI machines, allowing them to measure the brain volume of participants. The researchers found that exposure to lead as a child was linked with a significant loss of brain volume in adulthood, particularly in men. Furthermore, there was a “dose-response” effect, in which the greatest brain volume loss was seen in participants with the greatest lead exposure. What’s especially tragic is that the loss of volume was concentrated in the prefrontal cortex, a part of the brain closely associated with executive function and impulse control.

Several things come to mind:

1. Perhaps we simply need more scientific studies in order to sort this out yet a one cause answer to the drop in crime may be really difficult to find. Human behavior within a social context is complex.

2. I wonder if this hints at an already existing and likely to grow set of studies that discuss how everyday substances negatively affect us.

3. The person who is able to conclusively/definitively find the causes in the drop in crime will be either heralded, attacked from all sides, and perhaps both.