In the 1986 crime thriller, The Big Easy, two gunshot smugglers are said to be "Suffering from an acute case of lead poisoning." In his intriguing article, "America's Real Criminal Element, Lead" over at Mother Jones, Kevin Drum argues that chronic lead poisoning produced America's mid-20th century crime wave. Drum cites several studies that correlate crime and violence rates strongly with exposures to growing and then falling levels of the gasoline additive tetra-ethyl lead in the environment.
Drum begins by dismissing as inadequate various theories that supposedly account for the steep reduction in the U.S. crime rate over the past two decades, e.g., the broken windows theory of policing, higher incarceration rates, the burning out of the crack epidemic, fewer unwanted babies due to rising abortion rates, and so forth. Instead, Drum focuses on the analyses of econometrician Rick Nevin. In the 1970s, the U.S began phasing out leaded gasoline and it was no longer sold by the mid-1990s. Drum reports:
Intriguingly, violent crime rates followed the same upside-down U pattern [as lead emissions]. The only thing different was the time period: Crime rates rose dramatically in the '60s through the '80s, and then began dropping steadily starting in the early '90s. The two curves looked eerily identical, but were offset by about 20 years.
So Nevin dove in further, digging up detailed data on lead emissions and crime rates to see if the similarity of the curves was as good as it seemed. It turned out to be even better: In a 2000 paper (PDF) he concluded that if you add a lag time of 23 years, lead emissions from automobiles explain 90 percent of the variation in violent crime in America. Toddlers who ingested high levels of lead in the '40s and '50s really were more likely to become violent criminals in the '60s, '70s, and '80s.
Of course, Drum recognizes that correlation is not causation. So he looks at the research of Jessica Wolpaw Reyes who compared the crime rates in states where leaded gasoline was phased out faster and found that their crime rates also fell faster. Is the lead/crime hypothesis biologically plausible? Drum details research that shows that lead exposure causes all kinds of neurological havoc. The result is that ...
...even moderately high levels of lead exposure are associated with aggressivity, impulsivity, ADHD, and lower IQ. And right there, you've practically defined the profile of a violent young offender.
Needless to say, not every child exposed to lead is destined for a life of crime. Everyone over the age of 40 was probably exposed to too much lead during childhood, and most of us suffered nothing more than a few points of IQ loss. But there were plenty of kids already on the margin, and millions of those kids were pushed over the edge from being merely slow or disruptive to becoming part of a nationwide epidemic of violent crime. Once you understand that, it all becomes blindingly obvious. Of course massive lead exposure among children of the postwar era led to larger numbers of violent criminals in the '60s and beyond. And of course when that lead was removed in the '70s and '80s, the children of that generation lost those artificially heightened violent tendencies.
Blindingly obvious? As far as I know there are no national data series (other than crime statistics) related to societal levels of agressivity and impulsivity, but there are data on national trends in average IQs and ADHD. And those data cut against the lead/crime hypothesis. Take ADHD trends; even as blood lead levels have been dropping the diagnosed rate of ADHD has been rising steeply, up 66 percent in just the past 10 years. And despite the rise in ADHD, crime rates are still falling.
In addition, even as exposure to tetra-ethyl lead rose, average American IQ scores have been increasing at the rate of about 3 points per decade for nearly a century, up about 22 points since 1932 [PDF]. This increase is the well-known Flynn Effect, named after the New Zealand researcher, James Flynn, who first identified the steady rise in average IQ scores. Note that average IQ scores have been increasing ever since tetra-ethyl lead was first added to gasoline in the mid-1920s.
The good news is that the percentage of kids diagnosed with learning disabilities has fallen by 19 percent since 2001, although the absolute number of learning disabled students remains about the same. Part of the happy decline can be attributed, as the National Center for Learning Disabilities notes to the...
...Change in federal funding formula (beginning in 2000) no longer rewards high rates of special education identification in local districts combined with stagnant federal funding levels.
Interestingly, in a 2012 working paper Nevin argues that the increase in IQs in the early part of the 20th century resulted from lessened exposure to lead paint and that increases in the average IQ scores slowed down as tetra-ethyl lead exposure from gasoline rose. Perhaps Nevin would argue that the increase in the U.S murder rate from 1.2 per 100,000 in 1900 to 9.7 per 100,000 in 1933 can be attributed to rising lead paint exposure?
Drum is right that exposure to lead increases the chances that a person will suffer the sorts of neurological damage that lowers their intelligence and lower intelligence is well-known to correlate with increased criminality. Reducing such exposures has no doubt contributed to our happily falling crime rates. But it is likely that other factors including more policing, more incarceration, less crack, increased concealed carry, and other such efforts to control crime have contributed as well.