A recent article in Mother Jones discusses the potential role of lead in the increase in crime from the 1960s to the early 1990s, and the subsequent steady decline in crime rates since then. I have received numerous questions about this article and this possible connection between lead and crime. It is a well-written article, and an interesting question. Could one toxin really be responsible, among all the other possible causes, of the rise and fall in crime rates in the US?
Before we get to that, it’s interesting that this is not the first time in history this question has come up. There is a theory that the fall of Rome was due, in part, to chronic lead toxicity. The Romans used lead to make their water pipes (the origin of the word “plumber” as the root “plumb” refers to lead). But this was probably not the most significant source of lead for the Roman aristocracy, who also sweetened their wine and some of their food deliberately with lead. Analysis of lead levels in bones of burials from the time do show variable but often elevated lead levels, but the data is too scant to draw any firm conclusions.
They apparently knew of the toxic effects of lead, but thought that this was limited to acute lead toxicity – something that slave lead miners had to worry about, but not citizens. They did not think that low level chronic exposure was a risk, and apparently they were wrong.
However – there are problems with this nice story. Some scholars doubt that lead poisoning was as endemic in Rome as others claim. The evidence is complex. Lead was probably not added deliberately to wine, but wine was occasionally heated in lead containers. Terra cotta pipes were often used instead of lead for carrying water specifically because the risk of too many lead pipes was understood. And contemporary references in medical and veterinary writing make scant mention of lead poisoning, even though the syndrome was well recognized.
Beyond disagreements over the extent of lead poisoning in the Roman Empire, there is the far thornier problem of assigning cause to the fall of Rome. Most scholars criticize any attempt to find “the” cause of such a complex historical event, whether lead or anything else.
We find ourselves in a similar situation today. The Mother Jones articles centers around the rise and fall of crime from the 1960s to early 1990s in the US, and discusses research that argues that this rise and fall in the crime rate closely mirrors (although shifted by 23 years) the rise and fall of the use of leaded gasoline.
The article correctly points out that such ecological data is very difficulty to interpret. There are many potential factors that could correlate with the rise and fall in the crime rate. Confidence increases, however, when the correlation holds up in many possible ways. The article cites research by Rick Nevin and others showing that the correlation with environmental lead levels, mostly from leaded gasoline usage, holds up when compared state by state, city by city, and even by neighborhood. and also in countries other than the US. This makes the correlation much more compelling.
The literature seems to support a real connection between lead and crime. A recent review and study concluded that there is a correlation, and that lead levels in the air are predictive of crime rates, but especially when linked with resource availability. In other words, the effect of lead exposure on crime rate is greater when those individuals live in a neighborhood where they are less likely to be diagnosed and treated for lead poisoning, which makes sense.
There remains the possibility that lead exposure is just a marker for other variables that are truly the cause of crime. In other words – it’s possible that crime really has sociological causes, factors that also predict lead exposure. While this type of connection is probably true to some extent, it does appear that lead is an independent variable correlating with crime. Also a connection has been demonstrated with a prospective study, which partly controls for such confounding factors.
The lead connection is also biologically plausible, as it is well established that lead causes neurological changes including and increase in violence and decreased executive function.
While it is difficult to account for all possible confounding factors, the story that environmental lead exposure increases crime by causing chronic symptoms of violence and attention deficit is plausible and reasonably supported by existing evidence. The real question, it seems to me, is the magnitude of this effect, especially compared to other effects on crime.
The Mother Jones article, by Kevin Drum, cited a figure that 90% of the increase in crime since WWII might be due to lead. He was called out on this figure by blogger Deborah Blum, and Drum later printed a correction. He said the 90% figure is at the upper limit of the range of estimates, and that 50% is likely closer to the truth.
In the review I cited above, reference is made to research showing that “as much as 20%” of crime is “lead related.” One small point – Drum’s now 50% figure, as he points out, is the rise in crime, not the cause of all crime. The 20% figure cited in research is all crime – so these numbers may be compatible. Either way, the 90% figure likely overstates the connection.
Therefore, even accepting the 20% figure, that means 80% of crime has nothing to do (at least directly) with lead, and the sociologists are free to continue to speculate and study about the myriad of social causes of crime.
The connection between chronic lead exposure and neurological effects, including those that plausibly contribute to crime, is both plausible and reasonably supported by existing evidence. The magnitude of this effect is difficult to tease apart from the many variables that can potentially affect the crime rate. If we accept the 20% figure (crime that is lead related), which seems plausible, then this indicates a significant role for lead, but lead is certainly not the only important factor.
Also, because of the nature of this research there remains reasonable doubt about lead’s true role in the crime rate. This doubt, however, is not sufficient to argue that we should not pay attention to lead exposure or even take specific measures to limit it. The research is remarkably consistent in pointing to a real role for lead exposure. Multiple studies have also looked at the potential benefit of further reducing lead exposure (from soil and remaining lead pain, especially in window frames of old buildings). The research I can find all concludes that the benefit of lead reduction measures would be cost effective because of the potential benefits that would result.