Crime in the United States has been on the decline for more than twenty years.  According to the FBI’s crime statistics, violent crime in 2010 (the latest year for which statistics are available) was at its lowest level since 1972, and has been declining every year since 1991.  Property crime in 2010 was at its lowest level since 1967, and has also been declining since 1991.  There have been a number of attempts to explain this decrease, such as the “broken windows” theory of policing, which says that if law enforcement focuses on keeping general order and policing smaller crimes, the rate of more serious crime goes down.  But in recent years several studies have indicated that there may be another culprit for the drastic increase in all types of crime that occurred from the late 1960′s through the 1980′s–lead emissions.

Papers by Paul Stretesky/Michael Lynch (2004), Jessica Reyes (2007), and Rick Nevin (2000 & 2007), all examined a potential link between childhood lead exposure and the later commission of crime and all three reached the same conclusion:  there is a strong correlation between environmental lead levels and crime.  These papers had largely been ignored by the media until recent stories from Mother Jones and The Young Turks brought the work of Rick Nevin to the public’s attention.

The Mother Jones story, entitled “America’s Real Criminal Element: Lead” written by Kevin Drum for the January/February 2013 issue of the magazine, describes how former New York City mayor Rudy Giuliani and former police commissioner William Bratton were quite happy to attribute the city’s drastic drop in crime to their application of the broken windows theory.  Political scientist John DiIulio cautioned that the drop in crime would not last; that the children being born to baby boomers would produce a “demographic bulge” of young males, which he referred to as “juvenile super-predators.”  However, even though the predicted bulge arrived, crime continued to drop.  By 2010, crime in NYC had fallen to its lowest level in decades, showing a 75% drop from its peak in the early 90′s.

While most of the media was willing to buy into the belief that more effective policing was the main reason for the drop in crime, some researchers weren’t so sure.  For example, crime levels in NYC had begun to decline four years before Giuliani and Bratton came into office.  Also unexplained was the fact that crime wasn’t just dropping in New York, but cities across the country were reporting dramatic decreases in their crime rates, and all began at about the same time in the early 1990′s.

Rick Nevin was working as a consultant on lead paint removal in old houses when a client observed that a study had identified a potential link between lead exposure in childhood and later criminal activity.  This led Nevin to realize that the biggest source of lead exposure in children was not leaded paint, but emissions from leaded gasoline.

Nevin quickly discovered that the curves representing lead emissions and crime rates did not match up.  However, by shifting the crime rate curve by 23 years, the results are impressive:  the curves are almost identical.  This said to him that as more children were exposed to lead during childhood, the result would be an increase in crimes committed by those children as they reached their late teen and early adult years.  Critics of Nevin’s studies have pointed out that many things move in cycles, and that it is somewhat disingenuous for him to shift the crime graph until it matched the lead emissions curve.  These critics might have a point if Nevin had found it necessary to shift the crime graph some forty years or more to create a matching curve.  But the bulk of crime is committed by young adult males, and the time shift of 23 years that was necessary to make the curves match reinforces rather than detracts from his argument.

Other studies have backed up Nevin’s hypothesis.  In 2012 Howard W. Mielke and Sammy Zahran looked at lead emissions and crime on a city level.  They found that in areas where lead exposure was higher, crime was higher.  Where there was less lead, there was less crime.  As you can see in the chart below, their findings for various cities, in this case New Orleans, matched Nevin’s national data. (The curves have not been time shifted in this graph.)

These studies are all examples of “correlational research.”  This type of research is often confused with “causal-comparative research.”  The main difference between the two is that a correlational study suggests that a relationship exists between two things, but it does not address the nature of the relationship, whereas a causal-comparative study says that one thing definitely causes (or does not cause) another.  Kevin Drum posted an update to his Mother Jones article on January 28 with a link to a new paper by Rick Nevin, called “Lead and Crime: Why This Correlation Does Mean Causation.”  In this Nevin makes a case for why his statistical research (as well as that of others) should be considered as strong evidence for the link between lead and crime.

Correlational studies, especially ones which are based on statistical analysis such as Nevin’s is, have to be examined carefully for validity, and they often provide little or no evidence of causation.  However, there are no other explanations currently being offered for the drastic decline in crime rates over the past twenty years that hold up to scrutiny as well as does Nevin’s hypothesis.  More study is definitely needed, but Rick Nevin has presented a compelling argument that says that our attempts to remove as much lead as possible from the environment have been worth the cost, and may already be paying dividends to society.

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