An Astonishing Argument for Why Violent Crime Rates Have Dropped
Police stand next to the bodies of two Senegalese vendors who were killed in a market in Florence. The Italian city was in mourning Wednesday after an Italian far-right author shot dead two Senegalese men and wounded three others before killing himself in a daylight shooting spree that prompted outpourings of grief.
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It seemed, at first, preposterous. The hypothesis was so exotic that I laughed. The rise and fall of violent crime during the second half of the 20th century and first years of the 21st were caused, it proposed, not by changes in policing or imprisonment, single parenthood, recession, crack cocaine or the legalisation of abortion, but mainly by … lead.
I don’t mean bullets. The crime waves that afflicted many parts of the world and then, against all predictions, collapsed, were ascribed, in an article published by Mother Jones last week, to the rise and fall in the use of lead-based paint and leaded petrol(1).
It’s ridiculous – until you see the evidence. Studies between cities, states and nations show that the rise and fall in crime follows, with a roughly 20-year lag, the rise and fall in the exposure of infants to trace quantities of lead(2,3,4). But all that gives us is correlation: an association that could be coincidental. The Mother Jones article, based on several scientific papers, claimed causation.
I began by reading the papers. Do they say what the article claims? They do. Then I looked up the citations: the discussion of those papers in the scientific literature. The three whose citations I checked have been mentioned, between them, 301 times(5). I went through all these papers (except the handful in foreign languages), as well as dozens of others. To my astonishment, I could find just one study attacking the thesis(6), and this was sponsored by the Ethyl Corporation, which happens to have been a major manufacturer of the petrol additive tetraethyl lead. I found many more supporting it. Crazy as this seems, it really does look as if lead poisoning could be the major cause of the rise and fall of violent crime.
The curve is much the same in all the countries these papers have studied. Lead was withdrawn first from paint and then from petrol at different times in different places (beginning in the 1970s in the US in the case of petrol and the 1990s in many parts of Europe), yet, despite these different times and different circumstances, the pattern is the same: violent crime peaks around 20 years after lead pollution peaks(7,8,9). The crime rates in big and small cities in the US, once wildly different, have now converged, also some 20 years after the phase-out(10).
Nothing else seems to explain these trends. The researchers have taken great pains to correct for the obvious complicating variables: social, economic and legal factors. One paper found, after 15 variables had been taken into account, a four-fold increase in homicides in US counties with the highest lead pollution(11). Another discovered that lead levels appeared to explain 90% of the difference in rates of aggravated assault between US cities(12).
A study in Cincinnati finds that young people prosecuted for delinquency are four times more likely than the general population to have high levels of lead in their bones(13). A meta-analysis (a study of studies) of 19 papers found no evidence that other factors could explain the correlation between exposure to lead and conduct problems among young people(14).
Is it really so surprising that a highly potent nerve toxin causes behavioural change? The devastating and permanent impacts of even very low levels of lead on IQ have been known for many decades. Behavioural effects were first documented in 1943: infants who had tragically chewed the leaded paint off the railings of their cots were found, years after they had recovered from acute poisoning, to be highly disposed to aggression and violence(15).