The Lead/Crime Hypothesis

Back in April, the BBC ran an extensive look at the hypothesis that the removal from lead in gasoline during the 1970s led to the huge drop in crime in the United States beginning in 1990. Writer Dominic Casciani profiles economist Jessica Wolpaw-Reyes who published a 2007 paper on the apparent link between the removal of lead and the decline in crime in the U.S.

“Everyone was trying to understand why crime was going down,” she recalls. “So I wanted to test if there was a causal link between lead and violent crime and the way I did that was to look at the removal of leaded petrol from US states in the 1970s, to see if that could be linked to patterns of crime reduction in the 1990s.”

Wolpaw-Reyes gathered lead data from each state, including figures for gasoline sales. She plotted the crime rates in each area and then used common statistical techniques to exclude other factors that could cause crime. Her results backed the lead-crime hypothesis.

“There is a substantial causal relationship,” she says. “I can see it in the state-to-state variations. States that experienced particularly early or particularly sharp declines in lead experienced particularly early or particularly sharp declines in violent crime 20 years later.”

Apparently along with both an increase in crime after the lead began being added to gasoline in the early 20th century, the BBC report note that the UK removed lead from gasoline later than the United States and Canada did, and there is a corresponding later decrease in crime.

However, not everyone is buying it. The BBC quotes criminology professor Roger Matthews as being suspicious in general of biological explanations of crime,

“I don’t see the link,” he says. “If this causes some sort of effect, why should those effects be criminal?”

“The things that push people into crime are very different kinds of phenomena, not in the nature of their brain tissue. The problem about the theory is that a lot of these [researchers] are not remotely interested or cued into the kinds of things in the mainstream.

“There has been a long history of people trying to link biology to crime – that some people have their eyes too close together, or an extra chromosome, or whatever.

“This stuff gets disproved and disproved. But it keeps popping up. It’s like a bad penny.”

It seems like a bit of a stretch to compare widespread exposure to a known neurotoxin to beliefs about people who have particularly narrow gaps between their eyes. As another proponent of the lead-crime link, psychologist Dr. Bernard Gesch, tells the BBC,

One of the problems with criminal justice data is that very few of the factors are good at predicting rates of crime. But lead is.