BBC News asks a rather interesting question based on a curious correlation. Worldwide crime rates dropped, on average, at about the same time we stopped putting lead in gasoline:
For most of the 20th Century crime rose and rose and rose. Every time a new home secretary took office in the UK – or their equivalents in justice and interior ministries elsewhere – officials would show them graphs and mumble apologetically that there was nothing they could do to stop crime rising.
Then, about 20 years ago, the trend reversed – and all the broad measures of key crimes have been falling ever since.
Studies have shown that exposure to lead during pregnancy reduces the head circumference of infants. In children and adults, it causes headaches, inhibits IQ and can lead to aggressive or dysfunctional behaviour.
If you want to understand the causes of crime – and be tough on them – you need to start with lead, says Dr Bernard Gesch, a physiologist at Oxford University who has studied the effect of diet and other environmental factors on criminals.
In the early 1990s, economist and housing consultant Rick Nevin was pondering whether it would be worth the US spending vast sums cleaning leaded paint out of old housing in American cities. By then, everyone knew that lead did bad stuff to the brain and took years to leave the body.
And that got Nevin thinking. Would the accumulation of lead over time play a role in behaviour that would ultimately become criminal? Nevin calculated the rise and fall of the presence of lead from petrol and he compared that curve to the modern history of violent crime. What he came up with was rather startling.
When the amount of lead in the environment increased, Nevin showed a corresponding rise in violent crime two decades later. And when the amount of lead in the environment fell, violent crime also tracked down – again about 20 years later.
Fourteen years ago, Prof Jessica Wolpaw-Reyes, an economist at Amherst College Massachusetts, was pregnant and doing what many expectant mothers do – learning about the risks to her unborn child’s health. She started to read up on lead in the environment and, like Nevin before her, began pondering its link to crime.
“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.
So why isn’t this theory universally accepted?
Well, it remains a theory because nobody could ever deliberately poison thousands of children to see whether they became criminals later in life.