The Leading Cause of Violent Crime?What happens when you want to account for a global, decades-long, society-wide trend of change of behaviour, but against your best efforts you fail to find an explanation? Well, eventually you will give up and look for causes where you'd least expect to find them. And once you find causes, you will have two options: amend your theory of behaviour change, or abandon it.
Something along these lines happened in the 90's USA, when researchers tried to explain the plunge in violent crime rates compared to previous decades. A paper published in 2000 found that if a 23 year delay is introduced, there is as astounding correlation between lead emissions from automobiles and the variation in violent crime rates in the USA. The paper reports that "toddlers who ingested high levels of lead in the 40's and 50's were more likely to become violent criminals in the 60's and 70's": tetraethyl lead levels in the bloodstream explains crime rates better than any other measure of societal change.
|Correlaton of lead levels and violent crime - from Nevin, 2007|
As the article linked below puts it, "more prisons might help control crime, more cops might help, and better policing might help too (...) but the evidence is thin for all these as the main cause". And it is not just New York City where crime has dropped a shocking 75% from peak levels in early 1990's. Researcher Rick Nevin has found that lead and crime data from Australia, Canada, Great Britain, Finland, France, Italy, New Zealand and (West) Germany show similar correlations too. He also notes that no other theory can accommodate the data as well as the lead-hypothesis. Exposure to lead in childhood is linked to loss of grey matter and a degradation of the myelin sheath of neurons, leading to a drop in overall IQ besides adverse effects on emotional and impulse control.
So, as a take-away message: social science and theories of behaviour change can benefit from some thinking-outside-the-box. In this case the box turned out to be the usual framework of cognitive psychology, with an explanation to be found in the realm of the biochemistry of the brain.
For more information see the article and paper below: