What are the underlying causes of crime?
It is a question that has puzzled criminologists, psychologists, sociologists and criminal justice experts for centuries.
Genetic factors such as race, head shape, jaw structure, size of physical features and so on were once seen as relevant to determining whether a particular individual had a propensity towards crime.
Environmental factors such as geographical location, economic and social status, education level, and upbringing are now seen by some as important in assessing potential criminality.
Political factors such as ‘tough on crime’ policies which lead to the creation of more criminal laws, higher maximum penalties and mandatory sentencing regimes have also been linked to crime and criminality.
But overall crime rates in a whole host of developed countries including Australia, the UK, France, Finland, Italy, Germany, Canada and New Zealand have been decreasing since the early 1990s, despite differing social, economic and political conditions between and within those countries.
While it is difficult to pinpoint the reasons for falling crime rates and an individual’s propensity towards crime, some have offered an unusual explanation for why some people resort to violent criminal conduct: heavy metal absorption.
Before it became apparent that lead was poisonous, it had a multitude of every day uses – from water pipes in Ancient Rome, a face whitening solution for women in the middle ages, and well into the last century lead was found in paint, all kinds of ceramics and plates. Lead cans were even used to package food and soft drinks.
But perhaps most significantly, lead was added to petrol to boost motor engine power in the 1920s, right up until just a couple of decades ago. The lead in fuel is absorbed far more easily through breathing than lead that comes in contact with the skin or even mouth.
Lead absorption is now known to have seriously adverse health effects – especially for children.
Effects of over exposure to lead have been linked to lower IQ levels, hyperactivity, behaviour problems and retarded learning.
In 1999, researcher Nick Nevin wrote a study entitled: How lead exposure relates to temporal changes in IQ, violent crime and unwed pregnancy. He consolidated the results of several US studies measuring the levels of lead present in the blood of children at different periods of time throughout the last century.
He found that lead levels in children began to rise in the 1940s, after it became popular to add lead to petrol. US studies showed that between 1976 and 1991, the levels of lead present in blood in children under six declined by 75%.
The study concluded that there are significant links between petrol lead levels, blood lead levels in very young children, and violent crime.
A different study in 1996 similarly found that boys aged 7 to 10 with comparable IQ levels had higher levels of aggressive and violent behaviour if they had higher levels of lead in their blood.
Nick Nevin predicted that:
“Although crime… rates are obviously affected by a variety of factors, temporal trends in lead exposure appear to be a significant factor associated with subsequent trends in these undesirable social behaviours… if the association between gasoline lead and social behaviour continues into the future, then violent crime… could show dramatic declines over the next five to ten years.”
Since his 1999 study, crime rates have indeed fallen in countries where lead has been removed from fuel; and not just in Australia. Further studies, including one by Nevin himself, appear to confirm the correlation – if not the link – between lower levels of lead exposure and declining crime rates across the UK, US, France, Australia, Finland, Italy, West Germany and New Zealand.
Jessica Wolpaw Reyes from the US National Bureau of Economics came to a similar conclusion. In her 2007 paper, Wolpaw wrote:
“…Changes in childhood lead exposure are responsible for a 56% drop in violent crime in the 1990s… Childhood lead exposure increases the likelihood of behavioral and cognitive traits such as impulsivity, aggressivity, and low IQ that are strongly associated with criminal behavior. Under the 1970 Clean Air Act, lead was almost entirely removed from gasoline between 1975 and 1985.”
The obvious criticism of the findings is that a lot has changed over the past few decades – increased GDP, a general deference to authority which contrasts with the civil rights movements of the 1960s, better health and education etc – and that the drop in crime cannot just be attributed to the removal of lead.
So although evidence of a causal link may be tenuous, the correlation is certainly interesting.