The more lead that people are exposed to in childhood or in the uterus, the more likely they are to engage in criminal behaviour as teenagers or adults, according to a review of 17 studies.
“The evidence shows an excess risk for criminal behaviour years later,” says Maria Jose Talayero at the George Washington University in Washington DC.
Lead exposure has fallen in many countries, mainly due to the removal of lead additives from petrol (gasoline). However, there is no safe level – any amount of exposure is thought to be harmful.
It is estimated that 1 in 3 children globally have blood lead levels above 5 micrograms per decilitre, which can result in decreased intelligence, behavioural difficulties and learning problems. The effects are irreversible.
In Mexico, for instance, the main source of exposure is the continued addition of lead to the glazes on pottery, says Talayero. Other sources of exposure include lead piping, electronic waste and eating birds shot with lead pellets.
The decline in lead exposure in many countries correlates with a fall in crime levels, leading to suggestions that exposure to the metal increases criminal behaviour. But establishing this requires “bottom-up” studies that directly measure individuals’ lead exposure and look for links with criminal behaviour.
Talayero and her colleagues have assessed 17 studies of this kind done around the world, including in the US, Scotland, Brazil, South Africa and New Zealand. The studies varied widely in their methods and findings.
A few found no links between lead exposure and delinquency. One found a link between exposure and antisocial behaviour, but not arrests. But most did find links between exposure to lead and later arrests or delinquent or aggressive behaviour.
Overall, this shows there is an association, but Talayero says more studies are still needed to establish causality. “It’s really hard to prove,” she says. “There are so many things involved in criminal behaviour, it’s a really complex concept with many additional factors involved.”