Drop in crime rate 'due to less exposure to lead'
Removing lead from petrol and paints has sparked a dramatic drop in crime levels, suggests new research.
The heavy metal is a neurotoxin that can alter the formation of the brain and reduce grey matter in areas responsible for thinking and planning.
Now a study has found children under six exposed to the metal were much more likely to be suspended from school - or even locked up - as they grew up.
A one unit increase in blood lead levels - measured in millionths of a gramme per each tenth of a litre of blood - raised the probability a child would be suspended from school by up to 9.3 per cent.
And among boys, a one unit increase in blood lead levels raised the probability of incarceration by up to 74 per cent.
But a drop in exposure led to less antisocial behaviour - backing previous claims the banning of a lead is a significant factor behind decreasing crime over the past few decades.
Professor Janet Currie, of Princeton University, New Jersey, said: "Children who have been suspended are ten times more likely to be involved in criminal activity as adults."
Moreover, young people who are incarcerated for even a short period are less likely to graduate from high school and more likely to commit crimes as adults.
She added: "Our results support the hypothesis that reductions in blood lead levels may have been responsible for a significant part of the observed decrease in antisocial behaviour among youths and young adults in recent decades."
For most of the 20th Century crime rose across the UK and other Western countries.
Then, about 20 years ago, the trend reversed - and all the broad measures of key crimes have been falling ever since.
The fall in the use of lead-based paint and leaded petrol has been suspected of being behind the phenomenon but evidence has been scant - until now.
So Prof Currie and colleagues sought to find lead exposure's effect on school disciplinary problems and juvenile incarceration.
The study published on the National Bureau of Economic Research website analysed about 120,000 children born in Rhode Island because of the state's aggressive lead screening programme.
Nearly three-quarters of children have been screened at least once by the time they reach 18 months, far above the national average.
By age six, children in the study had been screened an average of three times.
Lead doesn't stay in children's bloodstreams for long before it's deposited in organs like the brain, and multiple blood screenings increase the chances of detecting exposure.
The researchers examined children born from 1990, which was shortly after leaded petrol was phased out, until 2014.
They accessed Rhode Island Department of Health blood lead level tests for preschool children conducted from 1994 to 2014.
They linked those records to school suspension records beginning in the 2007 to 2008 school year, as well as to juvenile detention records beginning in 2004.
Lead was banned from house paint in 1976, and leaded petrol was phased out between 1979 and 1986.
People who were exposed to lead up to the age of six are more likely to exhibit poor thinking skills and impulse control, to have trouble paying attention, and to behave aggressively.
These traits can lead to antisocial or criminal behaviour in adults.
Studies seeking links between adult crime and early childhood lead exposure have suggested that the drop in lead exposure could explain up to 90 percent of the sharp downward trend in US crime that started in the mid-1990s.
Lead can be absorbed into bones, teeth and blood. It causes kidney damage, inhibits body growth, causes abdominal pain, anaemia and can damage the nervous system.
More than a century ago, a royal commission recommended to British ministers that women shouldn't work in lead-related industry because of damage to their reproductive organs.
By the 1970s, studies showed that children could even be poisoned by chewing fingernails harbouring tiny flecks of old leaded paint from their homes and schools.
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.