Environmental pollution makes a big contribution to violent crime and antisocial behaviour, according to a provocative new analysis by an American political scientist. He believes that toxic chemicals, in particular metals in water supplies, can disrupt the neurological control mechanisms that normally inhibit our violent urges. Other experts are intrigued but want to see more evidence.

Conventional theories link crime with social, economic and psychological factors. But Roger Masters of Dartmouth College in Hanover, New Hampshire, says that these factors cannot fully explain why some counties in the US have only 100 violent crimes per 100,000 people each year, while others have over 3,000. Data on environmental pollution can account for a lot of the remaining variation, he claims.

Masters analysed a wide range of statistics including crime figures from the FBI and information on industrial discharges of lead and manganese, both into water and into the atmosphere, compiled by the Environmental Protection Agency.

After controlling for conventional variables such as income and population density, he found that environmental pollution seems to have an independent effect on the rate of violent crimes - defined as homicide, aggravated assault, sexual assault and robbery. Counties with the highest levels of lead and manganese pollution typically have crime rates three times the national average, says Masters. "The presence of pollution is as big a factor as poverty," says Masters, whose analysis will appear as a chapter in the book Environmental Toxicology, to be issued later this year by the publisher Gordon and Breach.

When brain chemistry is altered by exposure to toxic metals, Masters argues, our natural violent urges may no longer be restrained. "It’s the breakdown of the inhibition mechanism that’s the key to violent behaviour," he claims.

Masters points to experiments on cell cultures which have shown that lead partly incapacitates glial cells, which are responsible for "housekeeping" in the brain, mopping up unwanted chemicals ("Brain cells hit the big time", New Scientist, 5 February 1994, p 23). And in people suffering from calcium deficiency, which afflicts some of America’s poorest citizens, manganese inhibits the uptake of the neuro-transmitters serotonin and dopamine in parts of the brain. These chemicals are known to control impulsive behaviour.

Alastair Hay, a chemical pathologist at the University of Leeds, says that Masters’ theory is plausible, but notes that people who live in areas of high toxic discharges do not necessarily absorb more toxins.