An increase in violent crime in the 1970s and 80s is down to lead in the environment, research has claimed.

A study in the US which compared the level of crime and the earlier amount of lead in the atmosphere – from petrol, paint and other sources – found they appeared to be directly linked.

Researchers discovered that in cities where the amount of lead in the air went up, the crime rate went up around 20 years later.

When the amount of lead in the atmosphere came down, the number of robberies and attacks started to fall after about 20 years.

The authors of the study believe there could be something in lead that makes children who absorb more of it, more violent when they grow up.

The rate at which crime rose and fell was the same in all the six cities studied, regardless of what measures had been taken to prevent robbery and attacks.

The effect has been put down to changes in the amount of lead in the air from vehicles and industry as well as pollutants in the home like paint and water pipes.

Lead in petrol in the US was phased out from the mid-1970s onwards and in paint from the mid-1960s. Violent crime started to fall in the 1990s and has continued to fall since, despite a recession at the end of the last decade.

Other research has found that areas of US cities where lead levels have stayed high have continued to experience more robberies and attacks than other areas.

In Britain, violent crime has also been dropping since the 1990s. Lead in the atmosphere has been steadily decreasing, too, with one study showing it fell 90% between the mid-1970s and 1992. Since then it has continued to fall.

Professor Howard Mielke, of Tulane University, who studied the effect in New Orleans, said: "There is a very strong association between criminal activity and the environment in different parts of the city.

"The amount of lead in the environment ... was particularly strongly related to both learning problems and then violence.

He told BBC Radio 4's Today programme: "We've mapped the city of New Orleans and it's the mapping that has provided us with a tool for going back and looking at different kind of issues in the city.

"The police department is even using the maps as they find them very predictive of where the highest crime rates are being found."