For years the rate of violent crime has fallen in the United States. It's good news, but experts have never been able to explain why crime rates spiked in the 1980s and 1990s but then dramatically dropped in the 2000s. Theories ranging from improved police techniques to the "crack epidemic" to the legalization of abortion have all been proposed by researchers, but none seem to quite fit the facts.

Now, researchers say they may have found the perfect scapegoat for violent crime: leaded gasoline.

A new study has revealed that the rise and fall of leaded gasoline strongly correlates with the pattern of violent crime rates in America.

Past research have linked high lead levels to birth defects, lower intelligence and hearing problems, but now researcher are beginning to uncover evidence that it also causes high levels of aggression.

Tulane University toxicologist Howard W. Mielke found that children exposed to high levels of lead in the 1960s and 1970s resulted in a significant uptick in crime 20 years later.

Mielke found that when the use of leaded gasoline dropped in the 1980s, crime rates also declined at corresponding rates.

Further research found the chilling correlation in countries around the world and in six U.S. cities: Atlanta, Chicago, Indianapolis, Minneapolis, New Orleans, and San Diego.

According to Mielke, every one percent increase in the number of tons of lead released into the atmosphere corresponded with a half percent point increase in the aggravated assault rate 22 years later.

Specifically, the latest findings would mean that each metric ton of lead released in the atmosphere would result in an increase of 1.59 aggravated assaults per 100,000.

Meilke said that the data was able to explain 90 percent of the rise and fall of crime rates in the cities examined in the study.

The association between lead and violence is relatively new. In 1996, researcher from the University of Pittsburgh found that children exposed to high lead levels were significantly more likely to exhibit aggressive behavior compared to those exposed to normal levels.

Another study published in 2002 found that adolescents who had been arrested had on average significantly higher levels of lead in their bones than their peers who had never been arrested.

The latest study published in the journal Environmental International, supports growing research evidence that ties leaded gasoline to crime.

Mother Jones writer Kevin Drum wrote that obviously the millions of children who were exposed to high levels of lead didn't call become criminals, but he notes that those on the margin may have been "pushed over the edge from being merely slow or disruptive to becoming part of a nationwide epidemic of violent crime."

While leaded gasoline was quickly phased out by the 1980s and was banned for use in vehicles in the U.S. in 1996, it is still used in race cars, piston-powered airplanes and some off-road vehicles.

What's more, traces of lead can still be found in U.S. soil and an estimated 16 million U.S. houses is believed to have lead in its midst.

Cleaning up lead pollution would cost billions, but it may save far more money in the future and "could turn out to be the cheapest, most effective crime prevention tool we have," Drum wrote.

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