In the U.S., crime has been reduced significantly in the past several decades. Economists and researchers have hypothesized that this decline in crime is somehow linked to the removal of high lead levels from common things like gasoline or paint, and thus claim that lead poisoning at an early age may be cause for criminal activity later on.

Before Rudy Giuliani became the mayor of New York City in the early 1990s, the Big Apple was known for its high crime rates. Since the 1960s, rape, murder, and robbery rates had ballooned significantly. Giuliani came along with a plan to decrease crime with the “broken windows” theory, and it worked; crime dropped significantly. But it wasn’t just in New York City. Amazingly enough, it was happening all across the States during the last decade of the 20th century.

Why did the homicide rate decrease by over 40 percent by the end of the 1990s? Economists and criminologists have struggled to find a clear cut answer, and perhaps there is none. Some believe it’s because of an increase in police officers; others point to the fact that the number of criminals who are behind bars has risen. However, a fraction of researchers believe that the decline of crime is linked to lead.

Research has shown that having high levels of lead in blood could lead to decreased cognitive function, as well as aggressive or violent characteristics. According to the Centers for Disease Control and Prevention (CDC), having 5 micrograms per deciliter of lead in a child’s blood is considered abnormal. During the 1970s, the average U.S. resident had significantly higher levels of lead in their blood than the average person today.

So some economists have hypothesized that as the amount of leaded gasoline and lead paint increased during the 1970s, so did crime rates. Likewise, when lead levels began to drop, so did crime.

Correlation or Causation?

A 2000 report completed by economist and consultant Rick Nevin found that lead emissions from cars explained 90 percent of violent crime in America — when given a lag time of 23 years. Of course, correlation doesn’t equal causation; but other researchers have attempted to dig deeper to find a direct cause between the two. Jessica Wolpaw Reyes, for example, was a Harvard graduate student who became interested in the crime-lead conjecture, and she published a 2007 study that found that the correlation was visible not only in the States as a whole, but also state-by-state. Ultimately, more studies were completed at international, national, state and city levels, and they all pointed to a link between crime and lead poisoning.

According to the Environmental Protection Agency (EPA), lead can be harmful to humans when ingested or inhaled — and is especially dangerous to children under the age of 6. Lead poisoning can impair neurological development in children. It has been used in gasoline, paint, pipes, crystal, and ceramics; meanwhile, the amount of lead in mining and smelting sites tends to be much higher than levels in nature. Lead poisoning can cause symptoms like irritability, fatigue, stomachaches, and insomnia, and over time, if it’s not treated, it can lead to nerve damage, hearing and vision impairment, increased blood pressure, and behavioral problems in children.

"If you have a brain that's miswired, especially in areas involved in what psychologists call the executive functions — judgment, impulse control, anticipation of consequences — of course you might display aggressive behavior," Kim N. Dietrich, director of epidemiology and biostatistics at the University of Cincinnati College of Medicine, told Chemical & Engineering News. Dietrich conducted several studies as well, measuring the lead levels in children's blood and ultimately watching them grow up into their thirties. They found that 250 members of the study were arrested 800 times, and that their average blood-lead levels during childhood correlated with arrest rates.

"Overall, the evidence is sufficient that early exposure to lead triggers a higher risk for engaging in aggressive behavior," Dietrich told Chemical & Engineering News. "The question now is, what is the lowest level of exposure where we might see this behavior?"

Fortunately, lead poisoning is completely preventable. Making sure children don't have access to peeling paint or chewable surfaces that may be covered with lead-based paint is the first step, according to the CDC. Children and pregnant women, meanwhile, should not live in housing that was built before 1978 and is undergoing renovation. The CDC offers other tips, including avoiding eating candies from Mexico as well as removing toys and toy jewelry from children. Cold tap water also reportedly has less lead in it than hot tap water does.