Why has the crime rate gone down?

By the end of the 1990s, the crime rate in the U.S. had plummeted by more than 40%. There were a lot of hypotheses as to why this occurred, ranging from increased patrolling by police officers, to beautification of neighborhoods, to the legalization of abortion (the theory being that there were fewer unwanted, and hence poorly cared for, children being born). However, it’s likely that none of those factors holds the lion’s share of responsibility for the decline in crime. No, that distinction almost certainly goes to the banning of lead in gasoline and paint in the ‘70s. Lauren Wolf, associate editor of Chemical and Engineering News gives us the evidence.

The Environmental Protection Agency (EPA) currently states that the highest acceptable level of lead in a child’s bloodstream is 5 micrograms per deciliter (µg/dL). Above that limit, people begin to show impaired cognition. Before the ban on leaded gas and paint, the average U.S. resident had blood lead levels of 16 µg/dL. Yes, that’s over three times the safe limit. By 1991, most people had only 3 µg/dL of lead in their blood.

As you can see from the following chart, the crime rate tracks quite closely with the amount of lead exposure.

Economists hypothesize that regulation of leaded gasoline and lead paint in the 1970s caused crime rates to drop in the U.S. about 20 years later.CPSC = Consumer Product Safety Commission.
SOURCES: Rick Nevin, FBI Uniform Crime Reporting Statistics

Remember, there were lead-using cars on the road for many years after leaded gasoline was banned, and lots of people still live in older houses that contain lead paint today. This means that some regions of the country had high amounts of lead exposure for much longer than other areas. Sure enough those areas had higher crime rates. In fact, you can correlate lead exposure with crime not just in the U.S. as a whole, but by neighborhood.

Don't think this is where the evidence for the lead/violence connection ends though. There is good reason to accept that relationship. Studies confirm that lab animals that are exposed to higher amounts of lead do in fact become more aggressive. 

So how exactly does lead wreak all this havoc?

In the brain, lead interferes with communication between brain cells. In particular, lead can mess with the dopamine system (responsible for reward and impulse behavior) and the glutamate system (which is involved in learning and memory). So, too much lead gives you someone with diminished impulse control and limited ability to learn from mistakes. If ever I've heard a recipe for baking up criminals, this is it.

Lead is especially problematic for young children because it also interferes with brain development. Children with higher blood lead levels have less gray matter and lower IQs.

I’d like to point out that the acceptable level of lead has been dropping steadily. We may very well find out that 5 µg/dL is still far too high. If you add the fact that many neighborhoods still contain large amounts of lead dust, it’s clear that we cannot relax our standards or our desire to clean the air of all lead contamination.