Starting in the 1960s, America saw a huge increase in levels of violent crime that peaked in the early 1990s, then steadily declined, and continues to decline today. All kinds of theories have been promulgated to explain this peak and decline in crime, and plenty of politicians in the 1990s took credit for it. But in what I personally consider to be a tour de force of journalism, Kevin Drum of Mother Jones has summarized all of the available research. All of it points to one simple idea: violent crime rose as a result of lead poisoning because of leaded gasoline. It declined because of lead abatement policies.

There are three basic reasons why this theory should be believed. First, as Drum points out, the numbers correlate almost perfectly. “If you add a lag time of 23 years,” he writes. “Lead emissions from automobiles explain 90 percent of the variation in violent crime in America. Toddlers who ingested high levels of lead in the ’40s and ’50s really were more likely to become violent criminals in the ’60s, ’70s, and ’80s.”

Second, this correlation holds true with no exceptions. Every country studied has shown this same strong correlation between leaded gasoline and violent crime rates. Within the United States, you can see the data at the state level. Where lead concentrations declined quickly, crime declined quickly. Where it declined slowly, crime declined slowly. The data even holds true at the neighborhood level – high lead concentrations correlate so well that you can overlay maps of crime rates over maps of lead concentrations and get an almost perfect fit.

Third, and probably most important, the data goes beyond just these models. As Drum himself points out, “if econometric studies were all there were to the story of lead, you’d be justified in remaining skeptical no matter how good the statistics look.” But the chemistry and neuroscience of lead gives us good reason to believe the connection. Decades of research has shown that lead poisoning causes significant and probably irreversible damage to the brain. Not only does lead degrade cognitive abilities and lower intelligence, it also degrades a person’s ability to make decisions by damaging areas of the brain responsible for “emotional regulation, impulse control, attention, verbal reasoning, and mental flexibility.”

The bottom line, as Drum points out, is that “even moderately high levels of lead exposure are associated with aggressivity, impulsivity, ADHD, and lower IQ. And right there, you’ve practically defined the profile of a violent young offender.”

I’ve barely scratched the surface of Drum’s excellent article, and I’d seriously encourage you to hop over to Mother Jones right now and read the whole thing. I’ve been reading Drum for years and he’s been blogging about this topic for a long time. Indeed, I’ve been convinced of the lead/crime hypothesis for years thanks to Drum’s writing. But this article is a masterpiece. Read it. Talk about it. It’s an important piece of journalism.

In particular, it’s important because this is precisely the kind of problem that people are uncomfortable about believing. It’s hard for us to see the link between cause and effect when there’s a 20+ year gap between one and the other. Additionally, none of us like thinking that our autonomy as human beings can be destroyed by forces beyond our control that we can’t even see.

But such time lags between cause and effect do exist. Invisible molecules like tetraethyl lead can do us great harm. We need to understand this. It’s not enough to know this as an interesting fact. We have to know in our guts that these types of things are possible, because this is far from the only problem like it. And that kind of deep understanding that these problems are possible are what’s necessary to motivate us as people to do something about it