Kevin Drum at Mother Jones is convinced the reduction in violent crime in the U.S. over the last few decades can be attributed to the absence of lead in things like gasoline and paint. No one else out there has much to say on the subject, at least that I've seen, and the research seems to be scanty or well hidden. What's the straight dope? —Dean Lawler, Phoenix

I dunno, Dean. When a show like Duck Dynasty is gaining viewers and sponsors, you can't tell me brain damage in this country is on the decline.

The gist is that violent crime in the U.S., which rose sharply after 1960 and plummeted after 1990, tracked closely with per capita use of lead in gasoline, offset by 23 years. In other words, kids whose heads were messed up by high exposure to lead in infancy went on to become violent criminals as adults.

As Drum notes, a slew of theories have been advanced to explain the unexpected drop in crime over the past 20 years, ranging from better police work and more imprisonment to legalized abortion. Being a great believer in proof-by-graph myself, I'll say this—Drum makes a good case that the link between lead and crime warrants further study. To get the ball rolling, here's what I could dig up on Drum's key points:

• Childhood lead exposure significantly lowers IQ, and may lead to ADHD and other disorders. The harmful neurological impact of lead is well established. Although the ADHD link seems tenuous, studies have found IQ reductions of nearly five points by age 12 due to lead exposure.

• Elevated lead levels have been disproportionately found in criminals, especially juvenile offenders. There's abundant evident supporting this claim: A study of 250 Cincinnati children found those with higher average blood lead levels throughout early childhood had a 30 percent greater chance of being charged with violent crime as adults; those with higher lead levels at age six had an almost 50 percent greater chance.

• The correlation between childhood lead exposure and adult violent crime is consistent across jurisdictions at all levels, including cities, counties, states, and nations. One of Drum's primary sources, economist Rick Nevin, collected data from the U.S. and eight other countries and says in every case, allowing for the time lag, environmental lead and violent crime rose and fell at similar rates. Likewise, a study of air lead levels in 2,772 U.S. counties found a strong link between lead exposure and both violent and property crime. Nevin thinks exposure to lead paint in old, dilapidated urban housing has played a greater role in the baseline crime rate (which continues to decline), whereas lead in gasoline was largely responsible for the mid-to-late-20th-century spike in crime.

• Teen pregnancies followed the same up-and-down trajectory as violent crime, except the lag vs. lead exposure was 15 years, not 23. This is another example of proof-by-graph, but no question, the curves are a pretty close match. The difference in lag time is easily explained by the fact that teen mothers are (duh) teenagers, whereas criminals don't reach peak mayhem output till later.

• Lead exposure is a better explanation for the drop in crime than competing theories. For example, Drum notes that though New York mayor Rudy Giuliani and his police commissioner Bill Bratton were happy to take credit for the reduction in crime in their city during the 1990s, the drop had begun before they took office, and crime rates were falling similarly in many other cities too.

Although lead has long since been outlawed, Drum points out that many poor families still live in old homes with lead paint, and argues that a nationwide lead-abatement drive would pay off big. Second, lead paint and leaded gasoline were banned by the Consumer Product Safety Commission and the Environmental Protection Agency respectively, and it'd be nice to know that federal bureaucrats occasionally do some good.