The theory: lead exposure caused crime, and lead abatement efforts reduced it

The case for: This is another newly popular theory, in part because of coverage from Kevin Drum at Mother Jones and others. The lead paint ban, removal of leaded gasoline from America’s filling stations, and lead abatement efforts — which all decreased lead exposure particularly among children born from around 1975 to the late 1980s — correlates strongly to the cohort of children who hit peak criminal age in the 1990s and 2000s. And the data suggests that these specific cohorts were less likely to get arrested for crime. Given that there’s a body of psychological research tying lead exposure to more aggressive behavior, it’s likely reduced lead exposure played a role in reducing arrests and crime.

Unlike some of the other theories, evidence for the lead theory also comes from other developed countries, which also experienced a crime drop in the past few decades. “Put all this together and you have an astonishing body of evidence,” Drum wrote. “We now have studies at the international level, the national level, the state level, the city level, and even the individual level.”

The case against: The lead theory has the same problem as the abortion theory: In the 1990s, even people who had been exposed to lead as children started committing fewer crimes, albeit not to the same extent as younger cohorts. That indicates that while reduced lead exposure may have been a factor, even a big one, it may not explain the entire crime drop.

One study, published in the Journal of Quantitative Criminology, also found that the correlation between lead and crime seems to be based on faulty data from the FBI’s Uniform Crime Report, which collects reports from police departments around the country. The study suggested that when you look at more reliable National Crime Victimization Survey and homicide data, there is little to no correlation between lead exposure and crime. So, at the very least, lead doesn’t seem to explain the full crime rise and drop.

The bottom line: At least some effect. A 2015 Brennan Center analysis, which attempted to quantify the effect of several potential causes of the crime decline, didn’t have enough data to quantify lead’s impact. But past research makes a good case that it had at least some effect, particularly in the 1990s and going through the 2000s.