Let's turn now to a less contentious topic: gasoline lead and violent crime. As you know by now (yes, there will be a quiz), there are three fundamental types of evidence in favor of the lead-crime hypothesis:
- Retrospective statistical studies. These are called "ecological studies," and they rely on comparing past rates of lead poisoning in children to rates of violent crime two decades later. The more of these studies the better, but by themselves they're rarely convincing because it's very hard to disentangle all the possible causes of crime in an entire population based only on past statistics.
- Prospective studies. These are studies that pick a sample of babies and then follow them in real-time for 20 or 30 years. The researchers can measure blood lead levels every few months and then compare that to behavior later in life.
- Brain scan studies. This is just what it sounds like. You put people who have suffered from lead poisoning into an MRI machine and take a look at how lead affects specific parts of the brain.
All three of these suggest that lead poisoning in kids leads to more violent crime later in life. Today brings yet another bit of evidence. It's an ecological study done at Macquarie University in Australia, which makes it especially interesting. Here's the problem: As long as you do studies only of American crime, you can never know for sure if there's some hidden variable that affected all American kids and just happened to spread at about the same rate as lead poisoning. Maybe eating lots of McDonalds hamburgers causes crime rates to soar for some reason. But you'll never know because kids everywhere in the country ate more and more Mickey D's during the 50s and 60s and there's no data to tell you precisely who ate the most.
But if you study other countries, a lot of these confounding variables go away. Not all of them, perhaps, but most. If you think the problem is poverty or social welfare programs or policing styles or the unique American history of racism, then take a look at Canada. Or Italy. Or Australia. They have very different histories of all these things. If violent crime rates still match up with lead poisoning, that's good evidence that lead poisoning is truly the causal agent.
Rick Nevin has already done some of this work, comparing crime rates to lead poisoning in a number of other countries. But it's always better to get a detailed study from a local research team with access to more data. Interestingly, the Macquarie team didn't look at blood lead levels in children. They had direct access to atmospheric levels of lead, which Australia has tracked for decades, and this allowed them to look specifically at the effect of just gasoline lead. They were also able to make comparisons at the neighborhood, state, and national levels. Here are their conclusions:
Direct effects between air lead and assault rates across all suburbs were examined....For every additional μg/m3 of lead in air, assault rates 21 years later increased by 163 per 100,000 population. Lead in air was the strongest predictor in the model, accounting for 29.8 % of the variance in assault rates 21 years later.
....At the state level, strong positive correlations between petrol lead emissions and death by assault rates were found only for the states with the largest populations, highest population densities and greatest petrol lead emissions, namely, [New South Wales] and Victoria....Lead emissions in NSW accounted for 34.6 % of the variance in death by assault rates 18 years later....In Victoria...lead emissions accounted for 32.6 % of the variance in death by assault rates 18 years later.
....At a national level, the data also demonstrated a positive correlation between lead emissions and death by assault rates, but the association was weak. National lead emissions accounted for only 7 % of the variance in national death by assault rates 18 years later, as the health and behavioral effects of lead emissions are dissipated at larger geographic scales.
The chart on the right is typical of the suburban neighborhood-level data. Boolaroo is about an hour north of Sydney and has a history of high lead levels thanks to a local smelter that operated for decades. As you can see, the correlation between atmospheric lead and violent crime (lagged by 21 years) is very strong. In other suburbs, where lead was produced by gasoline and overall levels were lower, the correlation isn't quite as visually convincing, but it's still quite strong.
Australia banned leaded gasoline in 1985, and crime rates began dropping in the mid-2000s. That's quite different from the US, where leaded gasoline was phased out starting in the mid-70s and crime rates started dropping in the early 90s. This suggests that it's not just something about the specific time period from 1991-2010 that's responsible for our crime decline. Australia also has a very different racial history, a very different policing culture, a very different drug/gang culture, and a very different social welfare state. None of these are likely to be hidden variables that are coincidentally identical in both countries.
So we have a different country; a different time frame; and a different criminal justice culture. Yet crime still follows the trajectory of lead emissions, which appears to explain about a third of the change in assault rates. That's a lot. It's one more bit of data to add to the already persuasive pile of evidence in favor of the lead-crime theory.