When I read the New York Times writing about a 40 percent fall in Jamaica's murder rate (it's still very high) I naturally wondered about Kevin Drum's lead angle. And he says it fits the pattern: From 1990 to 2000, Jamaica started phasing out leaded gasoline. From 2009 to 2013, the crime rate has fallen forty percent. In other words, Jamaica is likely starting to see the beneficial impact of a youth cohort with lower levels of lead poisoning. And just based on the lead channel alone you'd expect to see meaningful further improvements over the next 5-10 years as the kids who were born after the complete eradication of leaded gasoline grow up.
That's not to ignore the other factors the article points to—particularly the crucial arrest of one particular drug lord—except to say that any kind of change in police tactics or strategies is going to bear more fruit against a general background of higher IQ, improved impulse control, and less violent behavior. And of course if Jamaica is lucky, it'll see some virtuous circles taking hold here. A lower level of violent crime should improve local business conditions, which should give people higher wages and more job opportunities outside of the criminal sector.
Of course to really be sure about this you'd want more detailed information about specific lead levels in Jamaicans' blood. But in light of the evidence from other countries it's fairly persuasive. And of course it suggests that the country has plenty more to gain from cleaning up contaminated soils. More broadly, though there's a lot of lip service paid to the concept of "environmental externalities" people rarely focus on it as a core economic policy issue. And yet in an industrial society where the most basic tasks (turn on the television, drive to the store) often involve directly or indirectly burning unpleasant substances the underregulation of toxins is a crucial problem. Climate change tends to outshine all other environmental worries these days, but the lead-crime link is a powerful reminder that a whole range of issues people care deeply about have significant environmental aspects.