When I wrote my big piece last year about the connection between childhood exposure to lead and rates of violent crime later in life, one of the big pushbacks came from folks who are skeptical of econometric studies. Sure, the level of lead exposure over time looks like an inverted U, and so does the national rate of violent crime. But hey: correlation is not causation.
I actually addressed this in my piece—twice, I think—but I always felt like I didn't address it quite clearly enough. The article spent so much time up front explaining the statistical correlations that it made the subsequent points about other evidence seem a bit like hasty bolt-ons, put there mainly to check off a box against possible criticism. That's not how I intended it,1 but that's how it turned out.
For that reason, I'm pleased to recommend Lauren Wolf's "The Crimes Of Lead," in the current issue of Chemical & Engineering News. It doesn't ignore the statistical evidence, but it focuses primarily on the physiological evidence that implicates lead with higher levels of violent crime:
Research has shown that lead exposure does indeed make lab animals—rodents, monkeys, even cats—more prone to aggression. But establishing biological plausibility for the lead-crime argument hasn’t been as clear-cut for molecular-level studies of the brain. Lead wreaks a lot of havoc on the central nervous system. So pinpointing one—or even a few—molecular switches by which the heavy metal turns on aggression has been challenging.
What scientists do know is that element 82 does most of its damage to the brain by mimicking calcium. Inside the brain, calcium runs the show: It triggers nerve firing by helping to release neurotransmitters, and it activates proteins important for brain development, memory formation, and learning. By pushing calcium out of these roles, lead can muck up brain cell communication and growth.
On the cell communication side of things, lead appears to interfere with a bunch of the neurotransmitters and neurotransmitter receptors in our brains. One of the systems that keeps popping up in exposure experiments is the dopamine system. It controls reward and impulse behavior, a big factor in aggression. Another is the glutamate system, responsible in part for learning and memory.
On the brain development side of things, lead interferes with, among other things, the process of synaptic pruning. Nerve cells grow and connect, sometimes forming 40,000 new junctions per second, until a baby reaches about two years of age. After that, the brain begins to prune back the myriad connections, called synapses, to make them more efficient. Lead disrupts this cleanup effort, leaving behind excess, poorly functioning nerve cells.
“If you have a brain that’s miswired, especially in areas involved in what psychologists call the executive functions—judgment, impulse control, anticipation of consequences—of course you might display aggressive behavior,” says Kim N. Dietrich, director of epidemiology and biostatistics at the University of Cincinnati College of Medicine....“Overall, the evidence is sufficient that early exposure to lead triggers a higher risk for engaging in aggressive behavior,” says U of Cincinnati’s Dietrich. “The question now is, what is the lowest level of exposure where we might see this behavior?”
There's more, including a number of items I didn't include in my article. The whole thing is worth a read if you'd like to learn a bit more than my piece covered about the brain science behind lead and crime.
1So why did I write it the way I did? No good reason, really. Partly it's because I told the story chronologically, and the really compelling parts of the brain science story are fairly recent. Partly it's because it just seemed to be easier to explain things doing it in the sequence I did it.