There is a particular quality to cognitive loss that makes it hard to detect in yourself. You do not experience the neurons that failed to connect when you were three. You experience only the adult you became, and that adult has no reference point for what the alternative would have looked like. The gap is invisible from the inside.

The Mexico City Prospective Lead Study gave us one way to measure this from the outside. They followed children from birth, measured blood lead, and administered cognitive tests, including the one below. Across the original study and every replication that followed, the finding holds: each microgram per deciliter of childhood blood lead costs roughly half an item completed in ninety seconds on this test.

The average American child in 1970 carried fourteen micrograms per deciliter. Peak urban exposure was higher. The average kid in an inner-city NHANES II sample was twenty-two. The test does not fix any of this. But it shows you, in a number you can hold in your hand, something real about what happened.