Lead Poisoning and Intelligence: A Search for Cause and Effect in the Scottish Mental Surveys

In 1932 and again in 1947, the Scottish Council for Research in Education conducted the Scottish Mental Surveys. Testing two cohorts, one in 1932 and another in 1947, researchers set out to measure—using the same validated test each time—the intelligence of every Scottish child 11 years of age. The stated impetus for the Surveys was a fear that average Scottish intelligence was declining. But when investigators compared the results of the 1947 Survey with those from 1932 their predictions were completely upended. Instead of average intelligence declining, it had risen, substantially. The author argues that based on a study of the relevant ecosystems in place in Scotland at the time the increase in intelligence resulted from a decline in lead body burden. There is no evidence that the children were tested for lead. The decline is thought to have closely followed a fall in occupational lead use, a heightened awareness of the dangers of lead-solvency, improvements in lead plumbing in working-class homes, and a national campaign to improve the nutrition of women and children. Evidence shows that milk consumption in Scotland increased sharply, especially among children, beginning in the mid-1930s, just prior to and following the birth of the second cohort. This provided a source of calcium in a diet that had shown signs of deficiency. Evidence also suggests that lead contamination, from lead water pipes and industrial sources, was widely prevalent in Scotland in the early part of the twentieth century.