One extra year of school in 1970s Britain was followed by fewer dementia diagnoses

Dementia research has a long-running puzzle about education. People who stay in school longer tend to be diagnosed with dementia less often, but they also tend to differ in wealth, health and family background. That makes it hard to tell whether school itself does anything.

A study in PNAS used a British law to get closer to an answer. In 1972, the UK raised the minimum school-leaving age from 15 to 16 for anyone born on or after September 1, 1957. A child born in late August of that year could leave at 15; one born days later had to stay another year. The reform raised the share of students staying until at least 16 by 14 percentage points.

People born just before and just after the cutoff should otherwise be very similar, so later differences between them can be tied more convincingly to the extra schooling. This approach, called regression discontinuity, comes closer to testing cause and effect than most dementia studies can. The researchers applied it to UK Biobank participants, identified Alzheimer's disease and related dementias through medical histories, hospital records and death registries, and followed people through age 65.

The extra education lowered the incidence of these dementias. It also weakened the link between genetic predisposition and dementia, which suggests a higher inherited risk does not settle the outcome. The additional year was also linked to higher household income and fewer strokes and heart attacks, though the study doesn't establish which of these, if any, connects schooling to the brain.

A natural experiment is still not a randomized trial. Stopping at 65 is a serious constraint too: dementia is uncommon at that age, so the estimates are imprecise. The open question is whether the gap from one extra teenage year persists, grows or fades into the ages when dementia becomes common.