These kids in the 1970s had to stay in school for another year. Decades later, the results are spectacular. : Science Alert

Much research shows that higher levels of education are linked to a lower risk of Alzheimer’s disease, and staying in school longer is also associated with health and longevity benefits.

But because virtually all evidence comes from observational studies, it’s difficult to know for sure whether education causes the positive health effects we observe, or whether other explanations are driving the changes — such as genetics, childhood circumstances, or socioeconomic factors.

“We have known for a long time that more educated people are less likely to develop dementia, but it is difficult to know whether education itself is responsible,” says Silvia Barcellos, a health economist at the University of Wisconsin-Madison.

However, thanks to a landmark legal reform in the UK in the 1970s, researchers have identified what they believe to be the first evidence of its kind demonstrating a causal link between education and reduced incidence of dementia.

In a new study published in PNASBarcellos and his fellow researchers studied the long-term effects of the Raising of School-Leaving Age Act (ROSLA) of 1972, which increased the minimum age at which students could leave school in England, Scotland and Wales from 15 to 16.

“The reform only affected students born after September 1, 1957, generating a discontinuity in the relationship between education and date of birth,” the researchers explain in their article.

This strict deadline created the conditions for a “natural experiment,” allowing researchers to compare the subsequent health status of students who were required to stay in school until age 16 (the “treated” students, who are now in their late 60s) with that of their peers just before the change took effect, who were allowed to leave school at age 15 (the “control” group).

“This setting provided a unique opportunity to study the causal effects of education on the incidence of Alzheimer’s disease and related dementias (ADRD),” the researchers explain.

“If, as hypothesized, there is a causal relationship between education and SARD, then one might expect to observe a corresponding discontinuity in the relationship between SARD and date of birth on September 1, 1957.

“Because individuals born just before and just after this date would have had a similar incidence of MDA in the absence of the policy, any observed discontinuity could be attributed to the difference in education caused by the policy.”

In another study published earlier this year, researchers drew on a similar type of historical “experiment,” examining the effects of strict sugar rationing on babies in the United Kingdom during World War II.

In this research, scientists used data from the large British Biobank study to see how the health of World War II children developed as they became adults, and how their health differed from that of children of the era who did not have sugar limits.

In their own research, Barcellos’ team also drew information from UK Biobank data, and this appears to have confirmed their hypothesis about education and dementia.

“Our results indicate that education has a negative causal effect on the incidence of MDA,” the researchers write.

“There is a sharp decline in the cumulative incidence of MDA among people born after September 1, 1957.”

In the researchers’ calculations, the effects of staying in school for an extra year reduced the cumulative incidence of MDA in the cohort by about 0.88 percentage points — an effect that the team recognizes as implausibly large, given that the prevalence of MDA in the sample was less than 1 percent overall.

The team suggests that the anomalous result could be due to statistical factors, but while they say they cannot provide an entirely precise estimate of the magnitude of the reduction in dementia cases, they say the evidence supports a causal effect.

“Although our sample is large, ADRD remains a rare outcome in these relatively young cohorts, and discontinuity regression designs require very large samples to obtain precise estimates,” they write.

“We can credibly estimate the direction of the causal effect of education, but not its precise magnitude.”

In addition to measuring the reduced incidence of dementia in children born after September 1, 1957, the researchers also examined the DNA of study participants, sorting them by whether they were at high or low risk for ADRD disease, based on whether or not they had certain genetic variations, including the Alzheimer’s risk gene APOE4.

For children born before the change in school leaving age, individuals at high genetic risk experienced a significantly higher incidence of ADHD, but among those born after the change, the additional year of schooling was associated with a significant reduction in the incidence of dementia in the high-risk group.

“For people who know they are at higher genetic risk for dementia, this is a hopeful finding,” says Leandro Carvalho, study co-author and health economist at the University of Southern California.

“Genes help shape disease risk, but they do not necessarily determine outcome.”

Although we cannot know for sure why this extra year of education brought health benefits, the researchers suggest that it may be due to the socioeconomic benefits produced by additional schooling, which could have improved their educational qualifications, income, and professional status.

Cumulatively over half a century, these benefits could have added up considerably, conferring protection against neurodegeneration, even in some high-risk individuals.

Related: Scientists Discover Blood Signature Linked to 23% Reduced Risk of Death

There is still much we don’t fully understand, and it’s worth noting that the discoveries here come from unique historical circumstances.

But if it happened once, maybe it can happen again.

“Our results suggest that experiences and circumstances that can be changed over the course of life can influence whether hereditary susceptibility ultimately translates into disease,” says Carvalho.

“Understanding which interventions can produce these protective effects is an important direction for future research.”

The results are reported in PNAS.