Most people assume their worst legacy from university will be an enormous student debt, but a shocking study has found higher education also comes with an increased risk of brain tumors. The result is surprising, since most health risks decrease with education. Nevertheless, it is based on a huge sample size. The effect is small, but easily meets standards of statistical significance. The explanation remains unclear.

Brain tumors are fortunately rare, so to study environmental factors that might increase their frequency requires a large sample size. Dr Amal Khanolkar of University College London certainly got that, using data from the superbly recorded Swedish health system, covering 4.3 million people born between 1911 and 1961 who were still Swedish residents at the end of that time.

Khanolkar published his findings of the observational study in the Journal of Epidemiology and Community Medicine and they make for sobering reading for those of us who think that “over-educated” is a contradiction in terms.

The sample group was tracked from 1993 to 2010, during which 12,836 of them developed one or more brain tumors.

Men with at least three years of university were 19 percent more likely to develop gliomas than their counterparts who left school after the 9-year minimum. For women the effect was even larger; a 23 percent increase in glioma, accompanied by a 16 percent increase in meningioma, a form of brain tumor that is usually not cancerous.

Controlling for factors such as income and marital status explained only a small part of the increase in men and none at all in women.

It is important to note that the increased risk, in absolute terms, is small. With less than half of 1 percent of subjects getting a brain tumor, no level of education represents a high risk. Considering the protective effects of education against conditions such as dementia, attending university is a net win for your health.

Nevertheless, the study raises intriguing questions about causation. Could tumors be a result of overuse of the brain? Is there something to the allegation some “think too much”?

One piece of the puzzle is indicated by the fact that men working as professionals or managers had substantially higher tumor rates – 50 percent more in the case of acoustic neuromas. Women also experienced an increased risk based on occupation, although to a lesser extent.

The variation between effects for different sorts of tumors might suggest results were cherry-picked, but the pattern is consistent for all tumor types, even though some do not reach statistical significance.

Moreover, as an until-recently homogenous nation, ethnic variations are unlikely to explain the observations. The paper also points out: “In principle, all patients diagnosed with a brain tumor should have equal access to the same standards of care as Sweden has a universal tax-funded healthcare system.” This removes the effect of under-diagnosis of uninsured people that would affect a similar American study.

Controlled trials and animal studies are certainly difficult in such a field, so explanations may be hard to come by.