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The link between smoking cigarettes and mental illness is well-established, but scientists have long pondered which causes which.

Now, using health data from more than 350,000 people, geneticists out of Denmark's Aarhus University have found significant evidence that smoking cigarettes can actually lead to mental disorders — a longstanding point of contention among researchers who have struggled to determine whether smoking makes people mentally ill or whether people smoke because they're mentally ill.

Using the gigantic UK Biobank, which contains genetic data on more than half a million people, researchers led by Aarhus' Doug Speed tracked the healthcare outcomes of people who began smoking in their late teens and found that they often weren't admitted to hospitals for mental distress until they were at least in their 30s.

On its face, that's strong evidence for the hypothesis that it's smoking leading to conditions ranging from depression to bipolar disorder, not the other way around.

"The numbers speak for themselves," Speed said in a press release. "Smoking does cause mental illness."

But that's not the whole story, he cautioned.

Most of the smokers carried genetic markers known as "smoking-related genes," which could determine who may be predisposed towards smoking — and who, in turn, may be susceptible to developing mental illness down the line.

"The people in the data set who carried the smoking-related genes but did not smoke were less likely to develop mental disorders compared to those who carried the genes and smoked," Speed explained. "Because the genetic variants also seem to be linked with the risk of mental illness, this used to be a bit blurry. But in this study, we demonstrate that it’s probable that the risk of starting to smoke causes the risk of developing mental disorders to increase due to the 'smoking-related genes.'"

As Speed and colleagues wrote in a recent paper in the journal Acta Psychiatrica Scandinavia noted, there's still way more study to be done to figure out how, exactly, this seeming correlation works.

"We still need to find the biological mechanism that causes smoking to induce mental disorders," Speed said in the Aarhus press release. "One theory is that nicotine inhibits absorption of the neurotransmitter serotonin in the brain, and we know that people with depression don't produce enough serotonin."

Another theory: that "smoking causes inflammation in the brain, which in the long term can damage parts of the brain and lead to various mental disorders."

"But as I said," the researcher equivocated, "we don't know for sure as yet."

More on mental health: Scientists Shocked That Middle School Is Horrible If You’re Ugly and Bad at Sports

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