Why the causes of poor mental health may share a common root

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These days, we are increasingly aware that mental health problems can be as debilitating as physical ones. Unfortunately, much of what we thought we knew about what causes mental illness has recently turned out to be wrong.

In a way, this shouldn’t be surprising, says Allen Frances at Duke University in North Carolina, lead author of a major US psychiatry textbook, the Diagnostic and Statistical Manual of Mental Disorders IV. “The brain is the most complicated thing in the known universe, and it reveals its secrets very slowly,” he says.

A couple of decades ago, we seemed on the verge of unlocking the secrets of several mental health conditions. The cause was thought to be irregular levels of various brain chemicals, which could be remedied with drugs. Depression, for example, was thought to stem from a lack of the brain signalling chemical serotonin, partly because the most common kind of antidepressants raise levels of this compound. Genetic studies also found that people with depression were more likely to have a gene variant that lowers serotonin. Schizophrenia was thought to be caused by excess activity of circuitry involving a different brain chemical, called dopamine.

But this neat picture turned out to be a mirage. The latest evidence suggests that low serotonin isn’t a cause of depression, and many early genetic studies that implicated single brain chemicals as a cause of specific conditions were carried out using scientific methods now considered to have been flawed.

Mental health conditions do have a strong heritable component. For instance, someone whose identical twin has schizophrenia has about an 80 per cent chance of developing …

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