DSM Diagnostic Line Moved Three Times, Disorder Stayed Same

Why haven't natural selection and evolution purged psychiatric disorders from human populations?

Twin studies show that schizophrenia, bipolar disorder, and depression cluster in families. And they also correlate with elevated intelligence and creative thinking. Traits linked to suffering persist, the story goes, because they're bundled with traits that confer advantage.

But this argument rests on an assumption it never actually defends. That assumption is that psychiatric disorder is a stable, biological category with a fixed boundary. Depression exists independent of how we measure it or decide who qualifies for it — or so we act. We pretend we can separate the disorder itself from our threshold judgment about when suffering becomes clinical.

Twin studies don't show that schizophrenia persists alongside intelligence. They show that genetic variants associated with schizophrenia diagnosis also predict higher IQ scores. But diagnostic criteria aren't biological facts. The DSM is a consensus document, revised multiple times, about which constellations of experience warrant clinical attention and treatment. Psychiatry agreed to draw a line somewhere on a continuous spectrum of human variation. That line moved in 1980, again in 1994, again in 2013.

If the same genetic variants increase both psychotic symptoms and cognitive ability, then schizophrenia isn't a thing persisting because of hidden benefits. It's a socially constructed category we've drawn around disability and distress at one end of a distribution that's biologically continuous. The genetic linkage isn't explaining why evolution kept this trait around. It's evidence that the trait itself isn't what we thought it was at all. This reframes every field dealing in categories borrowed from medicine. When you inherit a trait that makes you smarter and more anxious, disorder is what we call the version that brought someone to a clinic, not a property of the trait itself.