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The geneticist Kevin Mitchell has noted how specific mutations can manifest differently in different people: The same mutation can trigger epilepsy in some people while in others it triggers autism, schizophrenia, or nothing at all.

In 2010, the psychiatrist Thomas Insel, then director of NIMH, called for the research community to redefine schizophrenia as “a collection of neurodevelopmental disorders,” not one single disease. The end of schizophrenia as a monolithic diagnosis could mean the beginning of the end of the stigma surrounding the condition. What if schizophrenia wasn’t a disease at all, but a symptom?
“The metaphor I use is that years ago, clinicians used to look at ‘fever’ as one disease,” said John McGrath, an epidemiologist with Australia’s Queensland Centre for Mental Health Research and one of the world’s authorities on quantifying populations of mentally ill people. “Then they split it into different types of fevers. And then they realized it’s just a nonspecific reaction to various illnesses. Psychosis is just what the brain does when it’s not working very well.”

First, he needed more proof. In 1997, Freedman devised an experiment: He gave nicotine to people with schizophrenia, usually many pieces of Nicorette chewing gum, and then measured their brain waves with his double-click test. Sure enough, people with schizophrenia who chewed three pieces of Nicorette passed the test with flying colors. They responded to the first sound and didn’t respond to the second, just like people without schizophrenia. The effects didn’t last after the nicotine wore off, but Freedman still was stunned.