But now geneticists and neuroscientists are finding the exact same things. Or close to it. The idea of a continuum represents a major cognitive breakthrough for genetics. Researchers found the first hints that mental illness comes in mild forms when they examined relatives of seriously ill people. And awkward social behavior is more frequent in the biological parents of autistic children.
Schizophrenia offers the classic illustration.
Schizophrenics have less gray matter in the frontal cortex, the seat of higher thought, says psychiatrist Tyrone Cannon of the University of Pennsylvania. And in the hippocampus, which helps run memory and emotion, cells are out of place. People with mild schizophrenia, whose symptoms are not severe enough to keep them socially isolated, have mild versions of these brain abnormalities. People with severe schizophrenia, who are unable to distinguish fears and fantasies from reality, have severe versions. But as you increase the number of genes you pass over the threshold to clinical significance.