If you had an identical twin with schizophrenia, you would have about a 50-50 chance of developing the disease, regardless of whether the two of you were raised together or apart. That risk of developing schizophrenia is much higher than the 1 in 100 risk for the general population. The twin data tell us two things: first, schizophrenia has a strong genetic component, regardless of environment; and second, those genes can’t be acting alone, because the risk isn’t 100 percent.
