One possible cause of schizophrenia may be heredity, or genetics. Researchers believe that some people may inherit a tendency to schizophrenia. In fact, the disorder tends to run in families, but only among blood relatives.

People who have family members with schizophrenia may be more likely to get the disease themselves. If both biologic parents have schizophrenia, there is nearly a 40% chance that their child will get it, too. This happens even if the child is adopted and raised by mentally healthy adults. In people who have an identical twin with schizophrenia, the chance of schizophrenia developing is almost 50%.

In contrast, children whose biological parents are mentally healthy-even if their adoptive parents have schizophrenia-have about a 1% chance of getting the disease. That is about the same risk as for the general population shown in the chart.

Chances of Developing Schizophrenia

• General population: 1%
• Brother or sister has schizophrenia: 8%
• One parent has schizophrenia: 12%
• Fraternal twin has schizophrenia: 14%
• Both parents have schizophrenia: 39%
• Identical twin has schizophrenia: 47%

Schizophrenia is not passed on directly, like eye or hair color. Other conditions are necessary because other factors are involved. Otherwise, schizophrenia would always develop in both identical twins, since they have the same heredity.

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