Twin studies are a major source of information on the heritability of diseases and disorders, including schizophrenia. Identical twins, also known as monozygotic twins, share nearly 100% of their genes. Therefore, when studying identical twins, if one twin develops a genetic-based condition, researchers can estimate the likelihood of the other twin also developing the same condition, providing insights into the genetic component of the disease.
In the case of schizophrenia, research on twin studies has shown that if one identical twin has schizophrenia, the chance that the other twin will also develop schizophrenia is roughly around 50%. This high concordance rate suggests a strong genetic factor in the development of schizophrenia, though it is also clear that genetics is not the sole factor, since the concordance is not 100%.
So, if a person has an identical twin with schizophrenia, the estimated chance they will develop schizophrenia themselves is approximately 50%.
