MZ twins share 100% of their genes; DZ twins share 50% of their genes. If genes are a factor we would expect more identical twins to share the disorder than non-identical. Rosenthal took a case study which had a set of female quadruplets. They all developed schizophrenia although the onset and symptoms were very different. This could have been a result of having a troubled upbringing. This suggests a strong heritable component.
However, most first degree relatives and twins share the same or similar environments so it is difficult to separate genetic and environmental influences. Adoption studies compare people who have been raised in a different environment from their biological relatives. If they have similarities with their biological relatives this should be due to genes. Evidence from adoption studies, such as, Tenari’s study that had a longitudinal study of 155 schizophrenic mothers who gave up their children for adoption. These were compared with 155 adopted children who did not had a schizophrenic mother. Once these children reached adulthood there was a 10.3% chance of developing schizophrenia when