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A new study in Biological Psychiatry looks at the risk for schizophrenia in twins. Istock

The largest schizophrenia study of twins to date has found that as much as 79% of schizophrenia risk can be explained by genetic factors.

The research, published in the journal Biological Psychiatry, used a new statistical approach to address one of the factors that contributes to inconsistencies across previous studies, likely giving the most accurate estimate to date of schizophrenia's heritability.

Usually studies looking at the heritability – a term used to understand the influence of environmental vs. genetic factors – of schizophrenia require that people be classified as either having it or not. However, some people who are at risk could still develop the disease after the study ends.

"The new estimate of heritability of schizophrenia, 79%, is very close to the high end of prior estimates of its heritability," said Dr. John Krystal, Editor of Biological Psychiatry, referring to previous estimates that have varied between 50% and 80%.

"It supports the intensive efforts in place to try to identify the genes contributing to the risk for developing schizophrenia," he said. These efforts have been built on the idea that schizophrenia is highly heritable, a conclusion based on generations of twin studies.

The new study uses the Danish Twin Register – a record of all twins born in Denmark since 1870 – alongside information from the Danish Psychiatric Central Research Register, to assess the genes in more than 30,000 pairs of twins.

Because the diagnosis of schizophrenia is based on a narrow definition of symptoms, the researchers also estimated heritability using broader categorization techniques to include disorders on the schizophrenia spectrum. This gave them a similar estimate of 73%, indicating the importance of genetic factors across the full illness spectrum.

"This study is now the most comprehensive and thorough estimate of the heritability of schizophrenia and its diagnostic diversity," said Rikke Hilker, an author of the study. "It is interesting since it indicates that the genetic risk for disease seems to be of almost equal importance across the spectrum of schizophrenia," despite the clinical presentation of symptoms ranging from severe and long-lasting to subtle and temporary.

"Hence, genetic risk seems not restricted to a narrow illness definition, but instead includes a broader diagnostic profile," she added.