Although the average age of onset for the disease has not been determined, symptoms of schizophrenia usually appear between the ages of 16 and 30.
Psychosocial, environmental, and genetic factors are known to contribute to the risk of developing the disease, but to what extent? New research - carried out by scientists at the University of Copenhagen in Denmark - suggests that almost 80 percent of the likelihood of having schizophrenia may be genetic.
Rikke Hilker, Ph.D., of the Center for Neuropsychiatric Schizophrenia Research at the Copenhagen University Hospital, is the first author of the study, and the findings were published in the journal Biological Psychiatry.
Studying schizophrenia heritability
Dr. Hilker and her colleagues used data from the Danish Twin Register and the Danish Psychiatric Research Register to identify more than 31,000 pairs of twins born between 1951 and 2000.
This study sample was clinically followed for almost 11 years, and the scientists used complex statistical models to assess twin concordance rates.
Usually, statistical studies of heritability look at people who, by the end of the study, have either have been diagnosed with the disease or not.
However, they do not account for the risk of developing the disease after the research ends. But this research included a more recent statistical method called inverse probability weighting.
Having applied these methods, the researchers "estimated the heritability of [schizophrenia] to be 79 percent."
Also, when the researchers included schizophrenia spectrum disorders, such as schizoaffective disorders or schizotypal and schizoid personality disorders, the heritability rate was comparably high: 73 percent.
The significance of the study
These results, the authors note, are important for the medical research community, especially for "future genome-wide association studies."
The new findings also seem to reinforce previous studies. "The new estimate of heritability of schizophrenia, 79 percent, is very close to the high end of prior estimates of its heritability," explains Dr. John Krystal, who is the editor of Biological Psychiatry.
"It supports the intensive efforts in place to try to identify the genes contributing to the risk for developing schizophrenia," he adds. Medical News Today have previously covered such efforts, including a study that identified 80 new genes related to the illness.
Dr. Hilker also comments on the new findings, saying:
"This study is now the most comprehensive and thorough estimate of the heritability of schizophrenia and its diagnostic diversity. [...] It indicates that the genetic risk for disease seems to be of almost equal importance across the spectrum of schizophrenia."
"Hence," she added, "genetic risk seems not restricted to a narrow illness definition, but instead includes a broader diagnostic profile."
The study authors also comment on the strengths and limitations of their research, saying, "The key strength of this study is the application of a novel statistical method accounting for censoring in the follow-up period to a nationwide twin sample."
However, they concede that their study relied heavily on the consistency and validity of the diagnoses in the health registers.
There could also be undiagnosed cases of schizophrenia that were not accounted for, and the results are not applicable to cases wherein the disease developed after the age of 40.