“...we found evidence that genetic influences on schizophrenic and manic syndromes are partly shared.”

There is ongoing interest and controversy regarding whether, or how much, genetic influences are shared in common between psychotic disorders such as schizophrenia, schizoaffective disorder, and bipolar disorder/mania. Traditionally, in genetic research, single lifetime diagnoses are made, with schizophrenia at the top of the hierarchy. This means that, in a group of people diagnosed as having schizophrenia, there is no information about whether or not they also had illness episodes consistent with, say, mania, at some time in their lives. Taking this approach, most studies have not found a familial or genetic relationship between schizophrenia and bipolar disorder.

To our knowledge, this was the first twin modeling study to take a non-hierarchical approach, so that twins could qualify for more than one diagnosis during their lifetime. Taking this approach, we found evidence that genetic influences on schizophrenic and manic syndromes are partly shared. We also found evidence that genetic influences on schizoaffective disorder are entirely shared with the other two syndromes.

While doing fieldwork for an affected sib-pair linkage study of schizophrenia, in Cardiff, UK, I became interested in genetic influences on clinical variation within schizophrenia and other psychoses. I received a Medical Research Council (UK) Clinical Training Fellowship, during which I visited the Institute of Psychiatry in London, to work on collating information from the Maudsley Twin Register.

The success of the study came from being able to combine clinical research information from all twins with psychosis into a single dataset, thanks to support from senior colleagues, Peter McGuffin and Robin Murray; from the many people who have worked on the twin register since it began in 1948; and from a twin modeling approach suitable for a clinically ascertained sample, which was developed by Pak Sham and Frühling Rijsdijk.

The study contributed to a general move away from concentrating solely on single lifetime diagnoses as phenotypes, towards including information on, for example, co-occurrence of psychotic and affective symptoms, and symptom dimensions that cut across traditional diagnostic categories.