I have several posts on here related to schizophrenia and the claimed genetics of this psychiatric disorder and I thought it would be useful to combine twin studies, genetic studies, and polygenic scores in one post to give some perspective. Let me start with twin studies:
During medical school and my psychiatry residency training, the genetic nature of schizophrenia was often emphasized. The strongest evidence, we were informed, was that if an identical twin was diagnosed with schizophrenia, there was a 50% chance that the other identical twin would be diagnosed with schizophrenia. While this might leave some wondering why the other 50% do not get schizophrenia with an identical genetic profile, 50% is hard to just ignore. Well, as it turns out, it isn’t really accurate. Most of the studies that claimed such high concordance rates are from well over a half century ago. At that time, there was far more institutionalization and the diagnostic criteria were not really the same. As twin study critic Jay Joseph points out, if you take more modern studies the concordance rates are far lower, with an overall concordance rate for such studies after 1963 of 23% (some might recall a figure of 28%, but this does not include the Finnish study noted below, which seems to have been “disappeared”).
Depending on which of these studies you examine, that 23% figure might even be a bit inflated. Take, for example, a couple of the Scandinavian studies and although I may be accused of cherry-picking, these countries tend to have better national records to draw from, which include all twin pairs and all twin pairs that are concordant for schizophrenia, so I think they arguably are more accurate. I have recently posted on these studies. One is a Finnish study from 1984 and one is a more recent Danish study from 2018. I discussed them both at more length in previous blog posts, here and here. These studies both had a full twin registry available to them and were thus able to identify all individuals diagnosed with schizophrenia, as well as any which had a monozygotic or dizygotic twin and whether they were also diagnosed with schizophrenia. The Finnish study found an 11% concordance and the Danish study found a 14.8% concordance if you look at the actual numbers. These are shocking numbers for those of us who were led to believe that it was closer to 50%.
