E. Fuller Torrey recently published a journal article trying to cast doubt on the commonly-accepted claim that schizophrenia is mostly genetic. Most of his points were the usual “if we can’t name all of the exact genes, it must not be genetic at all” - but two arguments stood out:

Even though twin studies say schizophrenia is about 80% genetic, surveys of twin pairs show that if one identical twin has schizophrenia, the other one only has a 15% to 50% chance of having it.

The Nazis ran a eugenics program that killed most of the schizophrenics in Germany, eliminating their genes from the gene pool. But the next generation of Germans had a totally normal schizophrenia rate, comparable to pre-Nazi Germany or any other country.

I used to find arguments like these surprising and hard to answer. But after learning more about genetics, they no longer have such a hold on me. I’m going to try to communicate my reasoning with a very simple simulation, then give links to people who do the much more complicated math that it would take to model the real world.

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Some Unintuitive Properties Of Polygenic Disorders