The equal environments assumption in schizophrenia genetics

It is widely stated that schizophrenia is “genetic”. The main basis for this claim is the literature on twin concordance for schizophrenia in monozygotic (MZ) and dizygotic (DZ) twins. The validity of twin studies depends on the equal environments assumption. However, the low concordance rates in MZ and DZ twins prove two things: (1) schizophrenia can be at most only a little bit genetic, and (2) the equal environments assumption cannot hold. Proponents of the genetic contribution to schizophrenia must deal with a logical bind: the greater they make the gap between MZ and DZ twin concordance rates, the more they undermine the equal environments assumption; the smaller they make it, the more they undermine the genetic contribution to schizophrenia.