Heritability of mental disorders

Heritability of mental disorder is a difficult concept to understand. For example, it is commonly said that the heritability of schizophrenia is as high as 80%. People are different. Heritability is said to measure the extent to which the variability among people is accounted for by genetics. But is it really possible to quantify the relative importance of genetic and environmental influences? Is schizophrenia, for example, really 80% heritable?

The problem is that genetics and environment interact and are not simply additive factors. Both genes and environment are necessary. As Jay Joseph (see eg. his past guest post and recent post on his ‘The gene illusion’ blog) says in his article, “[B]oth genetic and environmental factors are essential interacting influences whose contributions jointly, as opposed to additively, combine to produce the observed behavioral characteristic (phenotype)”. It makes no sense to say that schizophrenia, or any other mental disorder is 80% genetic. Genes define the boundaries of the possible but environments determine the actuality.