Madness Explained: Psychosis and Human Nature by Richard P Bentall contains a chapter that also challenges the twin studies challenged below.
TWIN STUDIES IN PSYCHIATRY AND PSYCHOLOGY: SCIENCE OR
Jay Joseph, Psy.D.
Psychiatric Quarterly, Vol. 73, No. 1, Spring 2002 (°C 2002)
wileywitch sez: Though it is widely considered to be axiomatic that mental illness is genetically determined, upon analysis most of the research showing a concordance is, at best, questionable, and at times fraudulent.
Today, it is widely acknowledged by proponents of the genetic position that family studies are confounded by environmental factors, since family members share both common genes and a common environment. As a pair of psychiatric geneticists recently put it, family studies can provide only “the initial hint that a disorder might have a genetic component”
This is what I like to call, “Have I mentioned that I lived with my mother?” A statement that has too often been met with what I call the “therapeutic glaze.”
A question about identical twins separated at birth
…let us suppose that a 20 year-old pair of German reared-apart male MZ twins, separated at birth and brought up in different parts of the country, had been reunited in 1940. A researcher would probably notice several striking similarities. Both might be wearing swastika armbands, have short-cropped hair, give stiff arm salutes and rail against “the enemies of the Reich.” The values, beliefs, behaviors, and even wardrobes of these twins might be remarkably similar, yet the conclusion that these similarities are caused by common genes would be quite erroneous.