I would say there are valid reasons to be skeptical about claims for genetic "causes" of so-called "mental illnesses". I read that one of the early proponents for a "genetic theory" of "schizophrenia" was a Nazi from Germany named Ernst Rudin. The process of psychiatric "diagnosis" is done more subjectively than the diagnosis of physical illnesses, which uses more objective criteria such as laboratory tests. 

Furthermore, psychiatry has often labeled dissidents as being "mentally ill". An infamous example of this is from the now-defunct Soviet Union, in which dissidents were incarcerated in mental institutions and given debilitating drugs like haloperidol(sometimes dispensed under the brand name Haldol in the non-Communist world). This has been mentioned in Peter Breggin's book "Toxic Psychiatry". Dissident psychologist Jay Joseph has also written books critiquing genetic "theories" of "mental illnesses". Even the process of creating the psychiatric classifications themselves is often subjective, and it is often influenced by political fashions. Homosexuality was dropped as a psychiatric "diagnosis" due to a vote by the APA in 1973."Hysteria" was often a "diagnosis" applied against women during the Victorian era. Psychiatric "diagnoses" are matters of controversy even among mental "health" professionals. There are many that state that psychiatry's "diagnostic" manual, the DSM-V expands the psychiatric "diagnoses" too much. Allen Frances was one of these critics. The classification of "autism" has had a wider "diagnostic" net due to the inclusion of people said to have "Asburger's Syndrome", in which people are said to have "normal" language development, but are also said to be more introverted than "normal" people.