Some quotes.
In one set of studies researchers consistently found that others are judged much more blameworthy for behaviors when they follow from mental disorders than physical disorders. For example, researchers found that breaking a promise is judged much less harshly when the excuse involved a physical reason, like complications from a car accident, than a psychological reason, like suffering from serious depression.
Putting these discoveries together, the result is a folk psychological schema that represents the contents of mental disorders as beliefs and then fully embraces voluntarism about those beliefs. The theory presented in the present paper is that these features of the folk psychological system encourage the dualistic treatment of mental disorders.
Namely, the system encourages the representation of mental disorders and their content in terms of mental states, such as beliefs, and considers such states to be voluntarily controllable. This, in turn, explains why mental disorders, disordered thinking, and maybe even associated behavior are viewed as voluntary and drives a wedge between it and physical disorders or illnesses, which are not typically perceived as voluntary or controllable.