Psychiatric conditions in which symptoms arise involuntarily ('diseases') might be assumed to be more heritable than those in which choices are essential (behavioral disorders). We sought to determine whether psychiatric 'diseases' (Alzheimer's disease, schizophrenia, and mood and anxiety disorders) are more heritable than behavioral disorders (substance use disorders and anorexia nervosa). We reviewed the literature for recent quantitative summaries of heritabilities. When these were unavailable, we calculated weighted mean heritabilities from twin studies meeting modern methological standards. Heritability summary estimates were as follows: bipolar disorder (85%), schizophrenia (81%), Alzheimer's disease (75%), cocaine use disorder (72%), anorexia nervosa (60%), alcohol dependence (56%), sedative use disorder (51%), cannabis use disorder (48%), panic disorder (43%), stimulant use disorder (40%), major depressive disorder (37%), and generalized anxiety disorder (28%). No systematic relationship exists between the disease-like character of a psychiatric disorder and its heritability; many behavioral disorders seem to be more heritable than conditions commonly construed as diseases. These results suggest an error in 'common-sense' assumptions about the etiology of psychiatric disorders. That is, among psychiatric disorders, there is no close relationship between the strength of genetic influences and the etiologic importance of volitional processes.