A delusional disorder that occurred in one of a pair of monozygotic twins.

The authors reported a pair of male monozygotic twins in one of whom a delusional disorder occurred, and discussed the etiology of that disorder. The analysis of the twins indicates the following: 1. The disposition for the delusional disorder might be related to a psychological tendency to experience a delusion that might be based on a genetic factor. This might be strengthened by the development of a hypersensitive personality (Kretschmer, E.) promoted by environmental factors. 2. A hypersensitive personality might be a significant risk factor for the delusional disorder.