This group of disorders is characterized by delusions of misidentification, often accompanied by hallucinations of misidentification (Debruille and Stip, 1996). There are relatively ‘pure’ forms in which the delusion forms the greater part of the presentation and similar pictures in which the delusion is secondary to other disorders, such as schizophrenia, severe depression or Alzheimer's disease (Spier, 1992). In the past, cases have often been misdiagnosed as schizophrenia (Ellis and Young, 1990).

There are four main variants and experts in the field describe additional alternative forms of these. The types most commonly mentioned are:

1. The Capgras syndrome (Capgras and Reboul-Lachaux, 1923), in which the patient perceives someone in his environment (usually a closely related individual) to have been replaced by an almost, but not quite, exact double.

2. The Frégoli syndrome (Courbon and Fail, 1927) where the patient believes that persecutors have altered their facial appearance to resemble familiar people in the environment.

3. Intermetamorphosis (Courbon and Tusques, 1932), in which the patient thinks that the people around him or her have changed identities with each other, so that A becomes B, B becomes C, and so on.

4. Subjective doubles (Christodoulou, 1978) where the patient believes that exact doubles of him- or herself exist. This is said usually to co-exist with other forms of DMS and is rarely seen by itself.