Ganser syndrome is a psychiatric disorder characterised by approximate answers to questions. When asked how many legs a horse has they might say, "five," showing that they know the correct answer but are deliberately just off. The original description by Sigbert J.M. Ganser in 1898 pointed out their hysterical twilight state. They may also describe hallucinations which are usually more florid than those in schizophrenia. They may also have disorders of sensation similar to those in conversion disorder. They may be inattentive or drowsy.

This rare disorder is more common in prisoners or those who are otherwise confined. Some workers believe there is a genuine psychosis underlying this, others believe it is a dissociative disorder, while still others believe it is the result of malingering. Over the years, opinions have seemed to move from the first view more towards the last.

Although the subject appears confused in their answers, in other respects they appear to understand their surroundings. There may be a history of head injury.