Although insane' persons were incompetent as witnesses at early common law, the modern view is that the effect of mental illness upon competency is a preliminary question for the court in the absence of contrary statutory direction. An insane person is generally said to be a competent witness if he can understand the sanctions imposed to elicit the truth and can correctly recount the occurrence which is the subject of his testimony. Some courts exclude evidence of insanity offered for purposes of impeachment but most courts admit such evidence, treating medical and lay testimony with equal respect because of the refusal of the earlier cases to consider insanity as exclusively within the province of experts.
Thomas E. Watts Jr.,
Psychiatric Challenge of Witnesses,
9 Vanderbilt Law Review
Available at: https://scholarship.law.vanderbilt.edu/vlr/vol9/iss4/11