The Supreme Court of Pennsylvania has held that a psychiatrist's expert testimony is relevant in a prosecution for first degree murder when the testimony is offered to demonstrate that the defendant lacked sufficient mental capacity to formulate the specific intent required for a conviction of first degree murder.
Commonwealth v. Walzack, 468 Pa. 210, 360 A.2d 914 (1976).
William G. Merchant,
Criminal Law - Murder - Evidence - Psychiatric Testimony - Specific Intent - Diminished Capacity,
Duq. L. Rev.
Available at: https://dsc.duq.edu/dlr/vol16/iss1/13