This document outlines the four elements of the diminished responsibility defense in criminal cases in the UK: 1) the defendant had an abnormality of mental functioning, 2) the abnormality was a recognized medical condition, 3) the abnormality substantially impaired the defendant's ability to understand their actions, and 4) the abnormality provides an explanation for why the defendant carried out the killing. Case law has established that the impairment does not need to be total and is determined by a jury to be something between total and trivial or minimal. The abnormality and impaired state of mind must also be present at the time of the alleged killing.