If successfully pleaded, the partial defence of diminished responsibility reduces the defendant’s liability from murder to manslaughter. The defendant must prove that an abnormality of mental functioning, which arose from a recognised medical condition, substantially impaired his ability to understand the nature of his conduct, form a rational judgment, or exercise self-control (s.2 Homicide Act 1957, as amended by s.52 Coroners and Justice Act 2009). The term ‘recognised medical condition’ was intended to ensure that diminished responsibility defences were ‘grounded in a valid medical diagnosis linked to the accepted classificatory systems…’ (Ministry of Justice CP 19/08). The Court of Appeal has held that the presence of a ‘recognised medical condition’ is a necessary, but not always a sufficient, condition to raise the issue of diminished responsibility (R v Dowds  EWCA Crim 281). A recognised medical condition will only be regarded as having “substantially impaired” the defendant’s ability to do one of the specified things if it significantly or appreciably impaired his ability beyond something that is more than trivial or minimal (R v Golds  EWCA Crim 748). Expert evidence remains “a practical necessity” if the defence is to succeed (R v Bunch  EWCA Crim 2498). The absence of clear guidance as to what will be ‘sufficient’ to raise the defence and what level of impairment will be regarded as ‘substantial’ creates the risk that the defence will not be applied consistently by experts or the courts.