A person of D's sex and age, with a normal degree of tolerance and self-restraint and in the circumstances of D, might have reacted in the same or similar way (Asmelash)
Although there are no examples, RMC could include psychological and physical conditions (which affect mental function), e.g. chronic depression, OCD, PTSD, diabetes, brain damage. A Dr would give medicalevidence.
D's ability to (a) understand the nature of his conduct, (b) form a rational judgment, or (c) exercise self-control must be substantially impaired. SI means the impairment must be "important or weighty" (Golds).