Body Integrity Identity Disorder, or BIID, is a psychological condition in which the individual requests an elective amputation. Individuals with this condition experience the persistent desire to have their body physically match the idealized image they have of themselves. This desire forces individuals to deal with the paradox of losing one or more major limbs (i.e. arm[s] or leg[s]) to become whole. In their minds, “Less is more”.
Wikipedia has an article on this disorder also.