The overwhelming desire to actually become an amputee, by the removal of a healthy limb.
To actually become disabled, in which they are willing to sacrifice a perfectly healthy limb in order to achieve this goal. It does not just apply though, to amputations, but to other noticeable disabilities such as blindness, or other noticeable deformities.
Psychiatrists are of the opinion that this desire to be disabled, to be an amputee, is developed early on in one's childhood. Generally due to their early childhood where they encounter someone so afflicted, and notice how the adults respond with increased attention, or compassion.
They associate the attention, the increased compassion with the disability, desiring a similar response.
There is also the concept that it is a person having a low self esteem, and are trying to punish themselves.
Other theories are that a child feeling unloved, may seek that love later on, as they associate the attention given to a disabled person, as being love, so they desire to be in that position, to achieve what they didn't get as a child.
Noteworthy: Most surgeons will not perform such surgery, the removal of a healthy limb, which has caused some BIID people to actually self amputate a limb, or damage it so badly, that they actually require amputation.
However, these cases are rare, and usually most with this will act out being disabled when they can't actually become an amputee.