Chances are it's all going to be subjective anyway. Let's say a bricklayer becomes physically damaged and never gathered skills to be anything other than a bricklayer, but has nothing wrong with his speech or mental functioning. Is he disabled? From being a bricklayer, yeah. But from a variety of other jobs he could apply himself toward and strive for? Each case is unique and I'm betting a majority of disabled folks (assuming the disability is not due to psychosis or developmental disability/retardation/etc.) theoretically could strive toward other work but feel hopeless about their prospects, and so they hop around from doctor's office to doctor's office seeking endorsement to be on disability. This is all subjective and case-by-case, and I'm not going to make appeals to my own experience in this arena, but I am extremely confident disability can be entirely faked, as well as not necessarily faked but definitely bought into by people who convince themselves of their own hopelessness and feel honest in arguing they "can't" overcome their current state of misfortune.