The thing is, although the behavioural traits described by Williams Syndrome were present in him, some of them could be changed, or others can be found in people without the syndrome.
Schizophrenia seems to be the strongest point that psychology makes when it says it represents a real science, because they say schizophrenia can be a scientific proof that mental illnesses exists. So let´s talk about schizophrenia:
Excerpt from “The Myth of Schizophrenia as a Progressive Brain Disease" The authors, Robert B. Zipursky, Thomas J. Reilly,and Robin M. Murray, are all psychiatrists (source):
Sympthoms: “delusions of being controlled by an external force; the belief that thoughts are being inserted into or withdrawn from one's conscious mind; the belief that one's thoughts are being broadcast to other people; and hearing hallucinatory voices that comment on one's thoughts or actions or that have a conversation with other hallucinated voices”
“Schizophrenia has historically been considered to be a deteriorating disease, a view reinforced by recent MRI findings of progressive brain tissue loss over the early years of illness. On the other hand, the notion that recovery from schizophrenia is possible is increasingly embraced by consumer and family groups. This review critically examines the evidence from longitudinal studies of clinical outcomes, MRI brain volumes, and cognitive functioning.