The medical care of the developmentally disabled represents a challenge to society in many aspects.
In American sociaty, the first challenge is a close contact with the reality that humans are frail, flawed, and that often those flaws can lead to illnesses that require psychiatric care.
The consequence is that, a topic already disturbing to most Americans (the notion of psychiatric treatment) is then combined with the phenomena of a more than commonly imperfect human organism, within a society that prefers to have no awareness of human fraitly, flaws, or mortality.
We live in a society where the most prominent feature of our cutlure is denial. For this reason, to then try and address human suffering in the context of our own physical fragility is even more frightening (see the related article on Geriatric Psychiatry).
The simplest way of dividing up the forms of developmental disability are into two categories -- those that are linked to disturbances in genetics, and those that are instead, likned to injuries in the period preceding birth, during birth, or within infancy and childhood.
There are, of course, humans who fall into both categories. But for the sake of simplicity, we can utilize this simple division.
Of course, there are minimal resources for psychiatrists treating these patients, in the are of most need, which is solid clinical research.
This reflects the difficulty in establishing a broader scientific method, which may indeed reflect a similar problem for psychiatry as a whole. (http://psychiatry.wikia.com/wiki/The_Scientific_Method_and_Medicine).