Medicine

of cases are classed as being due to an environmental retiology, only 6% are considered as due to hereditary factors, 11'6% are classed as due to both, and 57% are classed as of unknown retiology. In this grouping, it should be noted that most subdivisions of Category VI are classed as of unknown retiology, since, as in most cases of Down's syndrome, though the immediate cause may be clear, the condition is not hereditary. Cases of cultural-familial retardation and psychosis are classed as being due to a combination of hereditary and environmental factors. Pitt and Roboz's survey therefore demonstrates that it is now possible to arrive at a fairly satisfactory classification of mental defect, though many of the headings do not imply an understanding of the ultimate causation.