Mental deficiency and its synonym “oligophrenia” are terms interpreted very differently in various countries; this has made it almost impossible to compare the statistical data of these countries. The concept the lay person in this country has of mental defect applies with few exceptions to individuals with intelligence quotients below 60 per cent., i.e., idiots, imbecile and obvious simpletons. When a person with this conception of mental defect—and we must admit that it corresponds fairly closely to the legal interpretation of the Mental Deficiency Acts in this country—is told that mental deficiency is a major social problem, the statement is received with some measure of incredulity. There is some justification for this incredulity. The statement is ambiguous and is based upon some rather muddled thinking. If we accept this legal and administrative interpretation of mental defect only about 1 per cent. of the population can be said to be mentally defective. Probably no other 1 per cent. of the population has such a high proportion of decent, docile and law-abiding citizens. If so, what meaning can we give to the statement that mental deficiency is a major social problem ?