Mr. Caradoc Jones this morning emphasized very clearly indeed the important distinction between high and low grade mental deficiency. He showed us some very striking figures suggesting, not that heredity is not involved in both, but that it is a different sort of heredity. It always seems to me that in considering this and related matters the analogy of stature is a helpful one. Many of us remember those posters of the last war but one, which said “Your King and Country Need YOU,” coupled with the statement that “You” had to be 5 ft. 4 in. high—a standard which went down afterwards. If one rejects for any purpose a segment of the population on a measurement of this kind one is rejecting people for very different reasons. The arbitrary standard cuts off, of course, the dwarfs; the achondroplasics, the midgets, the cretins, the rachitic dwarfs, and so on; but it cuts off far more of those who are simply short. In causation we can normally expect the dwarf's condition to be due to hereditary factors, actually a single factor in achondroplasia; or it may be something environmental, as in the rachitic dwarfs or the cretins, but when we come to the people who are just short, it has been shown fairly conclusively that in a civilized community in which the standard of nutrition is adequate, at least 90 per cent. of the differences are due to heredity; but it is a different sort of heredity. We have a whole host of genetic factors, each one of which has a small effect; but the effect is cumulative; some factors make for greater stature, some for smaller, and it is on the sum total received from the parents that the stature of the individual depends.
Some patterns and social impacts of external migration on a below-replacement population: Denmark by the turn of the millennium
