It is a natural supposition that there is a sudden increase in the proportion of feeble-minded at adolescence. On account of the increased rate of growth at this period we might expect to find greater instability for a few years. It may well be that there is a rather sudden influx of the unstable type of feeble-mindedness at this period. Such an increase may occur without being detected by a series of brief intellectual tests such as the Binet scale. It would be of the conative type of feeble-mindedness that cannot at present be diagnosed by objective tests, the type that requires diagnosis by expert opinion. It is to be noted, however, that Binet, who paid much attention to the unstable type, says: “Since the ill-balanced are so numerous at ten years of age, and even at eight, we conclude that in many cases the mental instability is not the result of the perturbation which precedes puberty. This physiological explanation is not of such general application as is sometimes supposed” (_77_, p. 18). Only when an emotional disturbance is so great as to be detectable by mental tests will this influx need to be taken into consideration in stating the borderline for objective tests. The evidence that few cases of feeble-mindedness are not detectable until after ten years of age is all the other way. With the Stanford measuring scale, Terman and his co-workers did not even find a noticeable increase in the variability of the groups at the ages of adolescence (_57_, p. 555). It is to be remembered also that we are not concerned here with mere instability which corrects itself with more maturity, such as has been described by Bronner among delinquents. This does not, of course, amount to an incurable conative deficiency and is not classified under feeble-mindedness. Goddard has suggested that possibly the moral imbecile group comes into our class of feeble-minded suddenly with a common arrest of development at about the stage reached by the nine-year-old. He notes that “of the twenty-three cases of this sort picked out for us (at Vineland) by the head of the school department, fifteen are in the nine-year-old group, five in the ten-year-old, two in the eleven, and one in the twelve” (_113_). He regards this evidence, however, as meager and only suggestive. Doll has given evidence of late appearance of retardation in rare cases (_100_ and _99_).