That little allowance for the variation from age to age need be made for the number of cases not discoverable at the beginning of school life is further indicated by report of the Minnesota State School for Feeble-Minded. It shows that in only 247 out of its 3040 admissions was the mental deficiency known to commence after six years of age ([154]). If the number of feeble-minded who should be isolated were found to increase after school age less than one in 10,000 of the population, as this suggests, it would surely be better to neglect this variation from age to age than to emphasize it in dealing with the problem of objective diagnosis and social welfare.
How rare is the onset of feeble-mindedness after five years of age is also shown by the frequency of hereditary causes. In his study of the 300 families represented at Vineland, Goddard places only 19% in his “accidental” group and 2.6% in the group for which the causes are unassigned. The rest are either in the hereditary group, probably hereditary, or with neurotic heredity. Half of the cases in the “accidental” group are due to meningitis. His histories show that only 9 of the “accidental” and unassigned groups were unknown at 5 years of age. This is only 3% of his total feeble-minded group. To these might be added, perhaps, a few from the hereditary groups who did not show their feeble-mindedness at so early an age, but so far as I can judge these would not be of the intellectually deficient type that would be detectable by the Binet scale at any age. They would test high enough intellectually to pass socially and require expert diagnosis to be classed as feeble-minded.
Certain diseases, epilepsy and meningitis, are undoubtedly causes of feeble-mindedness. The evidence, however, seems to be that they are so rare compared with the mass of mental deficiency that after 5 years they may well be offset by the excessive death rate among the feeble-minded. That recoveries from feeble-mindedness are insignificant is generally agreed. Among the 20,000 in institutions in 1910 only 55 were returned to the custody of themselves. This is further evidence of the fundamental, if not congenital, nature of the deficiency.
While the evidence submitted above makes it seem fair to assume that the increase in the frequency of a certain degree of intellectual deficiency with age is probably negligible, it is not clear that the decrease with age in the proportion of feeble-minded caused by an excessive death rate may be neglected even for the test ages 5 to 25. By searching the literature it has been possible to assemble the records for nearly 3500 deaths among the feeble-minded in institutions in this country and Great Britain distributed by ages in ten-year periods. This evidence is presented in Table I. The number of cases under five years of age living in the institutions is so small that the deaths under five years are certainly misleading. They have, therefore, been omitted from the table and the distribution calculated for those five years or over (123, 154, 204, 205). Comparison is made with a similar distribution of the total deaths for a period of five years from 1901 to 1904, inclusive, within the area of the United States in which deaths are registered, compiled from the special mortality report of the Bureau of the Census ([206]). This registration area has a population of about 32,000,000. The general agreement of the distribution of deaths among the four different groups of institutional inmates seems to make it reasonable to assume that the United States group of institutional deaths for the year 1910 is a conservative description of excessive death frequency at the early ages among the feeble-minded in institutions.
Table I. Age Distribution of Deaths in the General Population and Among Feeble-Minded in Institutions.
| Population | Ages | ||||||
|---|---|---|---|---|---|---|---|
| 5-14 | 15-24 | 25-34 | 35-44 | 45-54 | 55 & over | ||
| Gen'l—U. S. in death registration area | 1,897,492 | 6.1% | 9.6% | 12.8% | 13.0% | 13.6% | 44.9% |
| F. M. 1910 in Institut'ns in U. S. | 840 | 26.6 | 33.0 | 18.9 | 9.1 | 45 & over 12.3 | |
| F. M. British (Earlswood) | 997 | 34.3 | 41.1 | 10.4 | 6.5 | 3.5 | 55 & over 4.2 |
| F. M. British (Barr) | 613 | 34.7 | 46.8 | 9.5 | 35 & over 9.0 | ||
| F. M. Faribault Minnesota | 982 | 27.6 | 38.0 | 16.1 | 8.6 | 3.5 | 55 & over 6.2 |
Table II. Mortality of Institutional Deficients in the United States Compared with the General Population, Showing its Possible Effect on the Frequency of Deficiency at Different Ages.
| Ages | ||||||||
|---|---|---|---|---|---|---|---|---|
| 5 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | |
| General population | 1000 | 983 | 972 | 956 | 934 | 903 | 872 | 835 |
| Deficients in Institut'ns | 1000 | 795 | 696 | 606 | 503 | 428 | 349 | 290 |
| Per cent. deficient if 1% at age 15 | 1.40 | 1.11 | 1.00 | .75 | ||||
Fig. 1. Mortality among Feeble-Minded in Institutions Compared With the General Population