To quote from the Report:
The only specific causes, other than congenital, to which is due a greater proportion of the total cases of blindness among those whose parents were cousins than among those whose parents were not related, are: Catarrh (parents cousins 28.1, parents not cousins 8.7 per 1,000), scarlet fever (parents cousins 10.7, parents not cousins 10.1 per 1,000), scrofula (parents cousins 28.9, parents not cousins 19 per 1,000), and measles (parents cousins 28.9, parents not cousins 23.5 per 1,000). The difference in these proportions is but slight, and the relative number of cases of blindness attributed to each of the other causes is greater among those whose parents were not related.[[77]]
It will be noted that the greatest proportion is in the case of scrofula.
Since it is probable that a part of those who did answer the question as to consanguinity are in fact the offspring of cousins, the percentage in each case should be somewhat increased. Allowing for these the same proportion as for those who did answer the question we should have of all the blind 4.47 per cent as the offspring of cousins; of the totally blind 4.14 per cent and of the partially blind 4.88. While of the congenitally blind we should have 14.7 per cent as offspring of cousins.
It is interesting to note in this connection that in 1900, Dr. Lee Wallace Dean, of the University of Iowa examined the 181 blind children in the Iowa College for the Blind, and found that 9 or nearly 5 per cent were the offspring of first cousin marriages.[[78]] Dr. Dean continues,
If we exclude from the list those blind children who were blind because of blennorrhea neonatorum, sympathetic opthalmia, trachoma, etc., and consider only those who suffered because of congenital conditions, we should find that 14 per cent were the result of consanguineous marriage of the first degree.... Among the pupils who have entered the college since 1900 the percentage is about the same.
This was written in 1903, three years before the publication of Dr. Bell's report.
Statistics from foreign sources give even larger percentages of the blind as the offspring of consanguineous marriage. Dr. Feer quotes fourteen distinct investigations of the etiology of retinitis pigmentosa, embodying in all 621 cases, of which 167 or 27 per cent were the offspring of consanguineous parents.[[79]] Retinitis pigmentosa is perhaps more generally attributed to consanguineous marriage than any other specific disease of the eye, and it is to be regretted that the Census report does not give any data in regard to this cause. Retinitis pigmentosa in known to be strongly inheritable, as is albinism and congenital cataract.
Looking now at the other side of the problem, that of the probability of consanguineous marriages producing blind offspring, we have as our data the 2527 blind whose parents were cousins, and a conservative estimate which may be made from the data in Chapter II that 1,000,000 persons in continental United States are the offspring of cousins within the degrees included in the Census report.[[80]] In the general population 852 per million are reported as blind, and 63 per million as congenitally blind. The actual figures for the offspring of cousin marriages are 2527 per million for all blind and 632 per million for the congenitally so. In other words only 0.25 per cent of the offspring of cousin marriages are blind and only 0.05 per cent are congenitally blind. Although the probability that a child of related parents will be born blind is ten times as great (632 per million vs. 63 per million) as when the parents are not related, the numbers are so small that there seems to be very little basis for a belief that consanguinity does more than to intensify an inherited tendency, especially since over one half of the congenitally blind of consanguineous parentage are known to have blind relatives.
2. The Deaf. The extent to which the connection between consanguineous marriage and deaf-mutism has been studied is indicated by a table given by Mr. Huth, in which are set forth the results of fifty distinct investigations.[[81]] In this table the percentages of deaf-mute offspring of consanguineous marriage to the total number of deaf-mutes investigated, varies from 30 per cent to none at all. Of these studies not more than ten or eleven have the slightest statistical value, and four of these—the most reliable—are from the reports of the Census of Ireland in the years 1851, 1861, 1871 and 1881.