| TABLE XXVI. | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Consanguinity of Parents. | 1871 | 1881 | 1871 and 1881 | ||||||
| Number of marriages | Number of conginital deaf-mutes | Average per marriage | Number of marriages | Number of conginital deaf-mutes | Average per marriage | Number of marriages | Number of conginital deaf-mutes | Average per marriage | |
| First cousins | 72 | 128 | 1.78 | 74 | 123 | 1.66 | 146 | 251 | 1.72 |
| Second cousins | 50 | 89 | 1.78 | 29 | 46 | 1.58 | 79 | 135 | 1.71 |
| Third cousins | 24 | 40 | 1.67 | 9 | 21 | 2.33 | 33 | 61 | 1.85 |
| Fourth cousins | 5 | 11 | 2.20 | 1 | 1 | -- | 6 | 12 | 2.00 |
| Fifth and sixth cousins | 12 | 19 | 1.58 | not stated | 12 | 19 | 1.58 | ||
| Total | 163 | 287 | 1.76 | 113 | 191 | 1.69 | 276 | 478 | 1.73 |
| No relationship[A] | 2,842 | 3,609 | 1.27 | 2,474 | 3,229 | 1.31 | 5,316 | 6,838 | 1.29 |
| Grand total | 3,005 | 3,896 | 1.30 | 2,587 | 3,420 | 1.32 | 5,592 | 7,316 | 1.31 |
| [A] See Table XXV. | |||||||||
In Scotland Dr. Arthur Mitchell made inquiry of the superintendents of a number of deaf-mute asylums, and found that of 544 deaf-mutes, 28 were the offspring of 24 consanguineous marriages.[[85]] There were 504 families represented in all, so that the average per family was 1.17 among the consanguineous to 1.07 among the non-consanguineous.
In Norway, according to Uchermann, while 6.9 per cent of all marriages are consanguineous within and including the degree of second cousins, and in single cantons the percentages range as high as 31.0, only in one single district does the number of the deaf-mutes harmonize with that of the marriage of cousins. The district of Saeterdalen has the greatest number of consanguineous marriages (201 out of 1250), but not a single case of deaf-mutism. Hedemarken, which has the fewest consanguineous marriages has a great many deaf-mutes. Where deaf-mutism exists it seems to be intensified by consanguinity, but where it is not hereditary it is not caused by consanguinity. Of the 1841 deaf-mutes in Norway, 919 were congenitally deaf, and of these 212 or 23 per cent were of consanguineous parentage.[[86]]
Dr. Feer gives a table containing the results of a number of studies of deaf-mutism, which shows an average of 20 per cent as of consanguineous origin. Four investigations give the number of children to a family. Table XXVII from Feer seems to indicate that the Irish census is fairly accurate at this point.[[87]]
| TABLE XXVII. Average Number of Children to a Family. | |||
|---|---|---|---|
| Observer. | Consanguineous marriages. | "Crossed" marriages. | |
| Huth (Irish Census) | 1.68 | 1.17 | |
| Wilhelmi | 1.71 | 1.26 | |
| Mygind | 1.53 | 1.20 | |
| Uchermann | 1.41 | 1.19 | |
In the American Census the instructions to enumerators have been so diverse that statistics of the deaf have been very poor until recent years. Not until the Twelfth Census was the inquiry put upon a really scientific basis.
This reform, as also the more intelligent attitude of the American people in general towards the affliction of deafness, is due largely to the work of Dr. Alexander Graham Bell. An enumeration of Dr. Bell's services directly, and through the agency of the Volta Bureau, in this cause, cannot be given here. For our purpose the most important of his contributions is embodied in the Special Report of the Twelfth Census of the United States already referred to.
As in the investigation of the Blind, the circular letter sent to each person reported by the enumerators as deaf contained questions in regard to parentage and the existence of deaf relatives. It is unfortunate that in these returns it is impossible to distinguish between degrees of relationship, but in such an extensive compilation it was doubtless impracticable to attempt to unravel the intricacies of consanguinity. Judging from the returns of the Census of Ireland we may assume that about half of the cases returned as "cousins" were first cousins.
The replies to the inquiry as to deaf relatives were more carefully analyzed, and were divided into four groups, which are referred to throughout as (a), (b), (c) and (d) relatives. These groups are: (a), deaf brothers, sisters or ancestors; (b), deaf uncles, aunts, cousins or other relatives not (a), (c) or (d); (c), deaf children, (sons or daughters); (d), deaf husbands or wives. Thus a large proportion of the hereditary cases would be included in the first two categories, (a) and (b).[[88]]
The causes of deafness are given in detail, but as might be expected the returns are not as definite or as accurate as we should desire. The causes given have been grouped under five main heads; these again are subdivided, often into divisions numerically too minute for real statistical value. Table XXVIII includes the main groups and those specific causes which number more than 3000 cases. The extreme variation in the percentages of those who are the offspring of consanguineous marriages cannot be attributed to mere chance. There is clearly some fundamental connection between consanguinity and congenital deafness if 11.8 per cent of all the congenitally deaf are the offspring of consanguineous marriages, while of the adventitiously deaf but 3.1 per cent are the offspring of such marriages. In fact we are tempted to jump at the conclusion that consanguinity is in itself a cause of deaf-mutism. Furthermore 42.1 per cent of the deaf whose parents were cousins were congenitally deaf, while this was true of but 15 per cent of those whose parents were unrelated.