CONSANGUINITY AND THE SPECIAL SENSES
The most important source for this chapter is the special report on the Blind and the Deaf in the Twelfth Census of the United States.[[75]] This report was prepared under the direction of Dr. Alexander Graham Bell, as Expert Special Agent of the Census Office.
The enumerators of the Twelfth Census reported a total of 101,123 persons as blind, and to each of these Dr. Bell addressed a circular of inquiry. By this method he obtained verified returns of 64,763 cases of blindness in continental United States or 85.2 per 100,000 of the total population. In the same way he obtained data in regard to 89,287 persons with seriously impaired powers of hearing, or 117.5 Per 100,000 of the total population.
In each case the following questions among others were asked: "Were his (or her) parents first cousins? If not first cousins were they otherwise related by blood to each other, before their marriage? Were any of his relatives blind? If yes, what relatives? (Father, mother, grandparents, brothers, sisters, uncles, aunts, and how many of each, so far as known)." The results of this inquiry give us the best and most reliable statistical material which has ever been compiled on any phase of the problem of consanguineous marriage. The investigation of the deaf was similar to that of the blind, but even more complete.
I. The Blind. The question as to the relationship of the parents was answered in 56,507 cases, in 2,527 or 4.47 per cent of which the parents were reported as cousins. Of the 57,726 who answered the question in regard to blind relatives, 10,967 or 19 per cent replied in the affirmative.[[76]] The blind relatives were divided into two groups: (a) blind brothers, sisters or ancestors, and (b) blind collateral relatives or descendants. Table XXII concisely expresses the results most fundamental for this study.
| TABLE XXII. | |||||
|---|---|---|---|---|---|
| Consanguinity of Parents. | Totals | Having blind relatives Class (a).[A] | Having blind relatives Class (b).[A] | Having no blind relatives. | Not Stated. |
| The blind | 64,763 | 8,629 | 2,338 | 46,759 | 7,037 |
| Totally blind | 35,645 | 4,378 | 1,215 | 26,349 | 3,703 |
| Partially blind | 29,118 | 4,251 | 1,123 | 20,410 | 3,334 |
| Parents cousins.--The blind | 2,527 | 844 | 149 | 1,456 | 78 |
| Parents cousins.--Totally blind | 1,291 | 435 | 78 | 739 | 39 |
| Parents cousins.--Partially blind | 1,236 | 409 | 71 | 717 | 39 |
| Parents not cousins.--The blind | 53,980 | 7,395 | 2,095 | 43,368 | 1,122 |
| Parents not cousins.--Totally blind | 29,892 | 3,720 | 1,090 | 24,541 | 541 |
| Parents not cousins.--Partially blind | 24,088 | 3,675 | 1,005 | 18,827 | 581 |
| Consanguinity not stated.--The blind | 8,256 | 390 | 94 | 1,935 | 5,837 |
| Consanguinity not stated.--Totally blind | 4,462 | 223 | 47 | 1,069 | 3,123 |
| Consanguinity not stated.--Partially blind | 3,794 | 167 | 47 | 866 | 2,714 |
| [A] Symbols for Blind Relatives--(a) blind brothers, sisters or ancestors; (b) blind collateral relatives or descendants. | |||||
Of the 2527 blind persons whose parents were cousins, 993 or 39.3 per cent have blind relatives, 33.4 per cent having blind brothers, sisters or ancestors, and 3.9 per cent having blind collateral relatives or descendants. And 9 per cent of the blind who have blind relatives are of consanguineous parentage, while but 3.1 per cent of the blind who have no blind relatives are the offspring of cousins. These figures alone indicate a decided intensification of blindness through consanguinity, although it should be remembered that a relationship "works both ways," so that when a brother has a blind sister, the sister would have a blind brother. This fact has probably diminished the apparent number of sporadic cases of blindness.
Considered with reference to the degree of blindness the table shows that 1291 or 51.1 per cent of the blind of consanguineous parentage are totally blind, and 1236 or 48.9 per cent are partially blind. Among those whose parents were not cousins, 55.4 per cent were totally and 44.6 per cent were partially blind.
Of the 2527 blind of consanguineous parentage, 632 or 25.0 per cent were congenitally blind, of whom 350 or 55.4 per cent also had blind relatives of the degrees specified. Not counting those who did not answer the question in regard to blind relatives, we have 615 cases of which 51.5 per cent had blind relatives of class (a), and 5.4 per cent blind relatives of class (b). Taking the 53,980 blind whose parents were not so related the number of congenitally blind was 3666 or but 6.8 per cent, of whom 1023 or 27.9 per cent had blind relatives. Omitting as before the "blind relatives not stated," we have 23.4 per cent who had blind relatives of class (a), and 4.3 per cent relatives of class (b).
On the hypothesis that consanguinity in the parents intensifies a tendency toward blindness we should expect to find among the congenitally blind a larger proportion of consanguineous parentage than among those blind from specific causes. In Table XXIII a general classification of the causes of blindness is given together with the consanguinity of parents. Specific causes in which the percentage of consanguinity differs in a marked degree from the average, are given parenthetically.
| TABLE XXIII. | |||||||
|---|---|---|---|---|---|---|---|
| Cause of Blindness. | Total. | Consanguinity of Parents | Percentages | ||||
| Cousins | Not Cousins | Not stated | Cousins | Not Cousins | Not stated | ||
| Total | 64,763 | 2,527 | 53,980 | 8,256 | 3.9 | 83.4 | 12.7 |
| Opacity of the eye | 33,930 | 1,000 | 28,797 | 4,133 | 2.9 | 84.9 | 12.2 |
| a. Causes affecting cornea | 11,380 | 444 | 10,016 | 920 | 3.9 | 88.0 | 8.1 |
| (1) Measles | 1,451 | 73 | 1,267 | 111 | 5.0 | 87.4 | 7.6 |
| (2) Scrofula | 1,165 | 71 | 1,026 | 68 | 6.1 | 88.1 | 5.8 |
| b. Causes affecting iris | 1,307 | 33 | 1,093 | 181 | 2.5 | 83.6 | 13.9 |
| c. Causes affecting lens | 11,769 | 228 | 9,467 | 2,074 | 1.9 | 80.4 | 17.7 |
| d. Other causes | 9,474 | 235 | 8,221 | 1,018 | 2.5 | 86.8 | 10.7 |
| Nervous apparatus affected | 7,944 | 276 | 6,980 | 688 | 3.5 | 87.8 | 8.7 |
| Unclassified | 14,885 | 938 | 12,463 | 1,484 | 6.3 | 83.7 | 10.0 |
| (1) Congenital | 4,728 | 632 | 3,666 | 430 | 13.4 | 77.5 | 9.1 |
| (2) Other causes | 10,157 | 306 | 8,797 | 1,054 | 3.0 | 86.6 | 10.4 |
| Unknown | 8,004 | 313 | 5,740 | 1,951 | 3.9 | 71.7 | 24.4 |
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.
The Irish censuses of 1891 and 1901 give similar data, though not so detailed as in 1871 and 1881. Thus we have in these reports a census inquiry into a phase of the consanguineous marriage problem extending over the period of six successive censal years. Although we can hardly suppose that these figures are accurate in all respects, they throw a great deal of light upon the problem, and are worth quoting in some detail. The tables as given by Mr. Huth contain a number of errors of detail, the correction of which changes the results materially.[[82]]
| TABLE XXIV. | |||||||
|---|---|---|---|---|---|---|---|
| Censal year. | Total population | Congenital deaf-mutes | |||||
| Number | Number per million | Average number to a family[A] | Parents cousins | ||||
| Number | Per cent. | Average number to a family[A] | |||||
| 1851[B] | 6,574,278 | 4,127 | 628 | ---- | 242 | 5.86 | 1.66 |
| 1861 | 5,798,967 | 4,096 | 706 | 1.22 | 362 | 8.84 | 1.72 |
| 1871 | 5,412,377 | 3,503 | 647 | 1.30 | 287 | 7.35 | 1.76 |
| 1881 | 5,174,836 | 3,163 | 611 | 1.32 | 191 | 6.04 | 1.69 |
| 1891 | 4,706,448 | 2,570 | 546 | 1.40 | 297 | 11.56 | 1.92 |
| 1901 | 4,456,546 | 2,179 | 489 | 1.40 | 249 | 11.43 | 1.73 |
| [A] From Table XXV. | |||||||
| [B] 1851 data from Huth, "Consanguineous Marriage and Deaf-mutism." The Lancet, 1900. | |||||||
Table XXIV summarizes the most important points in the Irish data. It will be seen that while there has been an absolute diminution in the number of deaf-mutes in Ireland with the decrease in population, there has been a relative increase of deaf-mutism. There are two possible explanations for this phenomenon, both of which may have operated in part; first that in the great emigration the deaf-mutes have been left behind, and second that with the introduction of improved methods of census taking, the returns are more complete than a half century ago. Mr. Huth believes that there is still room for improvement in Irish census methods, and thinks there is reason to believe that in the enumeration of the deaf all children born deaf in a family are included whether living or not.
Since Ireland is strongly Roman Catholic, the proportion of consanguineous marriages is probably small, so that the percentage of deafmutes derived from consanguineous marriages, varying from 5.86 to 11.56 is very much greater than the percentage of these marriages in the general population. The average number of deaf children to a family in Table XXIV varies less than any other part of the table, and clearly shows a much higher average number of deaf children where the parents were cousins. They reveal the interesting fact that the occurrence of two or more deafmutes in a family is more than twice as probable where the parents are related as where they are not. Table XXV still better illustrates this point. Of the families where there was but one deaf-mute, only 4.3 per cent were the offspring of cousin marriages; where there were two in a family 12.9 per cent were of consanguineous parentage; three in a family, 13.3 per cent; four in a family, 19.0 per cent; more than four in a family, 21.1 per cent.
| TABLE XXV. Number of Congenital Deaf mutes to a Family in Ireland. | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Year. | Parentage. | Families in which deaf-mutes numbered. | ||||||||||
| 1. | 2. | 3. | 4. | 5. | 6. | 7. | 8. | 9. | 10. | 11. | ||
| 1851 | Parents cousins | 127 | 45 | 20 | 10 | 5 | 2 | .. | 1 | .. | .. | .. |
| 1871 | Parents cousins | 91 | 38 | 24 | 5 | 3 | 1 | 1 | .. | .. | .. | .. |
| 1881 | Parents cousins | 63 | 30 | 13 | 6 | 1 | .. | .. | .. | .. | .. | .. |
| 1891 | Parents cousins | 82 | 38 | 19 | 9 | 1 | 3 | 1 | 2 | .. | .. | .. |
| 1901 | Parents cousins | 79 | 34 | 23 | 7 | 1 | .. | 1 | .. | .. | .. | .. |
| 1851 | All families[A] | 2963 | 347 | 158 | 35 | 13 | 5 | .. | 1 | .. | .. | .. |
| 1871 | All families[A] | 2460 | 305 | 167 | 47 | 20 | 5 | 1 | .. | .. | .. | .. |
| 1881 | All families[A] | 2080 | 281 | 162 | 39 | 18 | 6 | .. | .. | .. | 1 | .. |
| 1891 | All families[A] | 1473 | 273 | 134 | 40 | 12 | 6 | 1 | 2 | .. | .. | 1 |
| 1901 | All families[A] | 1219 | 231 | 122 | 34 | 10 | 4 | 2 | .. | .. | .. | .. |
| [A] Number of the "Deaf and Dumb" to a family, "as far as could be ascertained." | ||||||||||||
In 1871 and 1881 the inquiry was more minute and the degrees of consanguinity were specified. Mr. Huth quotes some of the figures for these years, probably derived from the same sources as Table XXVI, and comments as follows: "An examination of this table will show that the statistics so much relied upon as proving the causation of deaf-mutism by consanguineous marriages show nothing of the sort. In 1871 fourth cousins produced more deaf-mutes per marriage than any nearer relationship. In 1881 third cousins produced more than any nearer relationship."[[83]] Mr. Huth forgets that he is basing these statements on five and nine families respectively, and does not take into consideration the probability that if the returns are biased, as he suspects, this bias would affect the more distantly related, relatively more than the first cousin marriages, for the same reason that this would be true of the cases collected by Dr. Bemiss.[[84]] Combining the figures of the two censal years helps to correct these averages, and the distantly related show approximately the same average as the first cousin marriages in spite of the vastly greater selection which must have obtained in the distantly related cases.
In Table XXVI it will be seen that 52.5 per cent of the deaf-mute offspring of consanguineous parents were the offspring of first cousin marriages. On the assumption that this percentage is fairly typical of each set of returns we may say that from three to six per cent of the Irish deaf-mutes are the offspring of first cousin marriages. If, then, the proportion of first cousin marriages is no greater than in England, the percentage of deaf-mute offspring is several times as great as in the average non-related marriage.
| 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.
| TABLE XXVIII. | |||||||
|---|---|---|---|---|---|---|---|
| Cause of Deafness. | Total. | Consanguinity of Parents. | Per cent. | ||||
| Cousins. | Not Cousins. | Not Stated. | Cousins. | Not Cousins. | Not Stated. | ||
| Total | 89,287 | 4,065 | 75,530 | 9,692 | 4.5 | 84.6 | 10.9 |
| Affections of external ear | 871 | 29 | 760 | 82 | 3.3 | 87.3 | 9.4 |
| Affections of middle ear | 34,801 | 1,238 | 30,824 | 2,739 | 3.5 | 88.6 | 7.9 |
| Affections of internal ear | 12,295 | 343 | 11,121 | 831 | 2.8 | 90.4 | 6.8 |
| Unclassified | 31,205 | 2,183 | 25,281 | 3,741 | 7.0 | 81.0 | 12.0 |
| Unknown | 10,115 | 272 | 7,544 | 2,299 | 2.7 | 74.6 | 22.7 |
| Scarlet fever | 7,424 | 285 | 6,647 | 492 | 3.9 | 89.5 | 6.6 |
| Disease of ear | 4,210 | 222 | 3,683 | 305 | 5.3 | 87.5 | 7.2 |
| Catarrh | 11,702 | 304 | 10,450 | 948 | 2.6 | 89.3 | 8.1 |
| Colds | 3,074 | 81 | 2,666 | 327 | 2.6 | 86.7 | 10.7 |
| Meningitis | 3,991 | 83 | 3,741 | 167 | 2.1 | 93.7 | 4.2 |
| Old age | 3,361 | 38 | 2,369 | 954 | 1.1 | 70.5 | 28.4 |
| Military service | 3,242 | 40 | 2,897 | 305 | 1.2 | 89.4 | 9.4 |
| Congenital | 14,472 | 1,710 | 11,322 | 1,440 | 11.8 | 78.2 | 10.0 |
But on the other hand, 53.4 per cent of the deaf whose parents were cousins had deaf relatives of the (a) and (b) groups, while of those whose parents were not cousins, only 29.9 per cent in these groups had deaf relatives. In Table XXIX the close connection between deaf relatives of these groups and consanguinity is shown. For the sake of simplicity no account is taken of (c) relatives (deaf children), and (d) relatives (deaf husbands or wives), for in the first case only 370 deaf are reported as having deaf children and at the same time no (a) or (b) relatives, and in the Second case (d) relatives are not ordinarily blood relatives at all.
| TABLE XXIX. | |||||||
|---|---|---|---|---|---|---|---|
| Class of Deaf Relative.[A] | Total. | Consanguinity of Parents. | Per cent. | ||||
| Cousins. | Not Cousins. | Not Stated. | Cousins. | Not Cousins. | Not Stated. | ||
| Total | 89,287 | 4,065 | 75,530 | 9,692 | 4.5 | 84.6 | 10.6 |
| Stated | 80,481 | 3,911 | 73,639 | 2,931 | 4.9 | 91.5 | 3.6 |
| Not stated | 8,806 | 154 | 1,891 | 6,761 | 1.7 | 21.5 | 76.8 |
| (a) relatives | 21,660 | 1,850 | 18,838 | 972 | 8.5 | 87.0 | 4.5 |
| No (a) relatives | 58,821 | 2,061 | 54,801 | 1,959 | 3.5 | 93.2 | 3.3 |
| (a) or (b) relatives | 25,851 | 2,171 | 22,552 | 1,128 | 8.4 | 87.2 | 4.4 |
| (a) and (b) relatives | 4,117 | 412 | 3,587 | 118 | 10.0 | 87.1 | 2.9 |
| (a) but no (b) relatives | 17,543 | 1,438 | 15,251 | 854 | 8.2 | 86.9 | 4.2 |
| (b) but no (a) relatives | 4,191 | 321 | 3,714 | 156 | 7.7 | 88.6 | 3.7 |
| No (a) or (b) relatives | 54,630 | 1,740 | 51,087 | 1,803 | 3.2 | 93.5 | 3.3 |
| [A] Symbols for deaf relatives: (a) deaf brothers, sisters and ancestors; (b) deaf uncles, aunts, cousins, etc. | |||||||
Table XXIX shows unmistakably that the connection between consanguinity and hereditary deafness is very close. Where there is the largest amount of deafness in the family the percentage of consanguinity is the highest. That is, of those who had both (a) and (b) relatives ten per cent were the offspring of cousins, while of those who had neither (a) nor (b) relatives only three per cent were the offspring of cousins. It is natural to assume that as a rule where the deaf have either (a) or (b) deaf relatives, deafness is hereditary, for the probability of two cases of deafness occurring in the same family, uninfluenced by heredity would be very small. It is likely also that a great many of the deaf who stated that they had no deaf relatives were mistaken, for few people are well enough informed in regard to their ancestry to answer this question definitely. Not one man in thousands can even name all of his great-grandparents, to say nothing of describing their physical or mental traits. Others may have understood the inquiry to refer only to living relatives and therefore have omitted almost all reference to their ancestors. These possible errors might easily explain all the excess of the percentage of consanguinity among those reported as having no deaf relatives over the probable percentage of consanguineous marriage in the general population. But this very probability that comparatively few deaf ancestors have been reported increases the probability that the greater part of the (a) relatives were brothers and sisters rather than ancestors. Now of the 26,221 deaf having deaf relatives, 17,345 have only (a) relatives, and if these are largely living brothers and sisters the relationship would "work both ways," so that if there were two deaf children in a family, each would have an (a) deaf relative. In the Census of Ireland figures above quoted it will be remembered that among families which were the offspring of cousins the proportion having two or more deaf children was three times as great as among those who were not the offspring of consanguineous unions. If this follows in America, it largely accounts for the high percentage of the congenitally deaf who are the offspring of cousin marriages, and especially of those who have (a) deaf relatives.
| TABLE XXX. | |||||||
|---|---|---|---|---|---|---|---|
| Class of Deaf Relative.[A] | Total. | Consanguinity of Parents. | Per cent. | ||||
| Cousins. | Not Cousins. | Not Stated. | Cousins. | Not Cousins. | Not Stated. | ||
| Total | 14,472 | 1,710 | 11,322 | 1,440 | 11.8 | 78.2 | 10.0 |
| Stated | 13,428 | 1,647 | 11,110 | 671 | 12.3 | 82.7 | 5.0 |
| Not stated | 1,044 | 63 | 212 | 769 | 6.0 | 20.3 | 76.7 |
| (a) relatives | 5,295 | 986 | 3,961 | 48 | 18.6 | 74.8 | 6.6 |
| (b) and (c) but no (a) relatives | 860 | 126 | 686 | 48 | 14.6 | 79.8 | 5.6 |
| No (a), (b) or (c) relatives | 7,273 | 535 | 6,463 | 275 | 7.3 | 88.9 | 3.8 |
| [A] Symbols for deaf relatives: (a) deaf brothers, sisters or ancestors; (b) deaf uncles, aunts, cousins, etc.; (c) deaf children. | |||||||
A further analysis of the congenitally deaf according to consanguinity of parents and deaf relatives, as in Table XXX, helps to determine to what extent the greater number of deaf children to a family among the offspring of consanguineous marriages has influenced the totals. From the report it cannot be determined how many of the congenitally deaf had (a), (b) or (c) relatives alone, but the existence of (b) and (c) relatives would almost certainly indicate that the deafness was hereditary. Of these 14.6 per cent were the offspring of cousins, while of those having (a) relatives 18.6 per cent were the offspring of consanguineous unions. Thus it would seem to be a more reasonable conclusion that where two or more deaf-mutes appear in the same family, at least a tendency toward deaf-mutism is hereditary in the family and is intensified by the marriage of cousins, rather than that consanguineous marriage is in itself a cause. The fact that in many cases the relationship would "work both ways" would not greatly affect the percentage of the offspring of cousins having (b) and (c) relatives, for the chance would be slight that the (b) or (c) relative would be himself the offspring of a consanguineous marriage. Among the congenitally deaf who reported no deaf relatives, the percentage of consanguineous parentage is still high, (7.3 per cent), but this excess can easily be accounted for by the ignorance of deaf relatives on the part of the informant, without contradicting the hypothesis of heredity.
Basing now our percentages on the totals of consanguineous and non-consanguineous parentage respectively, and including only those who answered the inquiry as to deaf relatives, it will be seen (Table XXXI) that while of all the deaf less than one third are returned as having deaf relatives, of the deaf who were the offspring of cousins over one half (55.5 per cent) were returned as having (a) or (b) deaf relatives.
Again taking into consideration only the congenitally deaf the results are still more striking. Table XXXII shows that 66.5 per cent of the congenitally deaf who are of consanguineous parentage are known to have deaf relatives.
| TABLE XXXI. | ||||||
|---|---|---|---|---|---|---|
| Class of Deaf Relatives. | Total. | Consanguinity of Parents. | Per cent. | |||
| Cousins | Not Cousins | Total | Cousins | Not Cousins | ||
| Deaf relatives stated | 80,481 | 3,911 | 73,639 | 100.0 | 100.0 | 100.0 |
| (a) relatives | 21,660 | 1,850 | 18,838 | 26.9 | 47.3 | 25.5 |
| No (a) relatives | 58,821 | 2,061 | 54,801 | 73.1 | 52.7 | 74.5 |
| (a) or (b) relatives | 25,851 | 2,171 | 22,552 | 32.1 | 55.5 | 30.6 |
| (a) and (b) relatives | 4,117 | 412 | 3,587 | 5.1 | 10.5 | 4.8 |
| (a) and no (b) relatives | 17,543 | 1,438 | 15,251 | 21.8 | 36.8 | 20.7 |
| (b) and no (a) relatives | 4,191 | 321 | 3,714 | 5.2 | 8.2 | 5.1 |
| No (a) or (b) relatives | 54,630 | 1,740 | 51,087 | 67.9 | 44.5 | 69.4 |
| Symbols for deaf relatives: (a) deaf brothers, sisters or ancestors; (b) deaf uncles, aunts, cousins, etc.; (c) deaf children; (d) deaf husbands or wives. | ||||||
| TABLE XXXII. | ||||||
|---|---|---|---|---|---|---|
| Class of Deaf Relatives. | Total. | Consanguinity of Parents. | Per cent. | |||
| Cousins | Not Cousins | Total | Cousins | Not Cousins | ||
| Deaf relatives stated | 13,428 | 1,647 | 11,110 | 100.0 | 100.0 | 100.0 |
| (a) relatives | 5,295 | 986 | 3,961 | 39.5 | 59.9 | 35.6 |
| (b) or (c), no (a) relatives | 860 | 126 | 686 | 6.4 | 7.6 | 6.2 |
| No (a), (b) or (c) relatives | 7,273 | 535 | 6,463 | 54.2 | 32.5 | 58.2 |
| Symbols for deaf relatives: (a) deaf brothers, sisters or ancestors; (b) deaf uncles, aunts, cousins, etc.; (c) deaf children. | ||||||
The percentage having (a) relatives, including brothers, and sisters, is nearly twice as great among the deaf of consanguineous parentage as among the offspring of unrelated parents. This is not inconsistent with the Irish returns which show the average number of deaf children to a family to be so much greater where the parents were cousins, than where they were not.
The statistics of the (c) relatives, or deaf sons and daughters of the deaf, are not very full. Of the 31,334 married deaf who answered the inquiry in regard to deaf relatives, 437 or 1.4 per cent reported deaf children and 30,897 or 98.6 per cent reported no deaf children. Of the totally deaf 2.4 per cent had deaf children, and of the congenitally deaf 5.0 per cent. The percentage of deaf children varied greatly according to the number and class of deaf relatives, as shown by Table XXXIII.
| TABLE XXXIII. | ||||
|---|---|---|---|---|
| Class of Deaf Relatives. | Percentage having deaf children. | |||
| Total. | Totally deaf. | Partially deaf. | Congenitally deaf. | |
| (a), (b) or (d) | 1.4 | 2.4 | 1.1 | 5.0 |
| (d) | 3.2 | 3.3 | 2.6 | 6.4 |
| No (d) | 1.1 | 1.4 | 1.0 | 2.5 |
| (a) and (d) | 6.3 | 6.7 | 4.3 | 7.8 |
| (d), but no (a) | 2.2 | 2.2 | 2.0 | 4.9 |
| (a), but no (d) | 1.4 | 2.3 | 1.3 | 2.6 |
| No (a) or (d) | 0.9 | 1.0 | 0.9 | 2.3 |
| (a), (b) and (d) | 9.5 | 9.9 | [A] | 9.0 |
| (a), (d), but no (b) | 5.5 | 5.9 | 3.6 | 7.4 |
| (b), (d), but no (a) | 2.5 | 2.4 | [A] | [A] |
| (d), but no (a) or (b) | 2.2 | 2.2 | 2.0 | 5.2 |
| (a), (b), but no (d) | 1.9 | 3.1 | 1.7 | [A] |
| (a), but no (b) or (d) | 1.3 | 2.1 | 1.2 | 2.8 |
| (b), but no (a) or (d) | 1.0 | 1.6 | 1.0 | [A] |
| No (a), (b) or (d) | 0.9 | 1.0 | 0.9 | 2.6 |
| [A] Percentages not given where base is less than 100. | ||||
| Symbols: (a) deaf brothers, sisters or ancestors; (b)deaf uncles, aunts, cousins, etc.; (d) deaf husbands or wives. | ||||
The striking feature of these percentages is the regularity with which they increase in proportion as the number of deaf relatives increases, until among the 242 persons who have (a), (b) and (d) relatives, 23 or 9.5 per cent also have (c) relatives. A consanguineous marriage within a family tainted with deafness would have the same effect as doubling the number of deaf relatives, which as we have seen greatly increases the percentage having deaf children.
It would seem that the number of the married deaf reported as having deaf children is much too small, especially since Dr. Fay[[89]] produces statistics of 4471 marriages of the deaf of which 300 produced deaf offspring. Counting only the 3,078 marriages of which information in regard to offspring was available these figures show an average of a little less than one such marriage in ten as productive of deaf offspring. The total number of children of these marriages was 6,782, of which 588 were deaf. These 3,078 marriages represented 5,199 deaf married persons as compared with the 31,334 reported in the Twelfth Census, or about one sixth. Increasing the 300 families who had deaf children in the same ratio we have 1800 as compared with the 437 reported by the census. But as it was inevitable that Dr. Fay's cases should be selected somewhat, he has probably collected records of more than one sixth of all the cases where deaf children were born of deaf parents. But we can hardly believe that he found three-fourths of such cases. The true number therefore must be considerably greater than 437, but less than 1800.[[90]]
Dr. Fay found that 31 out of the 4,471 marriages of the deaf were consanguineous, but he expresses the belief that the actual number and percentage of consanguineous marriages of the deaf are larger. The following table which combines several of Dr. Fay's tables sets forth the main results of his work. In each instance one or both parties to the marriage were deaf. The totals include only those of whom information as to the offspring was available.
| TABLE XXXIV. | ||||||
|---|---|---|---|---|---|---|
| Consanguineous Marriages of the Deaf. | Number of marriages | Marriages resulting in deaf offspring | Deaf children | |||
| Number | Per Cent | Number | Number Deaf | Per Cent Deaf | ||
| First cousins | 7 | 4 | 57. | 26 | 7 | 27. |
| Second cousins | 5 | 3 | 60. | 25 | 10 | 40. |
| Third cousins | 1 | 1 | -- | 1 | 1 | -- |
| "Cousins" | 14 | 3 | 21. | 36 | 7 | 19. |
| Nephew and aunt | 1 | 1 | -- | 4 | 3 | 75. |
| Distantly related | 3 | 2 | 67. | 8 | 2 | 25. |
| Total consanguineous | 31 | 14 | 45. | 100 | 30 | 30. |
| Not consanguineous, or no information | 3,047 | 286 | 9. | 6,682 | 558 | 8. |
| Grand total | 3,078 | 300 | 10. | 6,782 | 588 | 9. |
Obviously percentages based on these figures are of little value of themselves, especially since Dr. Fay's cases are not entirely typical, but in general this table points us to the same conclusion that we have reached by other means, namely that where a tendency toward deafness exists, a consanguineous marriage is more likely to produce deaf children than a non-consanguineous marriage. If more figures were available the percentage of deaf children would probably increase with the nearness of consanguinity and the number of deaf relatives, but with the present data a further analysis has no significance.[[91]]
If, then, consanguineous marriages where relatives are deaf have a greater probability of producing deaf offspring, and also a greater probability of producing plural deaf offspring, than ordinary marriages, and two thirds of the congenitally deaf offspring of consanguineous marriages do have deaf relatives, it does not seem necessary to look beyond the law of heredity for an explanation of the high percentage of the congenitally deaf who are of consanguineous parentage.
In those cases of deafness which, in the Census returns, are ascribed to specific causes, the factor of consanguinity is still noticeable, although the percentage of the non-congenitally deaf who are the offspring of cousins never exceeds 5.3 (Table XXVIII). But the influence of heredity is not removed by the elimination of the congenitally deaf. Many instances are known where successive generations in the same family have developed deafness in adult life, often at about the same age and from no apparent cause. The following case well illustrates this point. It is furnished me by a correspondent in whom I have great confidence. The facts are these: A—— aged 28 married B—— aged 19, his first cousin who bore the same surname as himself. Both lived to old age and were the parents of eight children, two of whom died in infancy. My informant further states:
Having personally known very well all of the surviving six children of this family, I can truthfully state that all were unusually strong, active and vigorous people and all the parents of healthy children. A—— was troubled with deafness as long as I can remember, and this physical trait he transmitted to all of his children, though some of them did not develop the same till well along in life. C—— (the youngest son), however, began to indicate deafness quite early. No one of his four children is in the least deaf.
It will be noticed here that whereas in the case of the cousin marriage the trait was so strongly inherited, it disappeared entirely in the next generation with a non-consanguineous marriage. The inheritance of tendencies or weaknesses may be more common than the actual inheritance of defects. Dr. Bell's words on this point are suggestive:
Where a tendency toward ear trouble exists in a family, it may lie dormant and unsuspected until some serious illness attacks some member of the family, when the weak spot is revealed and deafness is produced. We are not all built like that wonderful one-horse shay that was so perfectly made in all its parts that when at last it broke down it crumbled into dust. When an accident occurs it is the weak spot that gives way, and it would be incorrect to attribute the damage to the accident alone and ignore the weakness of the part; both undoubtedly are contributing causes.
In the case, then, of a deaf person who has deaf relatives, the assigned cause of deafness may not be the only cause involved, or indeed the true cause at all. It may be the cause simply in the same sense that the pulling of a trigger is the cause of the expulsion of a bullet from a rifle, or a spark the cause of the explosion of a gunpowder magazine; hereditary influences may be involved.[[92]]
It is thus possible to account for the large proportion of deafness among persons of consanguineous parentage by the simple action of the laws of heredity. Why then should we go out of our way to look for a cause of the defect in consanguinity itself? When two explanations are possible, the simpler explanation is the more probable, other factors being equal; but in the present problem the factors are not equal, for the evidence points strongly toward the simpler hypothesis of intensified heredity, while there is little or no evidence that consanguinity is a cause per se.
As to the probability then of a consanguineous marriage producing deaf offspring, it will readily be seen to be very slight, and in those cases where there is actually no trace of hereditary deafness in the family, perhaps no greater than in non-related marriages. While the census figures in regard to the deaf are not complete they probably include a great majority of the deaf in the United States. The 89,287 deaf would mean an average of 12 deaf persons to every 10,000 inhabitants and the 14,472 congenitally deaf, 2 persons to every 10,000. Assuming then, as before[[93]] that 1,000,000 persons in continental United States are the offspring of consanguineous marriages within the limits of the term "cousins" as used in the Census report, 41 out of every 10,000 persons of consanguineous parentage would be deaf, and 17 congenitally so. Thus less than one half of one per cent of the offspring of consanguineous marriages in the United States are deaf, and only one sixth of one per cent are deaf-mutes in the commonly accepted sense of the term.
It is interesting here to quote an opinion given by Dr. Bell in 1891, as to the probable results of the consanguineous marriage of deaf persons.[[94]]
1. A deaf person, not born deaf, who has no deaf relatives, will probably not increase his liability to have deaf offspring by marrying a blood relative.
2. A deaf person, born deaf, who has no deaf relatives, will probably increase his liability to have deaf offspring by marrying a blood relative.
3. A deaf person, whether born deaf or not, who has deaf relatives, will probably increase his liability to have deaf offspring by marrying a blood relative, especially if that relative should happen to be on the deaf side of the family. For example: If his father has deaf relatives and his mother has none, he will be more likely to have deaf offspring if he marries a relative of his father than if he marries a relative of his mother.
The laws of heredity seem to indicate that a consanguineous marriage increases or intensifies in the offspring whatever peculiarities exist in the family. If a family is characterized by the large proportion of persons who enjoy good health and live to old age with unimpaired faculties, then a consanguineous marriage in such a family would probably be beneficial, by increasing and intensifying these desirable characteristics in the offspring. On the other hand, if a large proportion of the members of a family betray weakness of constitution—for example: if many of the children die in infancy, and a large proportion of the others suffer from ill health, only a few living to old age with unimpaired faculties—then a consanguineous marriage in such a family would probably be hurtful to the offspring. A large proportion of the children would probably die in infancy, and the survivors be subject to some form of constitutional weakness.
As there are few families entirely free from constitutional defects of some kind, a prudent person would do well to avoid consanguineous marriage in any case—not necessarily on account of deafness, but on account of the danger of weakening the constitution of the offspring. Remoteness of blood is eminently favorable to the production of vigorous offspring, and those deaf persons who have many deaf relatives would greatly diminish their liability to have deaf offspring by marrying persons very remote in blood from themselves.
Children, I think, tend to revert to the type of the common ancestors of their parents. If the nearest common ancestors are very far back in the line of ancestry, the children tend to revert to the common type of the race. Deafness and other defects would be most likely to disappear from a family by marriage with a person of different nationality. English, Irish, Scotch, German, Scandinavian and Russian blood seems to mingle beneficially with the Anglo-Saxon American, apparently producing increased vigor in the offspring.