Consanguineous Marriages in the American Population Part 6

[Footnote 74: _Shattuck Memorials_, p. 118.]

CHAPTER VI

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.

[Footnote 75: U.S. Census, 1900, _Special Report on the Blind and the Deaf_.]

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 relations.h.i.+p 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.

[Footnote 76: U.S. Census, 1900, op. cit., p. 16.]

TABLE XXII.

----------------------------------------------------------------- | |Having |Having | | | |Blind |blind | | | |relatives|relatives|Having | Consanguinity of | |Cla.s.s |Cla.s.s |no blind | Not Parents. |Totals|(a).[A] |(b).[A] |relatives|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 relations.h.i.+p "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 cla.s.s (a), and 5.4 per cent blind relatives of cla.s.s (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 cla.s.s (a), and 4.3 per cent relatives of cla.s.s (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 cla.s.sification 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.

------------------------------------------------------------------------------- | |Consanguinity of | | | Parents | Percentages | |--------------------------------------------- | | | Not | Not | | Not | Not Cause of Blindness. |Total.|Cousins|cousins|stated|Cousins|cousins|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 ------------------------------------------------------------------------------- Uncla.s.sified |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]

[Footnote 77: U.S. Census, 1900, op. cit., p. 17.]

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.

[Footnote 78: _Effect of Consanguinity upon the Organs of Special Sense_, p. 4.]

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.

[Footnote 79: _Der Einfluss der Blutsverwandschaft der Eltern auf die Kinder_, p. 14.]

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.

[Footnote 80: From 1-1/2 to 2 per cent of all marriages were found to be between cousins within the degree of second cousins, and cousin marriages were found to be normally fertile.]

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.

[Footnote 81: _Marriage of Near Kin_, p. 229.]

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]

[Footnote 82: In a subsequent article Mr. Huth corrects some of these errors. See: "Consanguineous Marriage and Deaf-mutism," _The Lancet_, Feb. 10, 1900.]

TABLE XXIV.

------------------------------------------------------------------ | | Congenital deaf-mutes | |----------------------------------------------- | | | | | Parents cousins | | | | |---------------------- | | | |Average | | |Average | | |Number |number | | |number Censal | Total | |per | to a | |Per | to a year. |population|Number|million|family[A]|Number|cent.|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 ill.u.s.trates 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._ ---------------------------------------------------------------- | | Families in which deaf-mutes numbered.

| |---------------------------------------- Year.| Parentage. | 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|..|..|.. |..