Consanguineous Marriages in the American Population by George B. Louis Arner

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.
——————————————————————————- | |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 ——————————————————————————- 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]

[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 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._ —————————————————————- | | 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|..|..|.. |.. 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.

[Footnote 83: Huth, _Marriage of Near Kin_, p. 227.]

[Footnote 84: _Cf. supra_, p. 42.]

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.
————————————————————————– | 1871 | 1881 | 1871 and 1881 |———————————————————– | |Number| | |Number| | |Number| | | of |Aver | | of |Aver | | of |Aver |Number|conge-|age |Number|conge-|age |Number|conge-|age Consanguinity | of |nital |per | of |nital |per | of |nital |per of |mar- |deaf- |mar- |mar- |deaf- |mar- |mar- |deaf- |mar- Parents. |riages|mutes |riage|riages|mutes |riage|riages|mutes |riage ————————————————————————– 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 | | | | | | | | | relation- | | | | | | | | | ship[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.

[Footnote 85: Huth, op. cit., p. 226.]

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]

[Footnote 86: _Les Sourds-muets en Norvege_. Quoted by Feer, _Der Einfluss der Blutsverwandschaft der Eltern auf die Kinder_, p. 22.]

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]

[Footnote 87: Feer, op. cit., p. 22.]

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]

[Footnote 88: U.S. Census _Report on the Blind and the Deaf_, p. 127.]

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.
————————————————————————- | | |
| | Consanguinty | | | of Parents. | Per cent. | |——————–|—————– | | | | | | |
Cause of Deafness. | | | Not | Not | | Not | Not |Total.|Cous- |Cous- | Sta- |Cous-|Cous-| Sta- | | ins. | ins. | ted. | ins.| ins.| ted. —————————|——|——|——|——|—–|—–|—– | | | | | | |
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.
———————————————————————– | | |
| | Consanguinty | | | of Parents. | Per cent. | |——————–|—————– | | | | | | |
Class of Deaf | | | Not | Not | | Not | Not Relative.[A] |Total.| Cous-| Cous-| Sta- |Cous-|Cous-| Sta- | | ins.| ins.| ted. | ins.| ins.| ted. ———————————————————————– | | | | | | |
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.
———————————————————————— | | Consanguinity |
| | of Parents. | Per Cent. | |———————————————- Class of Deaf | | | Not | Not | | Not | Not Relatives.[A] |Total. |Cousins|Cousins|stated |Cousins|Cousins|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.
———————————————————————- | |Consanguinity |
| | of Parents. | Per cent. | |————————————- | | |Not | | |Not Class of Deaf Relatives. |Total.|Cousins|Cousins|Total|Cousins|Cousins ———————————————————————- Deaf relatives stated |80,481| 3,911 |73,639 |100.0|100.00 |100.00 ———————————————————————- (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.
————————————————————————- | |Consanguinity |
| |of Parents. | Per cent. | |————————————- | | |Not | | |Not Class of Deaf Relatives |Total.|Cousins|Cousins|Total|Cousins|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.
——————————————————————- | Percentage having deaf children. |————————————— Class of Deaf Relatives. | |Totally |Partially |Congenitally |Total.|deaf. |deaf. |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]

[Footnote 89: _Marriages of the Deaf in America_, chap. v.]

[Footnote 90: Of the 17 children of first cousins reported on my circulars as either totally or partially deaf, 9 are known to have had deaf ancestors.]

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.
———————————————————- | | Marriages |
| | resulting |
| | in deaf |
| | offspring | Deaf children |Number|———–|—————— Consanguineous |of | | | |Per Marriages |mar- | |Per | |Number|Cent of the Deaf. |riages|Number|Cent|Number|Deaf |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]

[Footnote 91: Mr. Edgar Schuster (_Biometrika_, vol. iv, p. 465) finds from Dr. Fay’s statistics that the average parental correlation (parent and child) of deafness is: paternal, .54; maternal, .535. English statistics of deafness give: paternal correlation, .515; maternal, .535. The fraternal correlation from the American data is .74 and from the English .70. See _infra_, p. 92.]

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]

[Footnote 92: U.S. Census _Report on the Blind and the Deaf_, p. 127.]

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.

[Footnote 93: _Supra_, p. 64.]

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]

[Footnote 94: _Marriage–An Address to the Deaf_, second edition, Appendix.]

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.

CHAPTER VII

SUMMARY AND CONCLUSION

Having thus considered the more important problems which have been connected with the marriage of near kin, we have only to discuss the bearing of the conclusions thus formed upon the social aggregate, and the effect which consanguineous marriages have upon the evolution and improvement of the human species.

It has been shown that the frequency with which consanguineous marriages occur varies greatly with the physical and social environments; that such marriages are more frequent in isolated and in rural communities than in cities; and that with the increasing range of individual activity and acquaintance the relative frequency of consanguineous marriage is decreasing.

Consanguinity in the parents has no perceptible influence upon the number of children or upon their masculinity, and has little, if any, direct effect upon the physical or mental condition of the offspring.

The most important physiological effect of consanguineous marriage is to intensify any or all inheritable family characteristics or peculiarities by double inheritance. The degree of intensification probably varies with the nature of the characteristic; degenerate conditions of the mind, and of the delicate organs of special sense being the most strongly intensified.

It is probable also that in the absence of degenerative tendencies the higher qualities of mind and body are similarly intensified by marriage between highly endowed members of the same family. Dr. Reibmayr believes that inbreeding is necessary to the higher evolution of the race: “A settled abode, natural protection from race mixture and the development of a closely inbred social class are the basic conditions of every culture period.” But inbreeding must not be carried too far: “In the course of generations the ruling class begins to degenerate mentally and physically, until not only is the class destroyed, but for lack of capable leadership the people (Volk) itself is subjugated and a crossing of blood again takes place.”[95]

[Footnote 95: Trans. from _Insucht und Vermischung beim Menschen_, p. 46.]

In the breeding of animals the closest inbreeding is frequently resorted to in order to improve the stock, and many examples can be given of the closest possible inbreeding for generations without apparent detriment, but it is universally admitted that the animals selected for such inbreeding must be sound constitutionally, and free from disease. After a certain number of generations however, degeneration apparently sets in. The number of generations through which inbreeding may be carried varies with the species, and the purpose for which the animals are bred. Where they are bred primarily for their flesh, as for beef, mutton or pork, it can be pursued farther and closer than where they are bred for achievement in which a special strength is required–for instance in the breeding of race horses. This would indicate that the more delicate brain and nervous system is sooner affected than the lower bodily functions.

In man, however, freedom from hereditary taint cannot so easily be secured. Individuals cannot be selected scientifically for breeding purposes. Furthermore, the human body is more delicately constructed than that of the lower animals, and the nervous system is more highly developed and specialized, so that it is reasonable to suppose that in man degeneration would set in earlier in the process of inbreeding, and that it would be impossible to breed as closely as with the lower animals. Instances are well known, however, where incestuous unions have been productive of healthy offspring, and successive generations of offspring of incestuous connection are not unknown; but, although statistics are lacking, it seems to be very often true that children of such unions are degenerate. It may be that the reason for this is that with the laws and social sentiments now prevailing in all civilized communities, only degenerates ever contract incestuous alliances. Desirable as it may be from a social point of view that this strong sentiment against incest should continue, it is not yet _proven_ that even the closest blood relationship between the parents is directly injurious to the offspring. The “instinctive horror of incest” is a myth, for although a horror of incest does very properly exist in civilized, and in some tribal societies, it is purely a matter of custom and education, and not at all a universal law.

Double heredity may account for all the observed ill effects of consanguineous marriage, including the high youthful death-rate, the higher percentage of idiocy, deafness and blindness, and probably also the scrofulous and other degenerate tendencies; nevertheless, there may be in some instances a lowering of vitality which this hypothesis does not fully explain.

The tendency of inbreeding in animals, it is well known, is to fix the type, the tendency of crossing, to variation. Inbreeding then, tends to become simple repetition with no natural variations in any direction, a stagnation which in itself would indicate a comparatively low vitality. Variation and consequent selection is necessary to progress. “Sex,” according to Ward[96] “is a device for keeping up a difference of potential,” and its object is not primarily reproduction, but variation.[97]

[Footnote 96: _Pure Sociology_, p. 232.]

[Footnote 97: Pearson (_Grammar of Science_, p. 373) points out that variation does occur in asexual reproduction. But that sex is at least a powerful stimulus to variation can hardly be questioned.]

It is organic differentiation, higher life, progress, evolution…. But difference of potential is a social as well as a physiological and physical principle, and perhaps we shall find the easiest transition from the physiological to the social in viewing the deteriorating effects of close inbreeding from the standpoint of the environment instead of from that of the organism. A long-continued uniform environment is more deteriorating than similarity of blood. Persons who remain for their whole lives, and their descendants after them, in the same spot, surrounded by precisely the same conditions, and intermarry with others doing the same, and who continue this for a series of generations, deteriorate mentally at least, and probably also physically, although there may not be any mixing of blood. Their whole lives, physical, mental, and moral, become fixed and monotonous, and the partners chosen for continuing the race have nothing new to add to each other’s stock. There is no variation of the social monotony, and the result is socially the same as close consanguineal interbreeding. On the other hand, a case in which a man should, without knowing it, marry his own sister, after they had been long separated and living under widely different skies, would probably entail no special deterioration, and their different conditions of life would have produced practically the same effect as if they were not related.[98]

[Footnote 98: Ward, op. cit., pp. 234-235.]

Professor Ward’s idea of “difference of potential,” or contrast, as essential to the highest vigor of the race as well as to that of the individual offspring, offers an alternative explanation of the observed results of consanguineous marriages, and one which does not necessarily conflict with the explanation already given. All the phenomena of intensification are simply due to a resemblance between husband and wife in particular characteristics, such as a common tendency toward deafness or toward mental weakness. This resemblance, which may or may not be the result of a common descent, renders more probable the appearance of the trait in the offspring. If the parents closely resembled each other in many respects they would be more likely to “breed true” and the children would resemble one another in their inherited traits, thus accounting for the high average of deaf-mutes to the family, observed in the Irish statistics.[99]

[Footnote 99: _Cf. supra_, p. 66.]

The theory of contrast and resemblance supplements that of intensified heredity where the resemblance is general, rather than in particular traits or characteristics. In such a case the absence of the stimulating effects of contrast might result in a lowering of vitality, which in turn would react upon the youthful death-rate.

Where then related persons differ greatly in mental and physical traits, and generally speaking, belong to different types, it is very improbable that there would be any ill effects resulting from the mere fact of consanguinity. A case in point is furnished me by a correspondent. A first cousin marriage which turned out exceedingly well was between strongly contrasted individuals; the husband was “short, stocky and dark complexioned” while the wife was “tall, slight of figure, and of exceedingly light complexion.” In other cases in which the results were not so good the husband and wife bore a close resemblance to one another, physically and mentally.

This, however, does not agree with the results obtained by Professor Karl Pearson. Basing his conclusions on the correlation of stature between husband and wife, he believes that homogamy is a factor of fertility. Taking 205 marriages from Mr. Francis Galton’s _Family Records_, Professor Pearson found the correlation between husband and wife to be .0931 +- .0467, while weighted by their fertility the correlation was .1783 +- .0210, practically doubling the intensity of assortative mating.[100] The value of these correlations, however, is impaired, as he says, by the insufficient number of observations, and by the fact that absolutely taller mothers are the more fertile.

[Footnote 100: _Royal Society Proceedings_, vol. 66, p. 30.]

In a subsequent investigation of from 1000 to 1050 pairs of parents of adult children, Professor Pearson found the correlation in stature to be .2804 +- .0189; of span .1989 +- .0204; and of forearm .1977 +- .0205; with cross coefficients varying from .1403 to .2023. If, as he believes, “The parents of adult children are on the average more alike than first cousins, then it follows that any evils which may flow from first cousin marriage depend not on likeness of characters, but on sameness of stock.”[101]

[Footnote 101: _Biometrika_, vol. ii, p. 373.]

But even if it were true, as is very improbable, that parents of adult children are more alike than first cousins, it would still be likely to follow that first cousins who married would be more alike than first cousins in general. A certain degree of resemblance is undoubtedly necessary to complete fertility: husband and wife must be physically compatible, and must both enjoy a certain degree of health and physical strength. These facts are admitted by all, but it does not follow that resemblance beyond a certain point is not in itself detrimental.

Professor Pearson’s own experiments in this line, however, do not give consistent results, for in correlating eyecolor with fertility, heterogamy seems to increase fertility. The highest average fertility (4.57) is in those cases where the father is dark-eyed and the mother light-eyed, while the lowest is where both parents have blue-green or gray eyes.[102]

[Footnote 102: _Phil. Trans. of the Royal Society_, vol. 195 A, p. 150.]

In a recent study an attempt has been made to measure the coefficient of correlation between cousins.[103] In the characteristics of health, success, temper and intelligence the coefficients ranged between .25 and .30. These values differ but little from those found to obtain for the resemblance between avuncular relatives for eye color (.265), or between grandparent and grandchild for the same characteristic (.3164).[104] Positive results were also found, with one doubtful exception, for the occurrence of insanity and tuberculosis in cousins. The writer concludes: “The grandparent, the uncle and aunt, and the cousin are on practically the same footing with regard to relationship or intensity of kinship as measured by degree of likeness of character; and it seems probable that any scientific marriage enactments would equally allow or equally forbid marriage between grandparent and grandchild, uncle and niece, aunt and nephew, and between first cousins.”[105]

[Footnote 103: Elderton and Pearson, “On the Measure of the Resemblance of First Cousins.” _Eugenics Laboratory Memoirs IV._ Reviewed in _Br. Med. Journal_, Feb. 15, 1908.]

[Footnote 104: _Phil. Trans. of the Royal Society_, vol. 195 A, p. 106.]

[Footnote 105: Elderton and Pearson, op. cit.]

As we should expect the resemblance between near relatives has been found to be much greater. From a measurement of from 4000 to 4886 pairs, the average correlation of the characteristics of stature, span, forearm length and eyecolor between parent and child was .4695. By similar computations and measuring the same characteristics, the fraternal correlation was found to be .508.[106] From measurements of a greater variety of characteristics in school children the mean fraternal correlation was .539.[107] In athletic power the coefficient was still higher, .72 between brothers, .75 between sisters and .49 between brothers and sisters. Measurements of mental characteristics–vivacity, assertiveness, introspection, popularity, conscientiousness, temper, ability and handwriting proved to be as easily correlated, the mean coefficients being; brothers, .52, sisters .51, brothers and sisters .52.[108]

[Footnote 106: Pearson and Lee, “On the Laws of Inheritance in Man,” _Biometrika_, vol. ii, p. 387.]

[Footnote 107: Ibid., p. 388.]

[Footnote 108: Pearson, “On the Laws of Inheritance in Man,” part 2, _Biometrika_, vol. iii, p. 154.]

The relative amount of degeneracy and disease among the offspring of consanguineous marriages has been enormously exaggerated, and the danger is by no means as great as is popularly supposed. Nevertheless, since it is undoubtedly true that on the average such marriages do not produce quite as healthy offspring as do non-consanguineous unions, and since public sentiment is already opposed to the marriage of cousins, it is perhaps just as well that existing laws on the subject should remain in force. From the standpoint of eugenics however, it is much more important that the marriage of persons affected with hereditary disease should be prevented. Dr. Bell has pointed out the danger of producing a deaf-mute race by the intermarriage of congenitally deaf persons,[109] and this warning should be made to apply to other congenital defects as well. Some states already prohibit the marriage of the mentally defective, and persons under the influence of intoxicants. Such provisions are wise, and are the most practical means of achieving eugenic ideals–by preventing the propagation of the unfit. The interests of society demand that the mentally and physically defective should not propagate their kind.

[Footnote 109: “Memoir upon the Formation of a Deaf Variety of the Human Race.” _Memoirs of the National Academy of Sciences_, vol. ii, pp. 177-262.]

From the broader viewpoint of social evolution the problems of inbreeding or crossing of stocks merge into the discussion of the endogamous and exogamous types of society. Whatever may have been the origin of exogamy, the survival of the exogamous type in progressive societies may easily be explained on the ground of superior adaptability, variability and plasticity, which enables such societies to survive a change of environment while the more rigid structure of the endogamous clan brings about its extermination.

Inbreeding leads to caste formation and a rigid and stratified social structure, which is in the end self-destructive, and cannot survive a change of environment. The governing caste may, as Reibmayr says, favor the growth of culture, but it is usually the culture of that caste, and not of the people at large. The ruling caste is usually the result of selection of the strongest and ablest, but after it becomes a caste, the individuals are selected on account of hereditary social position and not primarily on account of ability. Now biological experiments show that although artificial selection may be carried to a point where animals will breed true to a characteristic to within 90 per cent, yet if selection is stopped, and the descendants of the selected individuals are allowed to breed freely among themselves, they will in a very few generations revert to the original type. This is what happens in a social caste, unless, as in the case of the English aristocracy, it is continually renewed by selection of the ablest of the other classes.

The superposition and crossing of cultures, the development of secondary civilization, is necessary to social evolution in its broadest sense, and this usually involves crossing of blood as well as crossing of cultures. As a result of the unprecedented migrations of the last half-century we have in the United States the greatest variety of social types ever brought so closely together. An opportunity is offered either for the perpetuation of each racial type by inbreeding, with the prospect of an indefinite stratification of society, or for the amalgamation of all cultural and racial elements into a homogeneous whole, and the development of a race more versatile and adaptable than any the world has yet known. The general tendency will undoubtedly be toward amalgamation, but there are decided tendencies in the other direction, as for instance in the “first families of Virginia,” and in that large element of the New England population which prides itself upon its exclusively Puritan ancestry, and which has inherited from its progenitors that intolerance which characterized the early settlers of New England more than the pioneers of the other colonies. The dynamic forces of modern civilization are, however, opposed to caste–the West has long ago obliterated the distinction between the Pennsylvania German and the Puritan, the Scotch-Irish and the Knickerbocker Dutch. These same dynamic forces, which have prevented the formation of caste have at the same time been diminishing the percentage of consanguineous marriage and will undoubtedly continue to operate in the same way for some time to come. And when rational laws prohibit the marriage of the diseased and the degenerate, the problem of consanguineous marriage will cease to be of vital importance.

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GENEALOGIES CONSULTED.

* * * * *

Family. Author. Place and Date of Publication. —————————————————————– Augur, E.P. Augur, Middletown, Conn. 1904. Banta, T.M. Banta, New York, 1893. Bent, A.H. Bent, Boston, 1900.
Bolton, H.C. and R.P. Bolton, New York, 1895. Champion, F.B. Trowbridge, New Haven, 1891. Dewey, L.M. Dewey, Westfield, Mass., 1898. Faxon, G.L. Faxon, Springfield, Mass., 1880. Foster, F.C. Pierce, Chicago, 1899. Gates, C.O. Gates, New York, 1898. Giddings, M.S. Giddings, Hartford, 1882. Goodwin, J.J. Goodwin, Hartford, 1891. Hurlbut, H.H. Hurlbut, Albany, 1888. Kneeland, S.F. Kneeland, New York, 1897. Lee, E.J. Lee, Philadelphia, 1895. Mather, H.E. Mather, Hartford, 1890. Mead, S.P. Mead, New York, 1901. Potts, T.M. Potts, Canonsburg, Pa., 1901. Shattuck, L. Shattuck Boston, 1855. Tenney, M.J. Tenney, Boston, 1891. Udall, G.B.L. Arner, In _Genealogical Exchange_, Buffalo 1904-5.
Varnum, J.M. Varnum, Boston, 1907. Wood, C.W. Holmes, Elmira, 1901.

Consanguineous Marriages in the American Population by George B. Louis Arner (page 2)