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INTRODUCTION

Problems to be Treated--Degrees of Consanguinity--Literature of the Subject--Noah Webster--Bemiss--Dally--G.H. Darwin--Huth--Bell--Legal Status in the United States--Methods of Investigation--Genealogical--Personal--Isolated Communities

RATIO OF THE CONSANGUINEOUS TO ALL MARRIAGES

Previous Estimates--Mayo-Smith--Mulhall--Darwin--Application of Darwin's Method to American Data--Direct Method--Consanguineal Attraction--Same-name and Different-name Cousin Marriages--Summary

MASCULINITY

Constancy of the Sex-ratio--Consanguinity and Masculinity--Theory of Westermarck and Thomas--Duesing--Gache--Negroes in the United States--Genealogical Material--Other Compilations--Summary

CONSANGUINITY AND REPRODUCTION

CONSANGUINITY AND MENTAL DEFECT

Idiocy and Insanity--Inheritability of Mental Defect--Intensified Heredity--Barr's Investigations--Other American and English Data--Mayet's Prussian Statistics--Genealogical Data

CONSANGUINITY AND THE SPECIAL SENSES

United States Census Data--The Blind--Consanguinity of Parents--Blind Relatives--Degree of Blindness--Causes of Blindness--Retinitis Pigmentosa--European Data--Probability of Blind Offspring of Consanguineous Marriages--The Deaf--Irish Census--Scotland and Norway--United States Census--Consanguinity of Parents--Deaf Relatives--Causes of Deafness--Degree of Deafness--Direct Inheritance of Deafness--Intensification through Consanguinity--Dr. Fay's Statistics--Personal Data--Probability of Deaf Offspring from Consanguineous Marriages--Opinion of Dr. Bell

SUMMARY AND CONCLUSION

Summary of Results--Inbreeding and Evolution--Effects of Close Inbreeding--Crossing and Variation--"Difference of Potential"--Resemblance and Intensification--Coefficient of Correlation between Husband and Wife--Between Cousins--Between Brothers and Sisters--Consanguinity and Eugenics--Consanguinity and Social Evolution--Conclusion

INTRODUCTION

The purpose of this essay is to present in a concise form and without bias or prejudice, the most important facts in regard to consanguineous marriages, their effects upon society, and more particularly their bearing upon American social evolution. The problems to be considered are not only those which relate primarily to the individual and secondarily to the race, such as the supposed effect of blood relationship in the parents upon the health and condition of the offspring; but also the effect, if any, which such marriages have upon the birth-rate, upon the proportion of the sexes at birth, and the most fundamental problem of all, the relative frequency with which consanguineous marriages take place in a given community.

No thorough and systematic study of the subject has ever been made, and could not be made except through the agency of the census. The statistical material here brought together is fragmentary and not entirely satisfactory, but it is sufficient upon which to base some generalizations of scientific value. The sources of these data are largely American. Little attempt is made to study European material, or to discuss phases of the problem which are only of local concern. Some topics, therefore, which have frequently been treated in connection with the general subject of consanguineous marriages are here ignored as having no scientific interest, as for instance that of the so-called "marriages of affinity," which has been so warmly debated for the past fifty years in the British Parliament.

For obvious reasons it will often be impossible to distinguish between the different degrees of consanguinity, but wherever possible the degree will be specified. It is probable that where a number of marriages are vaguely given as consanguineous, few are more distant than second cousins, for in the United States especially, distant relationships are rarely traced except by genealogists. In designating degrees of relationship the common terminology will be used, as in the following table, expressing, however, the rather clumsy expression, "first cousin once removed" by the simpler form "1-1/2 cousin."

Much space is given to the alleged "innate horror of incest," and frequent appeals are made to Scripture, wrongly assuming that the marriage of cousins is prohibited in the Mosaic Law.

The origin of "prohibited degrees" is only conjectural. The Christian Church apparently borrowed its prohibitory canons from the Roman Law, and a dispensation is still necessary before a Catholic can marry his first cousin. However, such dispensations have always been easy to obtain, especially by royal families, and even the marriage of uncle and niece sometimes occurs, as among the Spanish Habsburgs, and as recently as 1889 in the House of Savoy.

The prohibition of the marriage of first cousins was removed in England by the Marriage Act of 1540, but by this time the idea of the harmfulness of kinship marriage was so thoroughly impressed upon the people that they were very prone to look askance at such unions, and if they were followed by any defective progeny, the fact would be noted, and looked upon as a chastisement visited upon the parents for their sin. Naturally the idea became proverbial, and in some places it has influenced the civil law.

Perhaps the first printed discussion of the subject in America is from the pen of Noah Webster, in an essay which should be as interesting to the spelling reformer as to the sociologist. He writes: "It iz no crime for brothers and sisters to intermarry, except the fatal consequences to society; for were it generally practised, men would become a race of pigmies. It iz no crime for brothers' and sisters' children to intermarry, and this iz often practised; but such near blood connections often produce imperfect children. The common peeple hav hence drawn an argument to proov such connections criminal; considering weakness, sickness and deformity in the offspring az judgements upon the parents. Superstition iz often awake when reezon iz asleep."

From about 1855 to 1880 much was written about the effect of consanguineal interbreeding. One of the first contributions came from America. In 1858 Dr. S.M. Bemiss, of Louisville, Kentucky, reported to the American Medical Association the results of his investigation of 833 cases of consanguineous marriage. His compilation remains to this day the largest single piece of direct statistical work on the subject. Unfortunately, however, his statistics have a strong, if unintentional, bias which seriously affects their value. In France one of the earliest discussions was by M. Boudin, who evidently obtained the Bemiss report , and enlarged greatly upon its fallacies. He also collected statistics of the deaf-mutes in Paris, and, by an amazing manipulation of figures, "demonstrated" that consanguinity of the parents was the cause of nearly one-third of the cases of congenital deafness. The savants of the Soci?t? d'Anthropologie took sides and the debate became very entertaining. Finally M. Dally came to the rescue, and published some very sane and logical articles which avoided both extremes, and first advanced the theory that any ill effects of consanguineous marriage should be attributed to the intensification of inherited characteristics.

In England similar discussions took place during the same period, complicated, however, by the presence of the patient and long-suffering "deceased wife's sister." The best of the English work has been the statistical study by George H. Darwin, and the classic "Marriage of Near Kin" by Alfred H. Huth, a book of 475 pages, including a very complete bibliography to the date of the second edition, 1885. Although Mr. Huth's book is not free from error, and is encumbered with a large amount of worthless material, it is now after thirty-three years, by far the best treatment of the subject.

In Italy Dr. Montegazza, in Spain Se?or Pastor and others, have made useful contributions. German writers have usually preferred more general subjects, but many of them have given much space to consanguineous marriage in sociological and biological works.

These laws probably have some effect in reducing the number of consanguineous marriages in these states, but the sentiment back of the law is more responsible for the decrease in the number of such unions than the law itself. For in the nature of things enforcement would be very difficult, and apparently little real effort is made in that direction. In Ohio, and probably elsewhere, the question as to consanguinity is not directly put to the applicants for a marriage license. The applicants are required to answer the usual questions in regard to age, parentage, residence, etc., and are then required to swear that their previous statements have been correct and that neither of them is "epileptic, imbecile or insane," that they are "not nearer of kin than second cousins, and not at the time under the influence of any intoxicating liquor or narcotic drug." Undoubtedly violations of the consanguinity clause are very frequent, and it is likewise easily evaded by going to another state where the laws are more liberal. One effect of the law is to provide a painless method of severing the marriage bond. A correspondent, who is a District Court Judge in Kansas, in reporting a case of first cousin marriage, adds that he "divorced them on the ground of consanguinity."

In the absence of direct investigation by the Census Bureau, or other public records of consanguineous marriages, perhaps the most promising field for research is in the genealogical records of American families. Several thousand volumes of such material have been published within the last half-century, and a large number of these are very carefully and scientifically prepared. The material gathered from such sources is very accurate in regard to the number of births, youthful deathrate etc., but mental or physical defects are rarely mentioned. The greatest objection to the utilization of this material, however, is the amount of labor necessary in order to glean the desired facts from the mass of irrelevant data. For example, in order to find one case of first cousin marriage it is necessary on an average, to examine the records of nearly two hundred other marriages.

The collection of data from personal sources is likewise open to grave objections. Not only is the informant likely to be biassed, but the cases which he will remember will be those in which something unusual has occurred. Herein lay the fallacy in the conclusions of Dr. Bemiss. I have endeavored to overcome this bias by restricting my requests for information to genealogists and others who would more naturally appeal to records, but my efforts have been only partially successful.

The number of cases of consanguineous marriage, embracing all degrees of consanguinity, which I have collected from these two sources, genealogies and correspondence, is 723, a number too small in itself to establish any definite conclusions; but by using this material in connection with other related data, I trust I may be able to add something to the comparatively small amount of real knowledge which the world already possesses in regard to the marriage of kin.

In the course of my investigations I visited Smith's Island, in the Chesapeake Bay, about twelve miles across Tangier Sound, from Crisfield, Maryland, and nearly opposite the mouth of the Potomac. Here is a community of about seven hundred people, who are principally engaged in the sea-food industry. Their ancestors have lived on the island for many generations and there have been comparatively few accessions to the population from the mainland. As a natural consequence the population is largely a genetic aggregation. Consanguineous marriages have been very frequent, until now nearly all are more or less interrelated. Out of a hundred or more families of which I obtained some record, at least five marriages were between first cousins. All of these were fertile, and all the children were living and apparently healthy. Since over thirty per cent of the inhabitants bear one surname , and those bearing the first four surnames in point of frequency comprise about fifty-nine per cent of the population, it will readily be seen that comparatively few absolutely non-related marriages take place. Yet in this community from September, 1904, to October, 1907, or during the residence there of the present physician, Dr. P.H. Tawes, there have been 87 births and but 30 deaths, the latter from the usual causes. During this period there has not been a single case of idiocy, insanity, epilepsy, deaf-mutism or even of typhoid fever on the island.

The evidence gathered from various other isolated communities is very conflicting. Huth describes a great many of them which have existed for many generations without crosses without ill results. Other writers quote instances where whole communities have become degenerate. Until the antecedents of a community are known it is of course impossible to estimate the effect of consanguinity. The exceptionally high percentage of deaf-mutism on Martha's Vineyard may to some extent be due to a high percentage of consanguineous marriage, but that inbreeding is not the primary cause is revealed by the records showing that among the first settlers were two deaf-mutes, whose defect has been inherited from generation to generation for two hundred and fifty years.

RATIO OF THE CONSANGUINEOUS TO ALL MARRIAGES

Towards determining the average frequency of occurrence of consanguineous marriages, or the proportion which such marriages bear to the whole number of marriages, little has as yet been done in this country. Professor Richmond Mayo-Smith estimated that marriages between near kin constituted less than one per cent of the total, and Dr. Lee W. Dean estimates that in Iowa they comprise only about one half of one per cent. But these estimates are little more than guesses, without any statistical basis.

In several European countries such marriages have been registered, though somewhat spasmodically and inaccurately. According to Mulhall the ratio of the consanguineous among 10,000 marriages in the various countries is as follows:

In Italy the ratio varies greatly in different parts of the country. Mulhall gives the following figures for the years 1872-75:

It will be noted that the lowest ratios are in provinces where the urban population is comparatively large. Wherever statistics have been gathered it is the rule that the percentage of consanguineous marriage is greater in rural than in urban districts. Table IV, also from Mulhall, illustrates this point.

In regard to the degree of consanguinity, it seems very probable that in the French, German, Italian, and English statistics and estimates few if any marriages beyond the degree of first cousins are returned as consanguineous, so in order to compare the Norwegian figures with the others they should probably be reduced by one half. Out of 1549 consanguineous marriages contracted in Prussia in 1889, 1422 were between "cousins" , 110 between uncles and nieces, and 16 between nephews and aunts. The ratio of such marriages to 10,000 in France during the fifteen years ending in 1875 was:

In Italy during seven years ending in 1874, of all consanguineous marriages 92 per cent were of cousins and 8 per cent were of uncle and niece or aunt and nephew.

This is rather higher than the average for urban districts, according to official figures, but Dally seems to consider it as typical. He gives examples of the carelessness and incompetency of the rural record keepers, and insists that the percentage is really much higher than the official figures would indicate. He estimates the consanguineous marriages in France not including second cousins, at from four to five per cent.

A very ingenious method of determining the approximate number of first-cousin marriages was devised by Mr. George H. Darwin. Noticing that in marriage announcements, some were between persons of the same surname, it occurred to him that there might be a constant ratio between same-name marriages and first cousin marriages. Some same-name marriages would of course be purely adventitious; so, to eliminate this element of chance, he obtained from the Registrar General's Report the frequency of occurrence of the various surnames in England. The fifty commonest names embraced 18 per cent of the population. One person in every 73 was a Smith, one in every 76 a Jones and so on. Then the probability of a Smith-Smith marriage due to mere chance would be 1/73^2 and of a Jones-Jones marriage 1/76^2. The sum of fifty such fractions he found to be .0009207 or .9207 per thousand. After the fiftieth name the fractions were so small as to have comparatively little effect upon the total. He therefore concluded that about one marriage in a thousand takes place, in which the parties have the same surname and have been uninfluenced by any relationship between them bringing them together.

These cases furnished by correspondents he calculated to be 3.41 per cent of all marriages in the families to which circulars were sent.

From the data collected from all these sources Mr. Darwin obtains the following proportion:

Here too, he fears that the denominator is too small, for by theoretical calculation he obtains by one method the ratio 2/7, and by another 1/1. He finally takes 1/4 for this factor. To express the proportion in another form:

The completed formula then becomes:

Applying this formula to the English statistics, Mr. Darwin computes the percentages of first cousin marriages in England with the following results:

In order to apply this formula to the American population I counted the names in the New York Marriage License Record previous to 1784, and found the number to be 20,396, representing 10,198 marriages. The fifty commonest names embraced nearly 15 per cent of the whole , or three per cent less than the number found by Darwin. Of these, one in every 53 was a Smith, one in 192 a Lawrence, and so on. The sum of the fraction 1/53^2, 1/192^2, etc., I found to be .000757 or .757 per thousand, showing that the probability of a chance marriage between persons of the same name was even less than in England, where Mr. Darwin considered it almost a negligible quantity.

Of these 10,198 marriages, 211, or 2.07 per cent were between persons bearing the same surname. Applying Darwin's formula we would have 5.9 as the percentage of first cousin marriages in colonial New York. This figure is evidently much too high, so in the hope of finding the fallacy, I worked out the formula entirely from American data. To avoid the personal equation which would tend to increase the number of same-name first cousin marriages at the expense of the same-name not first cousin marriages, I took only those marriages obtained from genealogies, which would be absolutely unbiassed in this respect. Out of 242 marriages between persons of the same name, 70 were between first cousins, giving the proportion:

as compared with Darwin's .57. So that we may be fairly safe in assuming that not more than 1/3 of all same-name marriages are first cousin marriages. Taking data from the same sources and eliminating as far as possible those genealogies in which only the male line is traced, we have it:

This is near the ratio which Darwin obtained from his data, and which he finally changed to 1/4. I am inclined to think that his first ratio was nearer the truth, for since we have found that the coefficient of attraction between cousins would be so much greater than between non-relatives, why should we not assume that the attraction between cousins of the same surname should exceed that between cousins of different surnames? For among a large number of cousins a person is likely to be thrown into closer contact, and to feel better acquainted with those who bear the same surname with himself. But since the theoretical ratio would be about 1/4 it would hardly be safe to put the probable ratio higher than 1/3, or in other words four first cousin marriages to every same-name first cousin marriage. Our revised formula then is:

Instead of Mr. Darwin's .35.

Taking then the 10,198 marriages, with their 2.07 per dent of same-name marriages, and dividing by .75 we have 2.76 per cent, or 281 first cousin marriages.

In order to arrive at approximately the percentage of first cousin marriages in a nineteenth-century American community I counted the marriage licenses in Ashtabula County, Ohio, for seventy-five years, . Out of 13,309 marriages, 112 or .84 per cent were between persons of the same surname. Applying the same formula as before, we find 1.12 per cent of first cousin marriages, or less than half the percentage found in eighteenth-century New York. This difference may easily be accounted for by the comparative newness of the Ohio community, in which few families would be interrelated, and also to that increasing ease of communication which enables the individual to have a wider circle of acquaintance from which to choose a spouse.

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