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,[[14]] and Dr. Lee W. Dean estimates that in Iowa they comprise only about one half of one per cent.[[15]] 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[[16]] the ratio of the consanguineous among 10,000 marriages in the various countries is as follows:
| table i. | |||
|---|---|---|---|
| Country. | Ratio. | Country. | Ratio. |
| Prussia | 67 | Alsace | 107 |
| Italy | 69 | France | 126 |
| England | 75 | Jews | 230 |
According to Uchermann the ratio is 690 or 6.9 per cent, including marriages between second cousins and nearer.[[17]] Dr. Peer says that 4 per cent of the marriages in Saxony are consanguineous.[[18]] The ratio seems to be increasing in France but diminishing in Alsace and Italy, as indicated in Table II.[[19]]
| TABLE II. | |||||
|---|---|---|---|---|---|
| Country. | Date. | Ratio.[A] | Country. | Date. | Ratio.[A] |
| France | 1853-60 | 97 | France | 1861-71 | 126 |
| Alsace | 1858-65 | 143 | Alsace | 1872-75 | 107 |
| Italy | 1868-71 | 84 | Italy | 1872-75 | 69 |
| [A] Per 10,000. | |||||
In Italy the ratio varies greatly in different parts of the country. Mulhall gives the following figures for the years 1872-75:
| TABLE III. | |||
|---|---|---|---|
| Province. | Ratio.[A] | Province. | Ratio.[A] |
| Venice | 24 | Sicily | 117 |
| Naples | 30 | Piedmont | 131 |
| Lombardy | 100 | Liguria | 183 |
| [A] Per 10,000. | |||
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.
| TABLE IV. | |||
|---|---|---|---|
| Country. | Rural. | Urban. | General. |
| England | 79 | 71 | 75 |
| France | 130 | 115 | 126 |
| Alsace | 121 | 41 | 107 |
| Norway[A] (Uchermann) | 810 | 260 | 690 |
| [A] Includes second cousins. | |||
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" (probably first), 110 between uncles and nieces, and 16 between nephews and aunts.[[20]] The ratio of such marriages to 10,000 in France during the fifteen years ending in 1875 was:[[21]]
| TABLE V. | |||
|---|---|---|---|
| Degree. | Urban. | Rural. | All France. |
| Nephew and aunt | 1.6 | 2.4 | 2.1 |
| Uncle and niece | 6.0 | 5.6 | 5.8 |
| "Cousins" | 96.0 | 119.0 | 113.1 |
| Total | 103.6 | 127.0 | 121.2 |
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.[[22]]
Dally[[23]] is very skeptical about the accuracy of the French figures, but says that in Paris the records are well kept. He found that in the years 1853-62 there were 10,765 marriages in the 8me arrondissement of Paris, and of these he finds:
| Marriages between cousins-german | 141 |
| Marriages between uncle and niece | 8 |
| Marriages between aunt and nephew | 1 |
| Total consanguineous | 150 |
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.[[24]] 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 1/732 and of a Jones-Jones marriage 1/762. 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.
The next step was to count the marriages announced in the "Pall Mall Gazette" for the years 1869-72 and a part of 1873. Of the 18,528 marriages there found, 232 or 1.25 per cent were between persons of the same surname. Deducting the percentage of chance marriages at least 1.15 per cent were probably influenced directly or indirectly by consanguinity.
Mr. Darwin then proceeded by a purely genealogical method. He found that out of 9,549 marriages recorded in "Burke's Landed Gentry," 144 or 1.5 per cent were between persons of the same surname, and exactly half of these were first cousins. In the "English and Irish Peerage" out of 1,989 marriages, 18 or .91 per cent were same-name first cousin marriages. He then sent out about 800 circulars to members of the upper middle class, asking for records of first cousin marriage among the near relatives of the person addressed, and obtained the following result:
| Same-name first cousin marriages | 66 |
| Different-name first cousin marriages | 182 |
| Same-name not first cousin marriages | 29 |
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:
| Same-name first cousin marriages All same-name marriages | = | 142 249 | = | .57 |
He is inclined to think that the ratio should be lower and perhaps .50 instead of .57. By a similar line of reasoning he obtains this proportion:
| _Same-name first cousin marriages_ Different-name first cousin marriages | = | _1_ 3 |
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:
| Same-name first cousin marriages All first cousin marriages | = | _1_ 5 |
The completed formula then becomes:
| All same-name marriages All first cousin marriages | = | 100 57 | X | _1_ 5 | = | .35 (nearly) |
Applying this formula to the English statistics, Mr. Darwin computes the percentages of first cousin marriages in England with the following results:
| London | 1.5 |
| Other urban districts | 2. |
| Rural districts | 2.25 |
| Middle class and Landed Gentry | 3.5 |
| Aristocracy | 4.5 |
In order to apply this formula to the American population I counted the names in the New York Marriage License Record previous to 1784,[[25]] and found the number to be 20,396, representing 10,198 marriages. The fifty commonest names embraced nearly 15 per cent of the whole (1526), or three per cent less than the number found by Darwin.[[26]] Of these, one in every 53 was a Smith, one in 192 a Lawrence, and so on. The sum of the fraction 1/532, 1/1922, 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:
| Same-name first cousin marriages All same-name marriages | = | _70_ 242 | = | .285 |
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:
| __Same-name first cousin marriages__ Different-name first cousin marriages | = | _24_ 62 | = | __1__ 2-7/12 | = | __1__ 2.583 |
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:
| __All same-name marriages__ All first cousin marriages | = | _3_ 1 | X | _1_ 4 | = | .75 |
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, (1811-1886). 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.
Adopting a more direct method of determining the frequency of cousin marriage, I estimated in each of sixteen genealogical works, the number of marriages recorded, and found the total to be 25,200. From these sixteen families I obtained 153 cases of first cousin marriage, or .6 per cent. Allowing for the possible cases of cousin marriage in which the relationship was not given, or which I may have over-looked, the true percentage is probably not far below the 1.12 per cent obtained by the other method.
The compiler of the, as yet, unpublished Loomis genealogy writes me that he has the records of 7500 marriages in that family, of which 57 or .8 per cent are same-name marriages. This would indicate that 1.07 per cent were between first cousins.
In isolated communities, on islands, among the mountains, families still remain in the same locality for generations, and people are born, marry and die with the same environment. Their circle of acquaintance is very limited, and cousin marriage is therefore more frequent. If we exclude such places, and consider only the more progressive American communities, it is entirely possible that the proportion of first cousin marriages would fall almost if not quite to .5 per cent. So that the estimate of Dr. Dean for Iowa may not be far out of the way.
Even for England Mr. Darwin's figures are probably much too large. Applying the corrected formula his table becomes:
| TABLE VI. | |||
|---|---|---|---|
| 1872. | Number marriages registered. | Per cent of same-name marriages. | Per cent of first cousin marriages. |
| London, Metropolitan Districts | 33,155 | .55 | .73 |
| Urban Districts | 22,346 | .71 | .95 |
| Rural Districts | 13,391 | .79 | 1.05 |
| Total | 68,892 | .64 | .85[A] |
| [A] Cf. Mulhall, .75 per cent, _supra_, p. 18. | |||
In regard to the frequency of marriage between kin more distant than first cousins figures are still more difficult to obtain. The distribution of 514 cases of consanguineous marriage from genealogies was as follows:
| TABLE VII. | |||||||
|---|---|---|---|---|---|---|---|
| First cousins | 1-1/2 cousins | Second cousins | 2-1/2 cousins | Third cousins | Distant cousins | Total | |
| Same-name | 70 | 24 | 49 | 19 | 20 | 26 | 208 |
| Different-name | 96 | 30 | 58 | 22 | 37 | 62 | 305 |
| Total | 166 | 54 | 107 | 41 | 57 | 88 | 513 |
Obviously this cannot be taken as typical of the actual distribution of consanguineous marriages, since the more distant the degree, the more difficult it is to determine the relationship. However it is very evident that the coefficient of attraction is at its maximum between first cousins, and probably there are actually more marriages between first cousins than between those of any other recognized degree of consanguinity. But the two degrees of 1-1/2 cousins and second cousins taken together probably number more intermarriages than first cousins alone. Allowing four children to a family, three of whom marry and have families, the actual number of cousins a person would have on each degree would be: First, 16; 1-1/2, 80; Second, 96; 2-1/2, 480; Third, 576; Fourth, 3,456. The matter is usually complicated by double relationships, but it will readily be seen that the consanguineal attraction would hardly be perceptible beyond the degree of third cousins.[[27]]
Omitting, as in the discussion on page 24, those genealogies in which only the male line is given we have the following table:
| TABLE VIII. | |||||||
|---|---|---|---|---|---|---|---|
| First cousins | 1-1/2 cousins | Second cousins | 2-1/2 cousins | Third cousins | Distant cousins | Total | |
| Same-name | 24 | 5 | 10 | 4 | 2 | 5 | 50 |
| Different-name | 62 | 15 | 33 | 12 | 23 | 26 | 171 |
| Total | 86 | 20 | 43 | 16 | 25 | 31 | 221 |
It would naturally be supposed that with each succeeding degree of relationship the ratio of same-name to different-name cousin marriages would increase in geometrical proportion, viz. first cousins, 1:3; second cousins, 1:9; third cousins, 1:27, etc., but on the other hand there is the tendency for families of the same name to hold together even in migration as may be proved by the strong predominance of certain surnames in nearly every community. So that the ratio or same-name to different-name second cousin marriage may not greatly exceed 1:4. Beyond this degree any estimate would be pure guesswork. However the coefficient of attraction between persons of the same surname would undoubtedly be well marked in every degree of kinship, and conversely there are few same-name marriages in which some kinship, however remote, does not exist.
The proportion of mixed generation cousin marriages (1-1/2 cousins, 2-1/2 cousins, etc.) is always smaller than the even generation marriages of either the next nearer or more remote degrees. For example, a man is more likely to marry his first or his second cousin than either the daughter of his first cousin, or the first cousin of one of his parents, although such mixed generation marriages often take place.
The conclusions, then, in regard to the frequency of consanguineous marriage in the United States may be summarized as follows:
1. The frequency varies greatly in different communities, from perhaps .5 per cent of first cousin marriages in the northern and western states to 5 per cent, and probably higher, in isolated mountain or island communities. The average of first cousin marriage in the United States is probably not greater than one per cent.
2. The percentage of consanguineous marriages is decreasing with the increasing ease of communication and is probably less than half as great now as in the days of the stage coach.
3. Although the number of marriageable second cousins is usually several times as great as that of first cousins, the number of marriages between second cousins is probably somewhat less than the number of marriages between first cousins, but the number of second cousin marriages combined with the number of 1-1/2 cousin marriages probably exceeds the number of first cousin marriages alone. So that the percentage of marriages ordinarily considered consanguineous is probably between two, and two and a half.
NOTE.—In an article entitled "Sur le nombre des consanguins dans un groupe de population," in Archives italiennes de biologie (vol. xxxiii, 1900, pp. 230-241), Dr. E. Raseri shows that from one point of view the actual number of consanguineous marriages is little, if any, greater than the probable number. The average number of children to a marriage he finds to be 5, the average age of the parents 33 and the average age at marriage 25. The Italian mortality statistics show that 54 per cent of the population lives to the age of 25, of which 15 per cent does not marry, leaving an average of 2.3 children in every family who marry. On this basis a person would have at birth 4,357 relatives within the degree of fourth cousins; at the age of 33 he would have 4,547; and at 66, 5,002. In 1897 out of 229,041 marriages in Italy, 1,046 were between first cousins, giving an average of one in 219. In 1881 the number of men between 18 and 50 and of women between 15 and 45 was 5,941, 495 in 8,259 communes with an average population of 3,500. In each commune there must be 360 marriageable persons of each sex, but to marry within his class a man would only have the choice of 180 women and vice versa. Adding the probable number who would marry outside the commune, the choice lies within 216 of the opposite sex. Of these 25 would be cousins within the tenth degree (fourth cousins) making the probability of a consanguineous marriage .11, reduced by a probable error in excess to .10. The probability of a first cousin marriage would be .82/216 or .0038, whereas the actual ratio is 1/219 or .0045.