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.
[Footnote 14: _Statistics and Sociology_, p. 112.]
[Footnote 15: _Effect of Consanguinity upon the Organs of Special
Sense_, p. 4.]
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
-----------------------------------
[Footnote 16: _Dictionary of Statistics_, p. 383.]
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]
[Footnote 17: _Les Sourds-muets en Norvège_. Quoted by Feer, p. 9.]
[Footnote 18: _Der Einfluss der Blutsverwandschaft der Eltern auf die
Kinder_, p. 9.]
[Footnote 19: Mulhall, _Dictionary of Statistics_, p. 383.]
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.
-------------------------------------------------
|_Ratio per 10,000 Marriages_.
-------------------------------------------------
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
-----------------------------------------------
[Footnote 20: Mulhall, op. cit., p. 383.]
[Footnote 21: Ibid., p. 384.]
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]
[Footnote 22: Ibid., p. 384.]
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
-----------------------------------------
[Footnote 23: "Recherches sur les Mariages Consanguins et sur les
Races Pures." in _Bulletins de la Société d'Anthropologie_, 1863, p.
527.]
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 1/73^2 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.
[Footnote 24: "Marriages between First Cousins in England and their
Effects," in _Journal of the Statistical Society_, June, 1875. pp. 154
_et seq_.]
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 142
-------------------------------- = --- = .57
All same-name marriages 249
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 1
------------------------------------- = ---
Different-name first cousin marriages 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 1
-------------------------------- = ---
All first cousin marriages 5
The completed formula then becomes:
All same-name marriages 100 1
-------------------------- = ----- X --- = .35 (nearly)
All first cousin marriages 57 5
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 1/53^2, 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.
[Footnote 25: _Names of Persons for whom Marriage Licenses were issued
by the Secretary of the Province of New York_.]
[Footnote 26: _Cf. supra_, p. 21.]
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 70
-------------------------------- = --- = .285
All same-name marriages 242
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 24 1 1
------------------------------------- = -- = -------- = -------
Different-name first cousin marriages 62 (2-7/12) 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 3 1
--------------------------- = --- X --- = .75
All first cousin marriages 1 4
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.
----------------------------------------------------
|Number |Per cent of|Per cent of
1872. |marriages |same-name |first cousin
|registered.|marriages. |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 | 1-1/2 |Second | 2-1/2 | Third |Distant|
|cousins|cousins|cousins|cousins|cousins|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]
[Footnote 27: See note, _infra_, p. 29.]
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 | 1-1/2 |Second | 2-1/2 | Third |Distant|
|cousins|cousins|cousins|cousins|cousins|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.