ining 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:

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Same-name first cousin marriages | 66
Different-name first cousin marriages | 182
Same-name not first cousin marriages | 29
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