Marriages Between First Cousins in England and Their Effects

It is well known that when the Census Act, 1871, was passing through the House of Commons, an attempt was made by Sir J. Lubbock, Dr Playfair, and others, to have a question inserted with respect to the prevalence of cousin marriages, under the idea that when we were in possession of such statistics we should be able to arrive at a satisfactory conclusion as to whether these marriages are, as has been suspected, deleterious to the bodily and mental constitution of the offspring. It is unfortunately equally well known that the proposal was rejected, amidst the scornful laughter of the House, on the ground that the idle curiosity of philosophers was not to be satisfied. It was urged, that when we had these statistics it would be possible to discover, by inquiry in asylums, whether the percentage of the offspring of consanguineous marriages amongst the diseased was greater than that in the healthy population, and thus to settle the question as to the injuriousness of such marriages. The difficulty of this subsequent part of the inquiry was, I fear, much underrated by those who advocated the introduction of these questions into the census. It may possibly have been right to reject the proposal on the ground that every additional question diminishes the trustworthiness of the answers to the rest, but in any case the tone taken by many members of the House shows how little they are permeated with the idea of the importance of inheritance to the human race. In the summer of 1873 the idea occurred to me that it might be in some measure possible to fill up this hiatus in our national statistics. In looking through the marriages announced in the Pall Mall Gazette, I noticed one between persons of the same surname; now as the number of surnames in England is very large, it occurred to me that the number of such marriages would afford a clue to the number of firstcousin marriages. In order to estimate what proportion of such marriages should be attributed to mere chance, I obtained the ‘‘Registrar-General’s Annual Report’’ for 1853, where the frequency of the various surnames is given. I here found that there were nearly 33,000 surnames registered, and that the fifty commonest names embraced 18 per cent. of all the population. It appear that one in 73 is a Smith, one in 76 a Jones, one in 115 a Williams, one in 148 a Taylor, one in 162 a Davies, one in 174 a Brown, and the last in the list is one Griffiths in 529. Now it is clear that in one marriage in 73 one of the parties will be a Smith, and if there were no cause which tended to make persons of the same surname marry, there would be one in 73, or 5,329 marriages, in which both parties were Smiths. Therefore the probability of a Smith—Smith marriage due to mere chance is 1 5329; similarly the chance of a Jones-Jones, a Davies-Davies and a Griffiths-Griffiths marriage would be 1 762 , 1 1622 and 1 5292, respectively. And the sum of fifty such fractions would give the probability of a chance marriage, between persons of the same surname, who owned one of these fifty commonest names. The sum of these fifty fractions I find to be 0.0009207, or 0.9207 per thousand. It might, however, be urged that if we were to take more than fifty of the common names, this proportion would be found to be much increased. I therefore drew a horizontal straight line, and at equal distances along it I erected ordinates proportional to 1 732 , 1 762 , . . . , 1 5292, The upper ends of these ordinates were found to lie in a curve of great regularity, remarkably like a rectangular hyperbola, of which my horizontal straight line was one asymptote; and the ordinate corresponding to Griffiths was exceedingly short. Observing the great regularity of the curve, I continued it beyond the fiftieth surname by eye, until it sensibly coincided with the asymptote, at a point about where the hundred and twenty-fifth name would have stood, and then I cut out the whole (drawn on thick paper) and weighed the part corresponding to the fifty surnames, and the conjectural part. The conjectural addition was found to weigh rather more than one-tenth of the other part; and as the chance of same-name marriages is proportional to the areas cut out, I think I may venture confidently to assert that in England and Wales about one marriage in a thousand takes place in which the parties are of the same surname, and have been uninfluenced by any relationship between them bringing them together. Now it will appear Darwin GH, Marriages between first cousins in England and their effects. Fortnightly Review 1875; 24:22–41. Published by Oxford University Press on behalf of the International Epidemiological Association