The Irish censuses of 1891 and 1901 give similar data, though not so detailed as in 1871 and 1881. Thus we have in these reports a census inquiry into a phase of the consanguineous marriage problem extending over the period of six successive censal years. Although we can hardly suppose that these figures are accurate in all respects, they throw a great deal of light upon the problem, and are worth quoting in some detail. The tables as given by Mr. Huth contain a number of errors of detail, the correction of which changes the results materially.[[82]]
| TABLE XXIV. | |||||||
|---|---|---|---|---|---|---|---|
| Censal year. | Total population | Congenital deaf-mutes | |||||
| Number | Number per million | Average number to a family[A] | Parents cousins | ||||
| Number | Per cent. | Average number to a family[A] | |||||
| 1851[B] | 6,574,278 | 4,127 | 628 | ---- | 242 | 5.86 | 1.66 |
| 1861 | 5,798,967 | 4,096 | 706 | 1.22 | 362 | 8.84 | 1.72 |
| 1871 | 5,412,377 | 3,503 | 647 | 1.30 | 287 | 7.35 | 1.76 |
| 1881 | 5,174,836 | 3,163 | 611 | 1.32 | 191 | 6.04 | 1.69 |
| 1891 | 4,706,448 | 2,570 | 546 | 1.40 | 297 | 11.56 | 1.92 |
| 1901 | 4,456,546 | 2,179 | 489 | 1.40 | 249 | 11.43 | 1.73 |
| [A] From Table XXV. | |||||||
| [B] 1851 data from Huth, "Consanguineous Marriage and Deaf-mutism." The Lancet, 1900. | |||||||
Table XXIV summarizes the most important points in the Irish data. It will be seen that while there has been an absolute diminution in the number of deaf-mutes in Ireland with the decrease in population, there has been a relative increase of deaf-mutism. There are two possible explanations for this phenomenon, both of which may have operated in part; first that in the great emigration the deaf-mutes have been left behind, and second that with the introduction of improved methods of census taking, the returns are more complete than a half century ago. Mr. Huth believes that there is still room for improvement in Irish census methods, and thinks there is reason to believe that in the enumeration of the deaf all children born deaf in a family are included whether living or not.
Since Ireland is strongly Roman Catholic, the proportion of consanguineous marriages is probably small, so that the percentage of deafmutes derived from consanguineous marriages, varying from 5.86 to 11.56 is very much greater than the percentage of these marriages in the general population. The average number of deaf children to a family in Table XXIV varies less than any other part of the table, and clearly shows a much higher average number of deaf children where the parents were cousins. They reveal the interesting fact that the occurrence of two or more deafmutes in a family is more than twice as probable where the parents are related as where they are not. Table XXV still better illustrates this point. Of the families where there was but one deaf-mute, only 4.3 per cent were the offspring of cousin marriages; where there were two in a family 12.9 per cent were of consanguineous parentage; three in a family, 13.3 per cent; four in a family, 19.0 per cent; more than four in a family, 21.1 per cent.
| TABLE XXV. Number of Congenital Deaf mutes to a Family in Ireland. | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Year. | Parentage. | Families in which deaf-mutes numbered. | ||||||||||
| 1. | 2. | 3. | 4. | 5. | 6. | 7. | 8. | 9. | 10. | 11. | ||
| 1851 | Parents cousins | 127 | 45 | 20 | 10 | 5 | 2 | .. | 1 | .. | .. | .. |
| 1871 | Parents cousins | 91 | 38 | 24 | 5 | 3 | 1 | 1 | .. | .. | .. | .. |
| 1881 | Parents cousins | 63 | 30 | 13 | 6 | 1 | .. | .. | .. | .. | .. | .. |
| 1891 | Parents cousins | 82 | 38 | 19 | 9 | 1 | 3 | 1 | 2 | .. | .. | .. |
| 1901 | Parents cousins | 79 | 34 | 23 | 7 | 1 | .. | 1 | .. | .. | .. | .. |
| 1851 | All families[A] | 2963 | 347 | 158 | 35 | 13 | 5 | .. | 1 | .. | .. | .. |
| 1871 | All families[A] | 2460 | 305 | 167 | 47 | 20 | 5 | 1 | .. | .. | .. | .. |
| 1881 | All families[A] | 2080 | 281 | 162 | 39 | 18 | 6 | .. | .. | .. | 1 | .. |
| 1891 | All families[A] | 1473 | 273 | 134 | 40 | 12 | 6 | 1 | 2 | .. | .. | 1 |
| 1901 | All families[A] | 1219 | 231 | 122 | 34 | 10 | 4 | 2 | .. | .. | .. | .. |
| [A] Number of the "Deaf and Dumb" to a family, "as far as could be ascertained." | ||||||||||||
In 1871 and 1881 the inquiry was more minute and the degrees of consanguinity were specified. Mr. Huth quotes some of the figures for these years, probably derived from the same sources as Table XXVI, and comments as follows: "An examination of this table will show that the statistics so much relied upon as proving the causation of deaf-mutism by consanguineous marriages show nothing of the sort. In 1871 fourth cousins produced more deaf-mutes per marriage than any nearer relationship. In 1881 third cousins produced more than any nearer relationship."[[83]] Mr. Huth forgets that he is basing these statements on five and nine families respectively, and does not take into consideration the probability that if the returns are biased, as he suspects, this bias would affect the more distantly related, relatively more than the first cousin marriages, for the same reason that this would be true of the cases collected by Dr. Bemiss.[[84]] Combining the figures of the two censal years helps to correct these averages, and the distantly related show approximately the same average as the first cousin marriages in spite of the vastly greater selection which must have obtained in the distantly related cases.
In Table XXVI it will be seen that 52.5 per cent of the deaf-mute offspring of consanguineous parents were the offspring of first cousin marriages. On the assumption that this percentage is fairly typical of each set of returns we may say that from three to six per cent of the Irish deaf-mutes are the offspring of first cousin marriages. If, then, the proportion of first cousin marriages is no greater than in England, the percentage of deaf-mute offspring is several times as great as in the average non-related marriage.