From the census statistics, then, it appears that the married deaf as a class do not have a large proportion of deaf children, and that this proportion is only a little more than twice as great when the deaf are married to the deaf as when they are married to the hearing. It appears also, however, that when there are deaf relatives involved in either kind of marriages, or when there is congenital deafness in the deaf parent, the effect is quite marked in the offspring.
Besides the census returns, we have the statistics presented in the reports of certain schools, which are found to point, as far as they go, to the same conclusions. In the Kentucky School, out of 83 pupils admitted in 1910 and 1911, there were none the children of deaf parents, though 35, or 30.1 per cent, had deaf relatives; and out of 42 admitted in 1912 and 1913, there were 2, or 4.8 per cent, the children of deaf parents, and 12, or 28.8 per cent, with deaf relatives. In the Iowa School, out of 62 admissions in 1911 and 1912, 4, or 6.5 per cent, had deaf parents, and 21, or 33.9 per cent, "defective" relatives. In the Michigan School, with an annual enrollment of some three hundred, there were from 1903 to 1908 but three children of deaf parents.[50] In the Colorado School, out of a total attendance since its founding to 1912 of 567, 3, or 0.57 per cent, were the children of deaf parents, though 83, or 14.6 per cent, had deaf relatives. In the Missouri School, out of a similar attendance to 1912 of 2,174 there were 52, or 2.4 per cent, with deaf parents, though there were 235, or 10.8 per cent, with deaf relatives.[51]
The most exhaustive study of the question of the liability of the deaf to deaf offspring is that of Dr. E. A. Fay in his "Marriages of the Deaf"—covering the majority of the marriages of the deaf in America at the time it was made (1898).[52] Statistical information is presented for 7,227 deaf persons and for 3,078 marriages with either deaf or hearing partners.[53] In the following table are summarized the results of this investigation.[54]
MARRIAGES OF DEAF PERSONS
| NUMBER OF MARRIAGES | NUMBER OF CHILDREN | |||||
| Partners in Marriage | Total | Resulting in deaf children | Per cent | Total | Deaf | Per cent |
| One or both deaf | 3,078 | 300 | 9.7 | 6,782 | 588 | 8.6 |
| Both deaf | 2,377 | 220 | 9.2 | 5,072 | 429 | 8.4 |
| One deaf, other hearing | 599 | 75 | 12.5 | 1,532 | 151 | 9.8 |
| One or both congenitally deaf | 1,477 | 194 | 13.1 | 3,401 | 413 | 12.1 |
| One or both adventitiously deaf | 2,212 | 124 | 5.6 | 4,701 | 199 | 4.2 |
| Both congenitally deaf | 335 | 83 | 24.7 | 779 | 202 | 25.9 |
| One congenitally, other adventitiously deaf | 814 | 66 | 8.1 | 1,820 | 119 | 6.5 |
| Both adventitiously deaf | 845 | 30 | 3.5 | 1,720 | 40 | 2.3 |
| One congenitally deaf, other hearing | 191 | 28 | 14.6 | 528 | 63 | 11.9 |
| One adventitiously deaf, other hearing | 310 | 10 | 3.2 | 713 | 16 | 2.2 |
| Both had deaf relatives | 437 | 103 | 23.5 | 1,060 | 222 | 20.9 |
| One had deaf relatives, other not | 541 | 36 | 6.6 | 1,210 | 78 | 6.4 |
| Neither had deaf relatives | 471 | 11 | 2.3 | 1,044 | 13 | 1.2 |
| Both congenitally deaf | ||||||
| Both had deaf relatives | 172 | 49 | 28.4 | 429 | 130 | 30.3 |
| One had deaf relatives, other not | 49 | 8 | 16.3 | 105 | 21 | 20.0 |
| Neither had deaf relatives | 14 | 1 | 7.1 | 24 | 1 | 4.1 |
| Both adventitiously deaf | ||||||
| Both had deaf relatives | 57 | 10 | 17.5 | 114 | 11 | 9.6 |
| One had deaf relatives, other not | 167 | 7 | 4.1 | 357 | 10 | 2.8 |
| Neither had deaf relatives | 284 | 2 | 0.7 | 550 | 2 | 0.3 |
| Partners consanguineous | 31 | 14 | 45.1 | 100 | 30 | 30.0 |
It is thus seen that 9.7 per cent of the marriages of the deaf result in deaf offspring, and that 8.6 per cent of the children born of them are deaf—proportions far greater than for the the population generally.[55] A striking fact to be noted, however, is that these proportions are greater when one parent is deaf and the other hearing than when both are deaf. The percentage of marriages resulting in deaf offspring when only one parent is deaf is 12.5, and when both are deaf, 9.2; while the percentage of deaf children born of them when only one parent is deaf is 9.8, and when both are deaf, 8.4. This is apparently a very strange result, though it probably may be accounted for in some part on the theory that it is not so much deafness itself that is inherited, but rather an abnormality of the auditory organs, or a tendency to disease, of which deafness is a result or symptom, and that with different pathological conditions in the parent there is less likelihood of deafness resulting.
The most significant part of the results seems to be found, as before, in respect to whether or not deaf parents are themselves congenitally deaf or have deaf relatives. On the one hand, when one or both of the parents are adventitiously deaf, the percentage of marriages resulting in deaf children is 5.6, and the percentage of deaf children is 4.2; when both parents are so, the percentages are lower: 3.5 and 2.3. The percentages rise when one parent is adventitiously deaf, and the other congenitally: 8.1 and 6.5. In respect to deaf relatives of parents, the percentages are very low when neither has such relatives: 2.3 and 1.2. The lowest percentages of all are in the case where both parents are adventitiously deaf and neither has deaf relatives: 0.7 and 0.3.
On the other hand, we find the proportion of marriages resulting in deaf offspring and the proportion of deaf children much greater when there is congenital deafness in one or both parents, when one or both have deaf relatives, and greatest of all when these influences are combined. When one or both parents are congenitally deaf, the percentage of marriages resulting in deaf offspring is 13.1, and the percentage of deaf children is 12.1; when both parents are so, the percentages are doubled: 24.7 and 25.9. When one parent has deaf relatives and the other has not, the percentages are 6.6 and 6.4; when both have, the percentages are nearly four times as great: 23.5 and 20.9. When both parents are congenitally deaf but neither has deaf relatives, the percentages are 7.1 and 4.1. When both are adventitiously deaf and both have deaf relatives, the percentages are 17.5 and 9.6. When both are congenitally deaf and one has deaf relatives, the percentages are 16.3 and 20.0; and when both have deaf relatives, the percentages are 28.4 and 30.3.
The evidence is very strong, then, with regard to the form of deafness and the presence or absence of deaf relatives. In cases where the parents are not congenitally deaf and have no deaf relatives, the proportion of deaf children is very low. When one or both parents are congenitally deaf or have deaf relatives—when the deafness is inherited or in the family—the likelihood becomes far greater, and greater still when the two influences are in conjunction. In general, in respect to the influences of heredity upon deafness, the main determinants seem to be found in the existence in the parties, whether hearing or deaf, of deaf relatives, and, to a less extent, in the existence in parties who are deaf of congenital deafness.