The law of Hofacker and Sadler cannot be regarded as possessing universal validity, although the figures on which it is based seem to show pretty clearly that we are justified in regarding the mutual interaction of the male and female reproductive elements at the moment of fertilization as effective in the determination of sex. In the investigations to which we have hitherto alluded it is only the relative ages of husband and wife that have been taken into account; but other researches have shown that the absolute age alike of the husband and of the wife has an influence in the determination of sex.
The influence of the absolute age of the mother in the determination of sex has been very clearly established. Ahlfeld was the first to draw attention to the fact that among the children of elderly primiparæ there was always to be found an excess of boys, and that there was an increase in this excess proportionate to the greater age of the mother. Among 102 children born to primiparæ over 32 years of age the sexual ratio was 137 : 100; and a later investigation made by the same author in conjunction with Schramm showed that among 1,038 children born to primiparæ over 28 years of age the sexual ratio was 124 : 100.
Hecker obtained similar results. Among 432 children born to primiparæ over thirty years of age the sexual ratio was 133 : 100. Winckel, dealing with primiparæ of the same ages, found a sexual ratio of 136.8 : 100.
Düsing, examining the records of the lying-in hospitals of Leipzig, Dresden and Jena and thus obtaining a very large number of instances whereon to base his conclusions, confirmed the view that elderly primiparæ give birth to an excess of boys, and further that the older they are the larger the excess of male births. He drew up the following table:
| Age of primiparæ. | Leipzig. Boys. Girls. | Dresden. Boys. Girls. | Jena. Boys. Girls. | Total Nos. Boys. Girls. | Sexual ratio. |
|---|---|---|---|---|---|
| 15 | 1 : − | 1 : 2 | 1 : − | 3 : 2 | 549 : 494 = 111.1 |
| 16 | 4 : 4 | 6 : 10 | 2 : 2 | 12 : 16 | |
| 17 | 23 : 13 | 20 : 15 | 9 : 7 | 52 : 35 | |
| 18 | 67 : 55 | 103 : 100 | 17 : 13 | 187 : 168 | |
| 19 | 110 : 103 | 152 : 141 | 33 : 29 | 295 : 273 | |
| 20 | 148 : 147 | 187 : 185 | 32 : 45 | 367 : 377 | 807 : 781 = 103.3 |
| 21 | 157 : 145 | 241 : 201 | 42 : 57 | 440 : 404 | |
| 22 | 120 : 133 | 191 : 207 | 48 : 53 | 359 : 393 | 903 : 962 = 93.9 |
| 23 | 106 : 108 | 168 : 149 | 51 : 51 | 325 : 308 | |
| 24 | 71 : 105 | 111 : 118 | 37 : 38 | 219 : 261 | |
| 25 | 79 : 57 | 73 : 72 | 35 : 27 | 187 : 156 | 531 : 469 = 113.2 |
| 26 | 45 : 35 | 30 : 43 | 20 : 20 | 125 : 98 | |
| 27 | 31 : 35 | 52 : 55 | 10 : 12 | 93 : 102 | |
| 28 | 32 : 33 | 26 : 33 | 19 : 16 | 77 : 72 | |
| 29 | 19 : 10 | 26 : 18 | 4 : 13 | 49 : 41 | |
| 30 | 9 : 15 | 30 : 13 | 9 : 6 | 48 : 34 | 155 : 104 = 150.0 |
| 31 | 3 : 8 | 15 : 11 | 3 : 3 | 21 : 22 | |
| 32 | 5 : 6 | 12 : 9 | 7 : 3 | 24 : 18 | |
| 33 | 2 : 2 | 5 : 5 | 5 : 2 | 12 : 9 | |
| 34 | 4 : − | 8 : 5 | 2 : − | 14 : 5 | |
| 35 | 2 : − | 9 : 3 | 2 : 1 | 13 : 4 | |
| 36 | 1 : − | 3 : 3 | 1 : 1 | 5 : 4 | |
| 37 | 4 : 1 | 4 : 3 | 1 : − | 9 : 4 | |
| 38 | − : − | − : 1 | 1 : − | 1 : 1 | |
| 39 | − : − | 4 : − | 1 : − | 5 : − | |
| 40 | 1 : 1 | 2 : 1 | 1 : − | 4 : 2 | |
| 41 | − : − | − : 1 | − : − | − : 1 |
As an explanation of this statistically proved fact, that elderly primiparæ gave birth to a large excess of boys, Düsing suggests that these women who conceive for the first time comparatively late in life, are, prior to the conception, in a state corresponding with that of a lower animal species suffering from a deficiency of males, and for this reason exhibit a tendency to procreate a larger number of individuals of the deficient sex. In multiparæ also it is possible to trace the influence of a deficiency of male individuals. When there is such a deficiency the interval between successive births is unduly protracted. Düsing found (once more from the records of the lying-in hospitals of Dresden, Leipzig and Jena) that the longer the interval between one parturition and the next the longer, that is to say, the mother has had to wait for her next conception, the greater is the excess of male births. Düsing therefore lays down the law: “Delayed impregnation gives rise to an excess of male births.”
| Age of primiparæ. | Leipzig. Boys. Girls. | Dresden. Boys. Girls. | Jena. Boys. Girls. | Total Nos. Boys. Girls. | Sexual ratio. |
|---|---|---|---|---|---|
| 1 | 162 : 158 | 194 : 178 | 58 : 45 | 414 : 381 | 108.6 |
| 2 | 366 : 307 | 374 : 361 | 168 : 145 | 908 : 813 | 111.6 |
| 3 | 198 : 196 | 207 : 194 | 116 : 94 | 521 : 484 | 107.7 |
| 4 | 127 : 109 | 132 : 106 | 59 : 45 | 318 : 260 | 115.7 |
| 5 | 59 : 54 | 55 : 54 | 38 : 38 | 152 : 146 | |
| 6 | 61 : 62 | 52 : 49 | 49 : 24 | 162 : 135 | 121.9 |
| 8, 9 and 10 | 18 : 16 | 41 : 23 | 16 : 24 | 75 : 63 | |
| 11 and more | 5 : 15 | 12 : 9 | 4 : 6 | 41 : 30 | |
| Totals: 4,903 births, 2,591 m.; 2,312 f.; sexual ratio = 112.06. | |||||
Bidder considers that his own observations entitle him to modify Ahlfeld’s dictum regarding the influence of age in primiparæ in giving rise to an excess of male births. He tabulates his results as follows:
| Age of Mother. | Number of cases. | Sexual ratio. |
|---|---|---|
| 17 to 20 | 80 | 122.2 |
| 20 to 22 | 405 | 130.1 |
| 22 to 24 | 369 | 109.9 |
| 24 to 26 | 1,138 | 104.6 |
| 26 to 30 | 2,049 | 105.5 |
| 30 to 32 | 878 | 112.5 |
| 32 to 36 | 1,120 | 119.6 |
| 36 to 39 | 676 | 123.1 |
| 40 and upward | 215 | 131.5 |
and formulates the following thesis: Very young primiparæ give birth to an excess of boys; primiparæ in the first bloom of womanhood give birth to an excess of girls; later, however, as the age of the primiparæ increases the excess of male births soon reappears and rapidly increases.