Hofacker’s data and the researches of Hampe agree with those of Bidder in showing that to very young primiparæ, as well as to elderly primiparæ, an excess of boys is born. Among the offspring of 363 mothers, at ages varying from 16 to 26 years, Hofacker found the sexual ratio to be 121; among the offspring of 1,056 mothers, at ages 26 to 36, the ratio was 101; and among the offspring of 567 mothers at ages 36 to 46, the ratio was 111. Hampe tabulated 5,992 instances as follows:
| Age of Mother. | Number of instances. | Sexual ratio. |
|---|---|---|
| Below 20 years | 56 | 107.7 |
| 20 to 25 years | 871 | 90.6 |
| 25 to 30 years | 1,633 | 114.9 |
| 30 to 35 years | 1,631 | 108.3 |
| 35 to 40 years | 1,185 | 117.1 |
| Over 40 years | 616 | 124.0 |
We learn, therefore, that if the age of the progenitors is to be regarded as one of the causes by which sex is determined, we must take into consideration not only the relative ages of husband and wife but, in addition, the absolute age of the wife.
Goehlert undertook a statistical investigation in which the absolute age of the husband was taken into consideration as well as that of the wife. From this it appeared that the maximum sexual ratio was exhibited when the father was between the ages of 30 and 35 years. When the age of the mother is treated as the determining influence, we find the maximum sexual ratio in the offspring of mothers between the ages of 25 and 30 years. Goehlert believes, however, that the paternal influence is more powerful than the maternal in the determination of sex. The respective influences are compared in the following table:
| Age of Father. | Age of Mother. | |||
|---|---|---|---|---|
| 20 to 30 years. | 30 to 40 years. | Over 40 years. | Totals. | |
| 25 to 35 years | 105.76 | 107.87 | 109.14 | 106.6 |
| 35 to 45 years | 102.8 | 105.1 | 105.3 | 104.7 |
| Over 45 years | 104.3 | 103.9 | 109.1 | |
| In general | 105.25 | 105.97 | 104.9 | 105.5 |
Geissler, studying the data obtainable regarding the sexual ratio during a 10–year period in the Kingdom of Saxony, ascertained that in families possessing two children or more there was a very definite distribution of the possible sex-combinations. Where there was an even number of children those families were in the majority in which the number of boys and girls was identical. If the number of children in the family is an unequal one, those combinations are most frequent in which the number of boys exceeds the number of girls by one; next in frequency are those combinations reversed to this, i. e., in which the number of girls exceeds the number of boys by one. All other combinations are comparatively infrequent in proportion as the discrepancy in number between boys and girls is larger. Rarest of all are families in which the children are all of the same sex; and among these, again, the most unusual are those consisting of boys only.
This distribution of the sex-combination is believed by Geissler to depend upon the fact that in the first birth and all the subsequent births there is generally speaking a slight advantage in favour of the male sex. It has not been proved that the sex of the first-born exercises a determining influence on the sex of the subsequent children. It does, however, seem clear that in the case of parents who have given birth in succession to several children of one sex only, there exists some definite obstacle to the procreation of children of the opposite sex. Putting these exceptions out of consideration, there seems to exist a tendency in the later births of a series toward the procreation, more especially of that sex which has been absent or deficient in the earlier births of the series. The strength of this equalizing tendency increases as soon as it has for the first time manifest itself. It is always greater when the sex deficient in the earlier births of the series has been the male.
I have myself undertaken a statistical investigation of the births occurring in the reigning families of Europe and in the families of the leading members of the aristocracy. The necessary particulars are to be obtained from the genealogical court calendars; and it is my belief that the data obtained regarding these uppermost strata of society are comparatively free from many sources of error by which the ordinary statistics of the subject are apt to be invalidated. For 556 marriages there were 1,972 births, comprising 1,023 boys and 949 girls, and thus exhibiting a sexual ratio of 107.7.
In relation to the relative ages of the parents, the following table was drawn up:
| Boys. | Girls. | Sexual ratio. | |
|---|---|---|---|
| Husband older than wife by one to five years | 294 | 283 | 103.8 |
| Husband older than wife by more than five and less than ten years | 327 | 306 | 106.8 |
| Husband older than wife by more than ten and less than fifteen years | 190 | 167 | 113.7 |
| Husband older than wife by more than fifteen years | 138 | 113 | 122.1 |
| Husband same age as wife | 34 | 42 | 80.9 |
| Husband younger than wife | 40 | 38 | 105.2 |