The fertility of female criminals was found by Lombroso to be undiminished. On the average, poisoners had given birth to 4.5 children, other murderesses to 3.2 children, child-murderesses to 2 children; thus the prisoners whose crime is commonly dependent on an abnormal eroticism had a fertility above the average.
The diminished fertility of prostitutes depends in part upon frequent venereal infection, in part upon the unfavourable influence of the mercury and iodide of potassium administered for the cure of such infection, also upon the frequency with which they consume excessive quantities of alcohol, upon the excessive frequency of coitus, which exercises a traumatic influence, upon the irregular mode of life, and upon their disinclination to be burdened with children.
Conjugal fertility, that is to say, the ratio between legitimate births and the number of married women between the ages of 15 and 50 years, has declined in Germany during the last decades. It was:
| During the years 1872 to 1875 | 29.7% |
| During the years 1879 to 1882 | 27.4% |
| During the years 1889 to 1892 | 26.5% |
This decline is small, but it is much more manifest in urban than in rural districts. This fact is shown by the following figures, relating to fertility in Prussia:
| 1872 to 1879. | 1894 to 1897. | |
|---|---|---|
| In all towns | 26.9 | 24.0 |
| In Berlin | 23.8 | 16.9 |
| In other large towns | 26.7 | 23.5 |
| In rural districts | 28.8 | 29.0 |
This difference depends principally on the fact that in the large towns of Germany (and still more in those of France) the use of means for the prevention of pregnancy is continually increasing, whereas the population of the rural districts is as yet less familiar with the use of these measures.
According to Hellstenius, conjugal fertility, that is, the number of children per married couple, is as follows:
| In the | Netherlands | 4.88 |
| Norway | 4.70 | |
| Prussia | 4.60 | |
| Bavaria | 4.55 | |
| Sweden | 4.52 | |
| Saxony | 4.35 | |
| England | 4.33 | |
| Belgium | 4.23 | |
| Denmark | 4.18 | |
| France | 3.46 |
Talquist, who has published a statistical investigation concerning the modern tendency to diminished fertility, arrives at lower figures than Hellstenius. According to him, conjugal fertility is: