Dr. Cattell's investigation of the families of 1,000 contemporary American men of science all of which were probably not complete however, shows that they leave, on the average, less than two surviving children. Only one family in 75 is larger than six, and 22% of them are childless. Obviously, as far as those families are concerned, there will be fewer men of inherent scientific eminence in the next generation than in this. The decline in the birth-rate is sometimes attributed to the fact that people as a whole are marrying later than they used to; we have already shown that this idea is, on the whole, false. The idea that people as a whole are marrying less than they used to is also, as we have shown, mistaken. The decline in the general birth-rate can be attributed to only one fact, and that is that married people are having fewer children. The percentage of childless wives in the American stock is steadily increasing. Dr. Crum's figures show the following percentage of childless wives, in the New England genealogies with which he worked: 1750-1799 1.88 1800-1849 4.07 1850-1869 5.91 1870-1879 8.10 J. A. Hill[124] found, from the 1910 census figures, that one in eight of the native-born wives is childless, as compared with one in five among the Negroes, one in nineteen among the foreign born. Childlessness of American wives is therefore a considerable, although not a preponderant factor, in this decline of the birth rate. Dr. Hill further found that from 10 marriages, in various stocks, the following numbers of children could be expected: Native-born women 27 Negro-born women 31 English-born women 34 Russian-born women 54 French Canada-born women 56 Polish-born women 62 The women of the old American stock are on the whole more sterile or, if not sterile, less fecund, than other women in the United States. Why? In answer, various physiological causes are often alleged. It is said that the dissemination of venereal diseases has caused an increase of sterility; that luxurious living lowers fecundity, and so on. It is impossible to take the time to analyze the many explanations of this sort which have been offered, and which are familiar to the reader; we must content ourselves with saying that evidence of a great many kinds, largely statistical and, in our opinion, reliable, indicates that physiological causes play a minor part in the decrease of the birth-rate.[125] Or, plainly, women no longer bear as many children, because they don't want to.