These figures are from a sufficient number of cases to be substantially reliable. For instance, there is not one chance in a thousand that the Harvard average is 10 per cent. too low. The existence and approximate amount of the decrease in the size of family is thus certain. Its substantial identity in Middlebury, a country college in Vermont with a local attendance, in New York University, a city college, and in Wesleyan University, a strongly sectarian college with an attendance drawn from the northeastern states, makes it probable that it has prevailed throughout the college population of the north Atlantic states. It must depend upon some fundamental cause.
City life and advanced age at marriage are out of question. The former cause would work to a far greater extent upon New York University or Harvard graduates than upon Middlebury graduates, all of whom come from and most of whom go back to life in small towns. Yet in the statistics there is little difference. An increase in the age at marriage can not have been the cause for the simple reason that such increase, as I have elsewhere shown, amounts only to a very few months. An increase in the age at marriage of the wives of our group of men would be a more efficient cause. I know of no available statistics to decide the question, but it would seem extremely unlikely that the age of wives should have increased much when the age of husbands has increased so little.
The most plausible explanation attributes the change to the custom of conscious restriction of offspring. Greater prudence, higher ideals of education for children, more interest in the health of women, interests of women in affairs outside the home, the increased knowledge of certain fields of physiology and medicine, a decline in the religious sense of the impiety of interference with things in general, the longing for freedom from household cares—any or all of these may be assigned as the motive for the restriction. The only other explanation which to the present writer seems adequate assigns the decreased productivity of college men to real physiological infertility of the social and perhaps of the racial group to which college men and their wives belong.
It is possible to do more than speculate about the relative shares of unwillingness and incapacity. The figures themselves tell a plain story to the student who examines them in the light of recent knowledge of the variability of physical traits.
If we tabulate the records by decades so as to show the percentages that families of 2, 3, 4, etc., children were of the total number of families, we can see just how the decrease in the averages has been brought about. Suppose for instance that we had in 1803-1814 and in 1865-1874 the following percentages:
