WY THILE intentional limitation of offspring cannot be regarded as the sole of the decline in fertility, it is safe to assume that it has been the most important factor in recent decades.' The purpose of this paper is to discuss theoretically those societal conditions and changes which appear to be the most important incentives to the diffusion of birth control practices. Since such practices are found under widely varying conditions in quite different types of societies, this theory claims validity only for the Western culture area and only for the period of declining fertility since the end of the nineteenth century. The rate of procreation, being largely dependent on human will, is essentially conditioned by the totality of social life and especially by the socioeconomic order of the society. The same pattern of behavior can be differently motivated in different types of society.2 Further, the more complex the structure of a society, the greater will be the variety of factors which can eventually induce limitation of offspring. Therefore, instead of attempting to formulate a law or to discover the final cause of birth control, one will have to point out the specific constellations of various possible causes with various peoples and with different groups and classes of these peoples. Furthermore, the changes within these constellations, which are conditioned by social developments, should be considered. Finally, among these various causes, those should be studied primarily which can be assumed to be effective in the broad masses of a population. Thus, it may be granted that the desire to have children may have been weakened in certain circles, especially in metropolitan populations, but since this is far from universal, it cannot account for the general and steady decline of the birth rate since the end of the nineteenth century. A very large proportion of those who practice birth control would probably have more children if they were not induced by strong reasons to resort to contraception or abortion.3