The American baby boom after the war made unconvincing U.S. advice to poor countries that they restrain their births. However, there has hardly been a year since 1957 in which birth rates have not fallen in the United States and other rich countries, and in 1976 the fall was especially sharp. Both East Germany and West Germany have fewer births than they have deaths, and the United States is only temporarily able to avoid this condition because the children of the baby boom are now an exceptionally large group of married couples.
    It is true that Americans do not typically plan their births to set an example for developing nations. We are more affected by women's liberation: once women see interesting and well-paid jobs are careers available, they are less willing to provide free labor for child raising. From costing nothing, children suddenly come to seem impossibly expensive. And to the high cost of children are added the uncertainties, introduced by divorce; couples are increasingly unwilling to subject children to the terrible experience of marital breakdown and themselves to the difficulty of raising a child alone.
    These circumstances—women working outside the home and the instability of marriage—tend to spread with industrial society and they will affect more and more countries in the near future. Along with them goes social mobility, ambition to rise in the urban world, a main factor in bringing down the births in Europe in the 19^th century.
    Food shortage will happen again when the reserves resulting from the good harvests of 1998 and 1999 have been consumed. Urbanization is likely to continue, with the cities of the developing nations struggling under the weight of twice their present populations by the year 2010. The presently rich countries are approaching a stable population largely because of the changed place of women, and they incidentally are setting an example of restraint to the rest of the world. Industrial society will spread to the poor countries, and aspiration will exceed resources. All this will lead to a population in the new century that is smaller than was feared a few years ago. For those anxious to see world population brought under control, the news is encouraging.  What influences the birth rate most in the United States is ______.