Advocates of zero population growth about four years ago began urging couples who wanted children to "stop at two." At first their campaign seemed to be having little impact. Last week, however, Government statistics showed that for the first time in U.S. history, the fertility level has dropped to 2.1 births per woman. This means that the fertility rate is down to the replacement level: if it continues at that level for a few more generations, and if emigration equals immigration, the number of births every year will just offset the number of deaths, and the U.S. population will become stabilized.
How much credit Z.P.G. can take for the drop is open to question. Smaller families had already become more fashionable by the time the group began its campaign, and economic factors and liberalized abortion laws have also had an effect. But whatever the reason, the fact is that the fertility rate for the first seven months of 1972 was lower than ever before: 72.7 births per 1,000 women aged 15 to 44, compared with a previous low of 75.8 in 1936 and a modern high of 122.9 in 1957. Moreover, the Government notes, the number of children that married women aged 18 to 24 now say that they expect to have averages out to 2.3 each. The exclusion of unmarried women from that statistic accounts for the fact that the expected trend for this age group is 2.1.
Even at the current rate, Z.P.G. is still some 70 years offand it may never come at all. One factor in the delay is that so many girls were born during the baby boom that the number of females in the prime childbearing ages will go on climbing until the early 1980s.
Another reason that the U.S. population will continue to grow is the current high proportion of young to old; although the number of aged people is growing, there are simply not enough oldsters around to die off and thus balance all the inevitable births among the nation's even larger population of young people.
Even more important, the desire for offspring is volatile; in a few years, big, or at least bigger families may become popular again. Because there are so many potential mothers (17 million women aged 20 to 29 now, compared with 12 million 20 years ago), a small upward revision in childbearing plans, say from two babies to three, could make a huge differenceabout 50 million by the year 2000in the total population.