Which is the more powerful agent of social change: fear or sympathy? Women in rich and middle-income countries may soon find themselves enrolled in a real-life experiment testing this proposition. That is because birthrates are dropping in much of the world. Demographics may soon rocket to the top of the political agenda, demanding an entirely new way of thinking about women and motherhood and the economy.

One reason for the shift was, as it were, born in the USA. That is because, for a long time, the United States has watched declining birthrates in places like Western Europe, Russia and even China with an air of superiority. The United States, lusty and fertile, was bucking the demographic trends.

Then, last week, new data showed that in 2011 the U.S. birthrate fell to the lowest level ever recorded: 63.2 babies per 1,000 women of childbearing age.

Crucially, immigrant women, whose fecundity had been holding up the U.S. figures, opted out of the maternity ward in the greatest numbers. According to analysis done by the Pew Research Center, the birthrate for women born in the United States fell by 6 percent between 2007 and 2010. For foreign-born women in the United States, the drop was 14 percent. Among Mexican immigrant women, the rate plunged 23 percent.

This is a big change for the United States, bringing the birthrate in the country more closely in line with those of the rest of the developed world. The total fertility rate in the United States, a measure of the total number of children the average woman is likely to have, was 1.89 in 2011.