Even when the total fertility rate falls below 2.1 children, the “momentum” effects of earlier fertility trends will keep a population growing for many decades. In cases when the absolute size of a national population declines, the drop often turns out to be short-lived, and in aggregate numbers usually is so slight as to be of little significance.

THERE are exceptions, of course. After the Soviet Union collapsed in 1991, Russia experienced an unusual combination of rapid fertility decline to very low levels (falling to about 1.2 in 1999) coupled with remarkably high mortality for adult males — the latter largely attributable to alcohol abuse. This singular combination did produce a modest, and widely reported, decline in the Russian population. The Russian fertility rate has rebounded to just over 1.6, and mortality among working-age men has declined, perhaps because of concerted alcohol-regulation measures.

In nearly all countries that are not desperately poor, women of childbearing age inevitably compare the burdens of bearing children against the potential rewards. And in many of these countries, young women especially, but also young men, increasingly see marriage and childbearing as major risks, given high divorce rates and the responsibility to support aging parents, who are enjoying longer lives. The task of assessing these risks is heaviest for young adults (ages 20 to 35), who happen to be the same people who produce the most births in all societies. In Brazil, Italy, Turkey and the United States, these young adults must prepare for careers that now require extended years of higher education and, often, high levels of debt. Even with college degrees, employment is uncertain and career paths are increasingly unstable.

Meanwhile, dual-earner partnerships have become essential, in countries rich and poor alike, even as earnings have become increasingly precarious. High home prices and rents, along with limited access to mortgages, have restricted the ability to create the independent homes typically considered essential for new families.

Rapidly declining fertility — especially if rates go very low — does pose challenges. Yet it also can provide substantial benefits that have received less attention.

First, as noted, fertility decline is associated nearly everywhere with greater rights and opportunities for women. The deferral of marriage and the reduction of births to two, one or none across so much of the world — and, again, in countries that are still far from rich — are broadly consistent with the higher educational attainment and career aspirations of young women. It is no surprise that the hand-wringers over fertility decline are usually men.