Lies, damned lies, and Lysistrata

“OUR women know what to do and when,” crowed Vladimir Putin, Russia’s president, after a recent uptick in the country’s fertility rate. A fall in America’s, by contrast, produced gloom. Both reactions were absurd. The conventional measure of fertility is a poor guide to how many babies people produce.

Imagine a Lysistrata scenario in which nobody procreates for a year. The commonly cited “total fertility rate”—the number of births each woman would have, assuming that in each year of her reproductive life she had the average number of children for women of that age in the current year—would fall to zero. Once the sex strike ended, though, it would bounce back, perhaps even rising higher than before, if couples try to make up for lost time. The “cohort fertility rate”—the actual number of children born to each woman in her lifetime—would change little.

Life is not a Greek comedy, yet something like this has happened in many countries. Parenthood now starts later than it used to. The total fertility rate falls as births are delayed; once older parenthood becomes widespread, it rises again. The ebb and flow of babies is a nuisance for hospitals and schools, but does not mean that babies are going in and out of fashion.

A cohort’s true fertility rate cannot be known until its women finish having children. But three demographers associated with the Max Planck Institute for Demography in Germany have tried to estimate it using various assumptions (see chart). The broad picture is the same: places with low total fertility also have low cohort fertility. But the new measure gives some countries cause for cheer. German women born in 1979 are expected to have more children, on average, than those born in 1970. The same is true of French ones. Even the Japanese seem to have become a little more fecund, starting with those born in the mid-1970s. But South Korea is still on a steep downward slope. The baby strike continues.