New Delhi, July 2 -- On a recent trip to Vienna in Austria, I found myself doing an odd calculation. Austria has a population of about nine million. Mumbai, the city I call home, houses more than 21 million people. A single Indian metropolis has more than twice the population of an entire nation that punches well above its weight on every measure of human development.

That contrast stayed with me. And it made political hand-wringing over India's declining fertility rate seem puzzling at best.

India's total fertility rate (TFR)-the average number of children a woman is expected to bear in her lifetime-has fallen from nearly 6 in the 1950s to about 1.9, just below the replacement level of 2.1. In demographic terms, this is the threshold a...