New Delhi, June 5 -- IN THE SPRAWLING slum in Delhi where Parul Gayen lived in the 1970s, children were everywhere. It was not unusual then that her mother had been one of six, or her grandfather one of 11. Swapan, the handsome boy whom she often saw cycling to work, and later married at 16, had six siblings-the seventh did not survive infancy. But times have changed, says Ms Gayen, who is now 58 and lives with Swapan in a one-bedroom flat nearby. Of the couple's three grown-up kids, only two decided to have children of their own. Both stopped at one. "One child feels lonely," she says.

In 1950 India's population was 360m. The average woman had six children-roughly the same as an American woman a century earlier. Today, with a population...