There is another way to investigate controversies over population: to go and see. In January, my mind full of academic disputes, I flew to Bombay, a city of 11 million squeezed onto a narrow peninsula jutting into the Arabian Sea. I assumed that in India, the nation best known for "overpopulation," theories come to ground.

I arrived at two o'clock in the morning. The fetid odor of waste was thick in the night air. From my taxi, I saw families sleeping on open ground by the road, still as corpses. This is the India that horrifies and fascinates Western visitors, an India where wealth hides behind walls and poverty hits you with a body blow.

Half of Bombay lives in slums; an estimated 200,000 sleep on the pavement. Water comes on only two to eight hours a day—sometimes not at all. Yet the city keeps gnawing, not so much pushed by a desperate rural population as pulled by the lure of city life. As Supriya S. Prasad, a program officer for the Christian Medical Association of India, later told me in Delhi, "Everybody says how happy they were in the village, but they don't want to go back."

I went to the Ministry of Health and Family Welfare, where state family-planning programs are administered. India's are the oldest in the world, dating from 1952, and so it felt inside the building. Long, dim, airless corridors had fluorescent lights strung like spiders' webs from impossibly high ceilings. "You felt that every day, in the name of cleaning, someone had rubbed the place down with a lightly grimed rag," V. S. Naipaul described a Bombay establishment, and he could have been picturing this one.

For several hours I wandered the building, bounced from one official to another. Through doorways I caught glimpses of clerks seated at ...