I n 1975 a typical Bangladeshi woman would have had seven children in her lifetime; today she would probably have three. This sudden decline, known as a fertility transition, is the most extreme case in a pattern that has emerged throughout South Asia. It occurred first in Sri Lanka, then in India and most recently, in Bangladesh and Nepal. The drop has demographers baffled. In the West, fertility started falling afterwards as advanced stage of development had been

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Some observers, such as Sajeda Amin of the Population Council in New York City, credit the Bangladeshi success to the government’s intensive family-planning program.

But although such efforts have provided essential access to contraception, they are working because Bangladeshis have also decided to have fewer children. In 1975, when asked how many children she wanted, a typical woman would reply four. Today she would say two.

Demographers agree that the fertility transition is ultimately caused by a drop in mortality. Once a couple realizes that their children are likely to survive, they can give birth to fewer infants and still be sure of being cared for in their old age. But according to Sonalde Desai of the University of Maryland, it used to be 50 years before a mortality dorp led to the fertility transition; now it is taking barely 10. And in Bangladesh, the connection is especially weak: infant mortality had remained at the rather high level of about 14 per 100 live births for two decades preceding the fertility transition.

Another oft-cited trigger for the transition is microcredit, an idea pioneered by Bangladeshi economist Muhammad Yunus. Since the 1970s, his Grameen Bank and another private organization, the Bangladesh Rural Advancement Committee (BRAC), have been making small loans to poor rural men and women. Monitoring by peers replaces collateral, leading to a repayment rate of more than 90 percent. Currently three million Bangladeshis, mostly women, have access to such credit, which they use to set up small ventures.

Although the programs have clearly been beneficial, their impact on fertility is hard to decipher. Both Grameen and BRAC require grantees to take a set of resolutions, one of which is to have small families. Women do use contraceptives more consistently when they belong to Grameen. More curiously, women in village, where Grameen operates are more likely to use contraceptives than women in other villages, even if they are not Grameen members.

Such an effect may come from an unconscious bias in Grameen’s choice of villages. On the other hand, it could be that the bank’s messages are diffusing throughout the community. Amin points out that microcredit programs were too small in the late 1970s, when the fertility transition began, to have been directly responsible for it. They might, she concedes, have had a catalytic effect.

Another factor for the transition, cited by Moni Nag of Columbia University, is less pleasant. The early 1970s were traumatic for Bangladesh. A bloody war with Pakistan led to the nation’s birth. Many women were raped in the war, and many more died in the floods and famines. The resulting upheaval in the social order-large numbers of women left their homes to become manual laborers-may, in a bizarre twist, have forced women to take more control of their fates. But Adrienne Germain of the International Women’s Health Coalition in New York City takes issue with such poverty-driven reasoning for the drop. Bangladesh, she points out, is no longer the basket case it was once labeled by former U.S. secretary of state Henry Kissinger: it has seen quite a bit of development. “Even though demographers can’t seem to measure it,” Germain adds, “there’s been an enormous change in the status of women.”

The final explanation for the population puzzle may lie in the information age. Bangladeshi radio provides six hours of health and family-planning programming a day. “People seem to think it is irresponsible to have large numbers of children because of overpopulation,” Amin remarks. Such awareness, remarkable in a people that cannot be sure of getting two square meals a day, suggests that media messages can on occasion replace literacy. Across the border in the Indian state of West Bengal, fertility has also dropped, in a radial pattern around the city of Calcutta. Evidently, urban centers serve to somehow disseminate the idea that small families are better.

Ultimately, Bangladesh offers few lessons that policy makers can apply to other regions of the world; everything seems to have played a role. Perhaps the good news is that even the simplest ideas are worth trying.