If you leave aside city states, Bangladesh is the world’s most densely populated country. At independence, its leaders decided that they had to restrain further population growth (China’s one-child policy and India’s forced sterilisation both date from roughly the same time). Fortunately, Bangladesh’s new government lacked the power to be coercive. Instead, birth control was made free and government workers and volunteers fanned out across the country to distribute pills and advice. In 1975, 8% of women of child-bearing age were using contraception (or had partners who were); in 2010 the number was over 60%.
In 1975 the total fertility rate (the average number of children a woman can expect to have during her lifetime) was 6.3. In 1993 it was 3.4. After stalling, it resumed its fall in 2000. After one of the steepest declines in history the fertility rate is now just 2.3, slightly above the “replacement level” at which the population stabilises in the long term. When Bangladesh and Pakistan split in 1971, they each had a population of 65m or so. Bangladesh’s is now around 150m; Pakistan’s is almost 180m.
Because of this Bangladesh is about to reap a “demographic dividend”; the number of people entering adulthood will handsomely exceed the number of children being born, increasing the share of the total population that works.This may be misplaced optimism on the part of the Economist. It is unlikely that fertility has dropped fast enough and low enough to have more than a modest impact (as opposed to a "handsome impact") on development. But it is better than the alternative, without question.
The second reason they mention is that the Green Revolution has kept rural families above water--in economic terms, if not always in hydrological terms. The third reason is the invention in Bangladesh of micro-credit, aimed largely at women. And the final reason, to which they accord the greatest weight, is the substantial role played in the country by NGOs. None of these other factors would matter, however, if fertility had not declined. Everything follows from that.