India‘s slowing population growth had a substantial impact on global population trends, a deeper look at new global population estimates published by the United Nations Population Division shows.A comparison of the latest World Population Prospects (2019 revision) with previous editions suggests that the downward revision in India’s population estimates for the future was the largest of any country, while the estimates for China’s population growth were simultaneously revised upwards.The estimates for when India is expected to surpass China as the world’s most populous country have been fluctuating every year.The new projections for India are the lowest they have been since the UN first began these projections a decade ago.Behind these downward revisions is the story’s of India’s rapidly accelerating demographic transition, a phenomenon whose pace has defied global expectations. Population projections emerge from the expected direction that trends in births and deaths take in a given country. Changes in the fertility rate - the expected number of children that a woman will have in her lifetime - affect the birth rate, while changes in disease environments (the slowing global spread of HIV/AIDS mortality, for instance) affect the death rate.India’s fertility rate has fallen faster than earlier predicted by international agencies, on the back of falling fertility in its poorest states. Even among high fertility social groups --- the most prominent and widely discussed of which are Muslims --- fertility is falling faster than anticipated.In demographic terms, 2.1 is the “replacement level of fertility”, meaning that if every woman had 2.1 children on average, the population would remain of the same size. By 2013, India’s Total Fertility Rate was already down to 2.3 children per woman, a further decline from 2.4 in 2012. The earlier UN population projections estimated a higher fertility rate for this period, throwing their population projections off.The TFR in 23 Indian states and Union Territories, including all of the south, is now below replacement level and only Bihar and Meghalaya still had TFRs above 3 children per woman as of 2016.But India’s poorest states with its highest fertility rates are moving the fastest at reducing fertility. In 2011, Sample Registration Survey data indicated that between 1999-2001 and 2009-2011, the states doing the best job at lowering fertility were ones with an already low fertility rate, raising concerns that fertility outcomes might be diverging instead of converging. However the most recent data has shown that between 2004-06 and 2014-16, the backward states were pushing forward, lowering their fertility fastest. In every state and at every age group, more educated women had the lowest fertility rates.Muslims in India continue to have a higher fertility rate than Hindus, as they have since the relevant Census data began to be recorded. However, Muslim fertility is falling faster than Hindu fertility, and the gap between the fertility rates of the two communities is closing, the most recent National Family Health Survey shows, according to an analysis by HowIndiaLives.com.States that are doing an overall better job on reducing fertility are also doing a better job of closing this gap between the two communities’ fertility rates, demographer Saswata Ghosh found - as these states get richer, better educated and healthier, all women in these states are beginning to have fewer kids. As a result, Muslim women in the southern states have a lower fertility rate than Hindus in the Gangetic belt states; high Muslim fertility is only a problem in states with high levels of fertility for all women. Even at the district level “[g]enerally, it has been observed that in areas with a considerable decline in fertility there is hardly any district with very high fertility among Muslims,” he found.In India, however, fertility also closely ties in with son preference. Over the same time period that India has lowered its fertility, its sex ratio at birth --- the proportion of girls to boys born to all women --- has become markedly skewed in favour of boys. Limiting family size is closely related to the sex and birth order of the family’s existing children. According to the latest National Family Health Survey, 89% of women aged 15-49 with two sons and no daughters were content with a family size of four. But for women who had daughters but no sons, a larger family was on the cards. Among women aged 15-49 with two living daughters and no sons, 63% wanted no more children, in comparison. With no real evidence that the Indian preference for at least one son is going away any time soon, slowing population growth may continue for the near future to mean that families, as far as possible, attempt to have fewer girls.Implicit in projections of global population, however, are assumptions about development, mortality and fertility that at times require the UN demographers to take leaps beyond what historical data predict, an exercise that can lead to revisions if reality does not pan out in precisely the same way. For example, the 2019 projections find that in some east Asian and southern European countries (such as Japan, Korea, Greece and Italy), fertility has fallen below 1.5 children per woman. However, since women in these countries express a desire for two children, the UN projects that in the future, with better childcare options, fertility in those countries might rise.Rukmini S. is a Chennai-based journalist.