October 31, 2011, was notable not only for the annual ritual of candy and costumes, but also for its designation by the United Nations as the date when global population reached seven billion. Although just an estimate – demographers are not able to count individuals in real time on such a large scale – the event was an important opportunity to present population trends to the media and public dialogue. Several babies born that day were named the “seven billionth;” in Russia, where various incentives have been implemented to try to boost an ultra-low fertility rate, Vladimir Putin visited a maternity ward to greet one of them.

Less than two years later, the biennial release of the UN’s revised World Population Prospects has been received with less hoopla, but is much more important for those who track demographic trends. This year’s revision is a reminder of the challenge of precisely accounting for human numbers and of how a slight change in current figures can have dramatic effects when compounded over decades through population momentum.

For the second revision in a row, the world’s projected population for 2050 has been revised higher in the UN’s medium-fertility variant; it is now projected to reach nearly 9.6 billion in 2050, up from 9.3 billion. As in earlier revisions, there is a very important caveat to this projection: It assumes a gradual convergence in countries’ fertility rates, the main driver of demographic trends, towards 2.1 children per woman, which would lead to a roughly stable global population. But for the last several revisions, these assumptions have proven too optimistic. The new 2050 projection is an increase of 400 million people relative to the 2008 revision and 250 million relative to the 2010 revision, changes that are largely due to the fact that fertility rates have remained higher than expected in much of sub-Saharan Africa and some other developing countries.

The Demographic Power of High Fertility Rates

Current fertility rates are higher in the 2012 revision compared to the previous for two reasons. In some countries with high rates, fertility has actually risen in recent years – a rare phenomenon, given that the demographic transition theory posits a steady and sustained decline once high fertility rates begin to fall. In others, especially in sub-Saharan Africa, the UN deemed its prior estimates too low based on new data.

The effect of higher-than-expected fertility rates is shown in Table 1 below, which lists the countries where projected populations for 2050 have increased by the largest amount, in percentage terms, compared to the 2010 revision. In several of these countries, new Demographic and Health Surveys (DHS), which provide the raw data that inform many of the UN estimates, have been published showing that fertility rates are higher than the UN previously estimated. In Burundi, a 2010 DHS reported a total fertility rate (TFR) of 6.4 children per woman; the UN had estimated 4.7 for 2005-2010. In Cameroon, a 2011 DHS found a TFR of 5.1, while the UN had estimated 4.7; in Zimbabwe, a 2010-11 DHS showed an unusual increase in fertility since the previous DHS, from 3.8 to 4.1 – both higher than the UN’s estimate of 3.5.

Not all new Demographic and Health Survey results are so gloomy. In Ethiopia, for example, modern contraceptive use has nearly doubled in recent years from 14 to 27 percent, according to a 2011 DHS. And total fertility rate is now below replacement level in the capital of Addis Ababa. These are indicators that progress through the demographic transition may be speeding up. However, the UN had projected that fertility would begin dropping even more dramatically to 3.9 nationally by 2010-2015. That projection has now been revised upwards to 4.6 in alignment with the recent DHS results.

Although not in the top 10 by percentage change in this revision, Nigeria is a particularly interesting example of the challenge of assuming that past fertility transitions will be replicated universally. In all, the largest country on the African continent has had its projected population for 2050 revised upwards by more than 50 percent over the past four years. The country’s projected population for 2050 has risen from 289 million in the 2008 revision to 390 million in the 2010 revision and now to 440 million. If the latest projection holds (certainly not a given), Nigeria will have 10 percent more people than the United States by mid-century.

The 2012 projection for Nigeria assumes a fairly gradual decline in fertility from 6.0 children per woman now to 3.8 by 2050. Given that the 2008 DHS reported that married women had an average ideal family size of nearly seven children, and only 10 percent were using a modern contraceptive method, preferences about and use of family planning would need to change significantly for this projection to be achieved. The findings of a new DHS conducted earlier this year, not yet available, will provide insight into any potential shifts in these indicators.

Demographic Diversity: Least Developed Countries Growing Fastest

Differential rates of growth mean that the population of the world’s poorest countries will soon well surpass that of the wealthiest countries. Currently, about 900 million people live in the “least developed countries,” which have an average per capita income of $750 per year. This figure is projected to more than triple to almost three billion by 2100, even assuming that fertility falls to replacement level.

Meanwhile, the population of the “more developed countries” (Europe, North America, Japan, Australia, and New Zealand) is projected to increase slightly to 1.3 billion, assuming that fertility rises by 15 percent. (Incidentally, without immigration, the UN projects the population of developed countries would decline.) Current estimates of fertility in some countries with very low rates, including Serbia, South Korea, and Thailand, have been revised downwards since the 2010 revision, but remain stable for others, including Portugal, Japan, and Russia.

The countries where projected 2050 populations have been revised downwards by the greatest percentage, shown in Table 2, are largely those where current fertility rates are lower than previously estimated. Although these countries’ fertility rates are still high, the revisions demonstrate that fertility trends are moving in many different directions, with some countries overcoming economic and social challenges to improve women’s access to and use of family planning and broader health services even as their neighbors do not.

These include Afghanistan, where a survey published in late 2011 reported rather surprising and welcome findings, including a contraceptive use rate higher than many African countries and a fertility rate of 5.1 children per woman, more than one child per woman lower than the UN’s estimate. The country’s projected 2050 population is now 57 million, more than 25 percent less than the previous projection.

In Malawi, a 2010 DHS estimated a fertility rate of 5.7 children per woman, lower than the UN’s estimate of 6.0. More than 40 percent of married women there are now using modern contraception, and President Joyce Banda is placing a strong emphasis on family planning, girls’ education, and women’s economic empowerment, all of which are likely to contribute to further demographic changes.

Will the New Revisions Hold True?

The UN’s assumption that fertility rates will decline significantly, steadily, and soon in the countries where they remain highest is somewhat controversial. This hypothesis is based on the experience of various regions that have passed through the demographic transition already, where declines in infant and child mortality were followed within a few decades by declines in fertility. However, sub-Saharan Africa, where remaining high fertility rates are concentrated, is not following this trajectory as uniformly or rapidly as other regions have.

Demographers have noted a phenomenon of fertility “stalls” in sub-Saharan Africa, when an initial decline in fertility stops for at least several years. The effect of such stalls is reiterated in the new UN projections. While some countries on the continent have made remarkable progress in expanding family planning services and use, recording 15 to 30 percentage point increases in the rate of modern contraceptive use over the past decade, data from countries such as Nigeria show that such successes cannot be taken for granted.

The revision’s authors recognize that their assumption of fertility decline is built upon another assumption, namely that the conditions that have facilitated such decline will become more widespread across sub-Saharan Africa. These include increases in contraceptive use, continued improvements in child health and women’s education, and changing preferences about family size. “To achieve such reductions, it is essential that access to family planning should expand, particularly in the least developed countries,” the UN demographers explain; otherwise, “the world population by 2100 could increase by nearly six times as much as currently expected.”