Assumptions about human behavior drive our knowledge of future global population trends. Demographers analyze population and other survey data in order to forecast trends, but uncertainty colors these projections.

In the 2008 Revision of World Population Prospects, the UN Population Division projects that our planet will grow to 9.15 billion people by 2050. Yet this medium-variant projection is just one of several possible scenarios released in this latest round of number crunching. The low- and high-variant projections—7.96 billion and 10.5 billion, respectively—could instead become reality, given slight shifts in fertility rates in developing countries, where growth rates remain higher than in more developed nations. Although both developing and developed nations are susceptible to shifts in fertility rates, uncertainties are greater in the developing world due to factors such as inconsistent data collection, weak health system infrastructure, and low government capacity.

Elizabeth Leahy and I investigate the underlying assumptions behind population projections in an article in the May/June edition of World Watch magazine. By comparing three of the leading population-forecasting institutions, we find that small variations in assumptions can lead to significant differences in projections.

Uganda’s demographic outlook is a prime example. Between 1960 and 2005, Uganda’s population grew by 22 million people, while the country’s fertility rate fell by less than 3 percent. The UN medium-variant population projection assumes the country will buck precedent and experience a 61 percent fertility rate decline between 2005 and 2050, resulting in a population of 91 million people. The U.S. Census Bureau, on the other hand, assumes a less drastic fertility decline and projects a population of 128 million people by 2050. The International Institute for Applied Systems Analysis (IIASA), an Austrian institution that projects population on a regional basis, recently revised its population projections to reflect greater growth in sub-Saharan Africa due to stalling fertility decline and stagnant educational-attainment rates.

Fertility rates rarely decline when governments have not made the proper investments in health and education. The UN medium-variant projection is commonly cited as an inevitable scenario; few people know that one of its underlying assumptions is that access to modern contraception will continue to expand. Without real-world development investments to match these assumptions, a very different scenario could easily materialize. By empowering women, bolstering access to education, and providing comprehensive family-planning services to citizens, governments and policymakers can translate these assumptions into reality.