Projecting population: population forecasts depend on assumptions--and can be wildly wrong.

• Author: Leahy, Elizabeth

In the Wasiko district of Uganda near Kampala, 35-year-old Yudaya and her husband, a peasant farmer, live with their six children, who range in age from 1 to 12 years old. Yudaya only attended school until her father died when she was 10, and remains illiterate. She and her husband do not use family planning. They would likely have an even larger family, but she has breast-fed each of her children for at least 18 months, a practice that suppresses fertility.

Living on a small patch of land in a two-room hut, the couple's limited resources are stretched thin. Their children are often turned away from school because their school fees remain unpaid. Although Yudaya and her husband agree that they would like no more children, unequal gender roles in her society, combined with Yudaya's scanty education and lack of accurate medical information, inhibit her use of family planning. When Yudaya tried to use contraceptives in the past, she experienced side effects and ultimately stopped because of her husband's resistance. "I had to stop using family planning because my marriage was at the verge of breaking up. In my culture, marriage is a bond, the man will use you whenever he wants and you should have no excuse to deny him your body."

Yudaya's story reflects how global demographic trends, which develop over a seemingly vast scale, are dependent on the decisions and opportunities of individual women, couples, and families. Yudaya's family of six children is actually slightly small in her country, where women average 6.7 children each. Uganda's population is growing by more than 3 percent annually and could double in less than 25 years. Yet the uncertainty of how the reproductive lives of Yudaya's children and others in future generations around the world will play out demonstrates that projections of our demographic future must always be viewed in the context of their assumptions and sometimes be considered with caution.

When the future of the world's population is referenced in mainstream media, it has nearly always been coupled with the magic number of 9.2 billion--until March, the projected total world population for 2050 under the commonly cited United Nations "medium projection." The medium projection is often treated as a certainty, but any projection is heavily dependent on a number of assumptions. When deciding where and how to invest in development, demographic projections cannot be taken for granted; the impact of current trends and policies will make all the difference. It is critical to ask what real-world policy changes will be required for this or any other population projection to materialize.

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UNCERTAINTIES

The differences between population projections and demographic trends are sometimes stark. The UN projections, for example, assume that world fertility rates will converge at a standard level of 1.85 children per woman, which seems rosily optimistic for countries at both extremes of high and low fertility when evaluated against recent trends. For instance, despite the general global progress along the demographic transition (with declines in birth rates following improvements in life expectancy), survey data show that two-thirds of countries in sub-Saharan Africa have seen no significant decline in fertility in recent years.

Because of the principle of population momentum, small variances in demographic trends today are compounded over time, magnifying their effect. Unexpected stalls in fertility decline, for example, can undermine the accuracy of population projections, as was the case recently in Kenya. Surveys reveal that between 1998 and 2003 the total fertility rate in Kenya rose from 4.7 to 4.8 children per woman. However, because the UN medium-fertility variant projections had assumed fertility would decline, the demographic momentum created by the stall in fertility nearly doubled the projection of Kenya's total population in 2050, from 44 million in the UN's 2002 revision to 83 million people in the 2004 revision.

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In some cases, the rapid and drastic projected, accelerations of fertility decline--and the much-improved access to and use of reproductive health services that will be required to achieve them--seem outright unrealistic. For example, Uganda's fertility rate declined by less than 3 percent between 1960 and 2005 as the country's population rose from 7 million to 29 million, according to the UN. However, its projections assume that fertility will decline by 61 percent between 2005 and 2050, to less than three children per woman. Even with this projected decline, Uganda's population would triple to 91 million by 2050. The U.S. Census Bureau takes an even less rosy view in its projection for Uganda, assuming gradually declining fertility over the next 40 years that only reaches 4.9 children per woman. This would lead Uganda's population...