Population projections undergird many important policy decisions, from the U.S. government’s Feed the Future program to the Sustainable Development Goals. But they’re not as straightforward as they appear. Demographers often base their estimates on complicated assumptions that aren’t obvious to the end user.

So when the Wittgenstein Center for Demography and Global Human Capital launched a major collection of new demographic projections last year, they put all their research online in a publically accessible data explorer. The portal allows users to map, graph, and download fertility, mortality, migration, and education data for every country from 1970 through 2100 from the Center’s peer review-style inquiry of over 500 experts.

When projecting population growth over decades and even centuries, demographers deal with uncertainty by making assumptions. Many projections assume that governments will make certain investments in family planning or health care systems, for example, and the UN Population Division assumes that in 2100, there is no net migration, since it’s nearly impossible to accurately predict patterns of human movement at the end of the century.

“But most users are not demographers,” said Jason Bremner, associate vice president and program director of population, health, and environment with the Population Reference Bureau, at the launch of the Wittgenstein Center book. Conveying these assumptions to policymakers so they fully understand what it takes to reach a certain projection is a major challenge. “This effort is an impressive step forward in looking at size and composition of every country,” Bremner said.

Stories About the Future

Often, demographers seek to convey the uncertainty in their findings by offering probabilistic projections. For example, the UN Population Division estimates that there’s an 80 percent chance that global population will reach between 9.6 and 12.3 billion by 2100.

“The problem with probabilistic projections is you don’t really have policy relevant scenarios,” said Wolfgang Lutz, director of the Wittgenstein Center and a lead author of the study, at the Wilson Center. “You give some objective distribution and you are not able to show what alternative interventions by politicians or more investment in education or in family planning what is it going to do.”

Instead, Wittgenstein demographers took a scenario-based approach, tailoring their projections to the Intergovernmental Panel on Climate Change’s shared socio-economic pathways – five scenarios that describe possible social and ecological evolutions over the next century. In the data explorer, users can compare how each pathway (in which the world moves towards sustainability, business as usual, inequality, conflict, or technological innovation), shapes a country’s various demographic indicators. The data explorer also lists the assumed total fertility rates, female life expectancies, and migration rates within every country under each pathway.

By using the shared-socioeconomic pathways, the team was able distill their complex projections into more relatable narratives, said William Butz, a co-author of the report. The goal was to demonstrate how several available sets of policies will likely influence fertility, mortality, and education trends, and in doing so, affect population size and societies’ adaptive and mitigative capacities.

“These pathways…are stories about what the future might look like,” Butz said. “[They] really lay out in some detail not just the assumptions about fertility migration and schooling, but also about how the rich countries of the world will relate to the poor countries of the world, and how the urban centers will relate to the periphery.”

Education is a centerpiece of each of those stories, said Butz and Lutz. They found that increased levels of education, particularly among young women and girls, not only lowered fertility and mortality, stabilizing population growth over time, but also improved a range of development outcomes, from food security to climate resilience and democratic transition.

Bremner, who at Population Reference Bureau helped craft an infographic explaining how assumptions can produce significantly different projections, said the opportunity for users to clearly visualize how the socio-economic pathways affect results is critical, and should be offered by all demographers, particularly as projections become more complex.