There’s always some uncertainty to making predictions, but Vogl said that population projections are usually “less uncertain” than other social and economic projections. This is because researchers already know roughly how many humans there are now, as well as how old everyone is, so they can guess, with some confidence, how many people will be of childbearing age in the next couple of decades—which means they can then guess how many children those people will have.

Even if future fertility rates stray a bit from expectations, Vogl said, it wouldn’t “change the fact that sometime in the next [100 years], the world’s population is going to peak.” And his hunch is that the population is unlikely to go up from there, barring some major increase in fertility rates (perhaps as a result of a political movement that encourages people to have a lot of babies, which is what happened in China in the mid-20th century).

Because some determinants of what the population will be 80 years from now are locked in today, it’s possible to anticipate broad demographic shifts. “By the time the world population stabilizes, Africa is going to be the largest world region in terms of population … and Islam is going to be the world’s largest religion,” Vogl said.

And crucially, the human population will, as a whole, get older. The UN’s data suggest that during the eight decades from 2020 to 2100, the number of people aged 80 or older will rise from 146 million to 881 million; during roughly that same span of time, humans’ median age will increase from 31 to 42.

When the population of a single country gets older like this, “that typically poses big problems for the country’s politics,” Vogl said. In this scenario, working people have to support a growing number of retirees, both on the society level, in terms of funding national retirement-benefits programs, and on the household level, where aging relatives might need family members’ care. (Japan is a frequently cited example of a country currently facing these issues as a result of low fertility rates and long life spans.)

“When that happens on a global level, it means that that pension crisis is going to happen in many countries independently, at different points along that global path,” Vogl said. As each country encounters this problem, immigration—bringing in younger, work-ready people from countries with a lower concentration of older people—could counteract the aging dynamics. But today’s politics indicate that immigration is not a simple fix. Less controversial ways for countries to offset this problem include growing their economy (because there would be more money to go around) and creating more opportunities for women in the labor market (which would alter a country’s ratio of workers to retirees).

The population trends of the rest of the century will also alter the texture of family life. As the fertility rate declines in many parts of the world, families will get smaller. This means children will tend to have fewer siblings, and parents can invest more of their resources and attention in each child, perhaps paying more to send them to a better school, notes Parfait Eloundou-Enyegue, a development-sociology professor at Cornell University. “And also, the culture moves in the direction in which the families become nuclear rather than extended,” he adds. “So in Africa, for instance, you have extended family systems where people would live with cousins and in-laws.” In many households, that family structure might start to give way to a smaller one.