Almost the world over, women are having fewer children than ever before. But new research suggests fertility rates can and perhaps will bounce back.

The reason is an oft-overlooked correlation in fertility research that suggests people who come from large families tend to have large families of their own. Over time, these people may come to dominate the population, reversing the trend of having only one or two children, researchers report Jan. 28 in the journal Proceedings of the Royal Society B.

Predicting the future of fertility is tough, said lead researcher Martin Kolk, a doctoral student in demography at Stockholm University.

"What we do know," Kolk told LiveScience, "is that ignoring this role of fertility correlations across the generations, that is probably wrong." [Crowded Planet: 7 (Billion) Population Milestones]

Fewer babies

Approximately 11 billion people will walk the planet by 2100, a population likely to tax Earth's water supply, waste-management and food resources. Nevertheless, the trend of declining fertility has its own set of problems: With more older people needing medical care and fewer younger people working to support the aging population, governments struggle to pay for their citizens' needs.

This population contraction has led to baby boosterism in some countries. In Japan, women have a fertility rate of 1.39 the number of children expected per woman in childbearing years, according to 2010 data from the World Bank. There, Prime Minister Shinzo Abe has promised reforms to make child care cheaper and to promote flexible work hours for women. France, the country with the second-highest fertility rate (2.03 in 2011) in Europe behind Ireland, has fought to keep birthrates high with government grants to mothers and paid maternity leave, among other policies.

The United States had a fertility rate of 1.88 in 2012, below the replacement rate of 2.1, meaning more people are dying than are being born, according to the Centers for Disease Control and Prevention. (Immigration keeps the population growing slightly.) Fertility is dropping even in less-developed countries. The only exception to the global trend is in sub-Saharan Africa, where birthrates are still high.

Another baby boom?

Before the advent of reliable contraception and relaxed social roles for women, pretty much everyone had lots of children. For the last two generations or so, children have been a choice, and families can grow according to their preferences. (Economics explain some, but not all, of people's family size choices, research finds.) [The History and Future of Birth Control: 12 Tales]

As a result, a new correlation has emerged: People from small families tend to have only a few kids, and people from large families tend have large broods. Kolk and his colleagues wanted to know what effect this correlation would have on the fertility rate in the long term.

The researchers built a mathematical model much like the ones used by biologists to study evolution over time. First, they set up the model so that children inherited either high or low fertility preferences from their parents. The environment was set up so that people could generally achieve their choices.

The result of this model showed, within three generations, a group of people who preferred to have lots of children and did so and a group of people who preferred to have few children and did so. Because those who preferred to have lots of kids passed on their preferences to more people than those who preferred to have a few, big families dominated and the population began to grow.

This model could be accurate if the cultural change that produced small families is a one-time thing, Kolk said. But it's also possible that cultural change is continuous. New leisure activities, new career opportunities and increasing diversity of choice could lead to more and more people choosing fewer children, even if their parents had many kids.

Predicting fertility's future

To model that possibility, the researchers altered their first model to include random "mutations" the possibility that some kids would buck their parents' preferences. They found that in this model, there was a similar initial drop in fertility, but with only a small rebound compared with the first scenario.

It's impossible to say which of the two scenarios will occur, Kolk said. And the model doesn't take into account factors like the planet reaching its carrying capacity, after which populations have to stop growing. Still, he said, the idea that fertility will stay low forever is not a given. The process of recovery is slow in the models, taking five to six generations, but it could occur.

"Maybe in some ways, it's good to reassure people that think that childbearing would become very, very low," Kolk said. "If childbearing would became very low, like in Germany or Japan, maybe something like we described could increase fertility, even though it could take a long time."

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