(Phys.org) —A trio of researchers at Stockholm University is suggesting that the current worldwide trend of low birth rates could reverse itself due to intergenerational fertility correlations. In their paper published in Proceedings of the Royal Society B: Biological Sciences, the team describes how they built mathematical models that use correlations between family size and birth rates to illustrate population growth rates in the future.
Human history has shown over thousands of years that there is a correlation between family size and birth rates—people that come from large families tend to have more kids than people who come from small families. Over the past 200 years, however, that trend has been altered—birth control and societal roles has led to radically smaller family size in most countries of the world (the lone exception being sub-Saharan Africa). If the trend continues, it could spell trouble for modern societies as older people begin to outnumber the young—it's young people, after all, who generally do most of the work in a society. But low birth rates may change, the researchers suggest, due to natural tendencies. They've created mathematical models that indicate worldwide population is likely to take one of two paths.
In the first scenario, if people from large families have more kids, even if there are far fewer of them, eventually, a shift will occur as each generation produces more offspring than average—at some point they would overtake the lower birthrate people, causing a growth in population.
In the second scenario, societal barriers appear that override intergenerational fertility correlations—similar to those that have caused low birth rates today. If the population grew too large for the planet to support, for example, natural processes would cause lower birth rates (or higher death rates) to occur—people would ignore the size of the family they were born into and intentionally choose to have fewer kids, causing population growth rates to slow.
Obviously no one knows if societies of the future will keep adding disincentives for having larger than average families–overriding intergenerational fertility correlations—but the models make it clear that if nature is allowed to take its course, it's very likely that several generations into the future, fertility rates may climb again, presenting governments with a whole new set of problems. The researchers suggest it's time that other researchers and governments stop ignoring such correlations, as the evidence for it is overwhelming.
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More information: Martin Kolk, Daniel Cownden, and Magnus Enquist." Correlations in fertility across generations: can low fertility persist?" Proc. R. Soc. B March 22, 2014 281 1779 20132561; DOI: 10.1098/rspb.2013.2561 1471-2954
Correlations in family size across generations could have a major influence on human population size in the future. Empirical studies have shown that the associations between the fertility of parents and the fertility of children are substantial and growing over time. Despite their potential long-term consequences, intergenerational fertility correlations have largely been ignored by researchers. We present a model of the fertility transition as a cultural process acting on new lifestyles associated with fertility. Differences in parental and social influences on the acquisition of these lifestyles result in intergenerational correlations in fertility. We show different scenarios for future population size based on models that disregard intergenerational correlations in fertility, models with fertility correlations and a single lifestyle, and models with fertility correlations and multiple lifestyles. We show that intergenerational fertility correlations will result in an increase in fertility over time. However, present low-fertility levels may persist if the rapid introduction of new cultural lifestyles continues into the future.