STOCKHOLM, January 30, 2014 ( – A team of researchers at Stockholm University has found that critically low fertility rates, especially in developed countries, may actually be reversed in the long term due to what they call “intergenerational fertility correlations,” which suggest that the generational trend of children from large families tending to have large families of their own will eventually result in an overall increase in fertility.

In a paper published January 29 in Proceedings of the Royal Society B: Biological Sciences, lead researcher Martin Kolk, a doctoral student in demography in the Department of Sociology at Stockholm University, describes how his research team built two mathematical models that use correlations between family size and birth rates to illustrate potential population growth rates in the future.

Specifically, the researchers wanted to see if the current trend of low fertility rates would continue, or if the number of children in families might be expected to increase.

“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,” Kolk states in the introduction to the study report.

The research team set up one mathematical model to look at generational trends with children inheriting either high or low fertility preferences from their parents. The model accounted for personal choices outside the influence of the parents.

This model found that people from the group that preferred to have lots of children, even though there were far fewer of them, produced children who also tended to have large families. This passed on the large family preference to more people, with the result that large families eventually outnumbered small families, causing a growth in population.

“Though at first the low fertility option of having fewer children was widespread in the population, because these individuals passed their preferences onto a smaller number of children the high fertility lifestyle soon over took,” the researchers explained.

The team then set up a second model that integrated more societal lifestyle influences, which they called “novel cultural innovations,” that outweighed the intergenerational fertility correlations that lead to higher fertility.

This model showed an initial fertility rate decline, reflecting accurately the current decline in fertility. However, this was also followed by a positive but less-pronounced increase in family size.

“We show that intergenerational fertility correlations will result in an increase in fertility over time,” the researchers conclude, but warn that, “present low-fertility levels may persist if the rapid introduction of new cultural lifestyles continues into the future.”

An abstract with links to the full text of the study titled “Correlations in fertility across generations: can low fertility persist?” is available here.


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