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29 January 2014Title:Correlations in fertility across generations: can low fertility persist?Authors:Martin Kolk, Daniel Cownden and Magnus EnquistJournal:Proceedings of the Royal Society B
A study published in Proceedings of the Royal Society B reveals the possibility for a reversal of the twentieth-century decline in fertility.
The models the scientists designed suggest that fertility rates are likely to rise with average family sizes increasing. Image credit: comedy_nose
Although the world population has now risen beyond 7 billion, fertility has declined over the last 200 years with women bearing fewer children on average than in the past. This decrease, which scientists call the ‘fertility transition’, has been especially dramatic in the developed world where fertility dropped by more than half between the 1850s and early 1900s as people began having smaller families.
Demographers have also reported an increasingly significant link between the number of children a women might have and the number her children will have. As well as inheriting genetics from our parents we can inherit cultural preferences which could explain this link. However, the rapid drop in fertility over the past two centuries suggests a faster change than could be caused by parental influence alone. Scientists suggest that other role models must also influence the fertility choices of an individual.
A team of scientists at Stockholm University set about making a mathematical model to show how these factors could influence overall fertility rates. In particular they were interested to see if the trend of low fertility rates would last or if the number of children families have might be set to increase.
Issues around fertility are complex but for their model the scientists had to make simple approximations. They assumed that children directly inherit from their parents a simple lifestyle preference- either for a larger or smaller family. To model the influence of wider social factors each modelled individual was also affected by a group of randomly selected role models. Individuals could only adopt their preferred lifestyle if they had an example from at least one of their role models. For instance, though a person might inherit a preference for a larger family, unless one of their role models had this lifestyle they could not adopt it themselves.
Using these basic parameters the team found that fertility rates were likely to increase. The scientists started with a population with high fertility rates but a novel widespread preference to have fewer children- designed to replicate real life conditions 200 years ago as fertility began to decline. The team ran their model for 25 generations, and found that fertility rates initially dropped, resembling the fertility transition period we have seen in reality, before picking up and continuing to increase. 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.
These results, though simplified, demonstrate that despite a current dip, we might expect fertility rates to increase again. To see if a low fertility rate could be sustained the team designed a second model which introduced a more complicated selection of lifestyle influences, reflecting an evolving culture of new cultural innovations. Rather than just adopting the lifestyle of their role models, individuals could adopt their chosen lifestyle even if didn’t match with anyone from their social group. Some individuals’ preferences were also allowed to disagree with those of their parents. In this more complex model, fertility rates also saw an initial decline followed by an increase; in contrast to the first model however, rates did not rebound to their initial highs.
Despite the simplicity of these mathematical models- where lifestyles are discreet choices, generations do not overlap and the influence of limited resources, government institutions and mass media are absent- the study may give a glimpse in to the future of our fertility. The team concluded that ‘only through continuous cultural change, introducing novel lifestyles associated with reduced childbearing’ will low fertility rates persist.
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