Evolutionary biologists have long puzzled over this because natural selection is expected to have selected for organisms that try to maximise their reproduction. But in industrialised societies around the world, increasing wealth coincides with people deliberately limiting their family size – the so-called 'demographic transition'.

In a study published in Proceedings of the Royal Society B: Biological Sciences, researchers from the London School of Hygiene & Tropical Medicine, the Centre for Health Equity Studies (Stockholm University/Karolinska Institutet) and UCL (University College London) reject a popular theory put forward to explain the phenomenon. This 'adaptive' hypothesis proposes that low fertility may boost evolutionary success in the long term by increasing offspring wealth, which in turn eventually increases the number of long-term descendants because richer offspring end up having more children.

The researchers found that having a small number of children increased the economic success and social position of descendants across up to four generations, but reduced the total number of long-term descendants. They conclude that the decision to limit family size can be understood as a strategic choice to improve the socioeconomic success of children and grandchildren in modern societies, but this socioeconomic benefit does not necessarily translate into an evolutionary benefit.

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