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Small families: more wealth, less evolutionary success

Evolutionary biology would seem to dictate that the more wealthy a family gets the more children they have - in fact, the exact opposite is true.

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A new study looks into the correlation between wealth and family size. (Kevork Djansezian/Getty Images)

Evolution would seem to dictate that the wealthier one becomes, the more likely they are to maximize their reproduction.

Yet, even anecdotal evidence seems to point in the opposite direction: the more wealthy one becomes the smaller the family they produce.

This is particularly true of richer countries in Europe, North America, and Japan where families have steadily shrunk for decades.

Researchers in Britain and Sweden, in a disruptive new study, partially reject the popular "adaptive" hypothesis that posits that low fertility boosts evolutionary success over time by increasing the wealth of offspring and finally increasing the number of descendants who will have more babies.

If you managed to follow that, the new research found that, although some of this explanation may be correct, the quantity-quality trade-off involved only applied to descendants' economic success, rather than their reproductive success, reported ABC Science.

"Under natural selection, you would expect organisms to use their resources to produce more genetic descendants, and so increase their Darwinian fitness. The demographic transition is a puzzle because at first sight it doesn't look like people are doing this," said study author Anna Goodman, a research fellow at the London School of Hygiene and Tropical Medicine, reported Science Daily.

"One adaptive explanation for the puzzle is that there exists a quantity-quality trade-off, such that having more children leads to those children being less able to reproduce in turn - i.e. higher 'quantity' leads to lower biological 'quality'."

To find this, the researchers used data from the Uppsala Multigenerational Birth Cohort Study, a massive study that tracked the lives of 14,000 Swedes born in the early 20th century and the lives of all of their descendants.

Socioeconomic success of each generation was determined by looking at grades, educational attainment and income, while reproductive success was measured in survival, marriage and fertility.

The researchers found that smaller family size and greater socioeconomic success were both related to high grades, better educations and higher income, said Science Daily.

The advantages of both factors were passed from generation to generation.

Yet, researchers found that unlike the adaptive theory, smaller, wealthier families had no positive effect on reproductive success beyond the first generation - even appearing to be negative in some cases, reported ANI.

The study was published in Proceedings of the Royal Society B: Biological Sciences.