Family Size in Affluent Cultures: Another Failed Prediction of Darwinian Theory - Evolution News & Views

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Family Size in Affluent Cultures: Another Failed Prediction of Darwinian Theory

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If natural selection really is the creative driving force behind the evolutionary development of species, you ought to find it programming creatures to maximize the number of their descendants. But this prediction of Darwinian theory is foiled by research newly reported in the Proceedings of the Royal Society B. Scientists in Sweden and the UK studied a cohort of 14,000 Swedes born between 1915 and 1929, plotting the relationship between family size, social and biological success ("Low fertility increases descendant socioeconomic position but reduces long-term fitness in a modern post-industrial society").

I take this subject personally since, with five kids, my wife and I consider ourselves as having a packed house. However, that's only relative to the culture around us -- Seattle -- which competes with San Francisco for the title of America's most childless city. In other places our family would be considered modest in size. My brother-in-law and his wife in Jerusalem, for example, have 18 (eighteen) kids.

The so-called demographic transition describes the way that as societies become more affluent, people have fewer kids. In Seattle, instead of children, people have dogs. The seeming evolutionary anomaly -- as folks find they can afford to support a larger family, they in fact curtail their reproduction -- has traditionally been explained as a longterm strategy of natural selection. You have fewer kids but you can care for them better so that in the long run you in fact ensure the spread of your genes to future generations. It's a tradeoff, quantity for quality.

That rationalization turns out to be wrong. Smaller families and their descendants are wealthier and more successful in social and educational terms. Measured for "descendant reproductive success," though, the strategy is at best a wash and at worst a flat contradiction of what natural selection is supposed to be selecting for.

From the Abstract:

Adaptive accounts of modern low human fertility argue that small family size maximizes the inheritance of socioeconomic resources across generations and may consequently increase long-term fitness. This study explores the long-term impacts of fertility and socioeconomic position (SEP) on multiple dimensions of descendant success in a unique Swedish cohort of 14,000 individuals born during 1915-1929. We show that low fertility and high SEP predict increased descendant socioeconomic success across four generations. Furthermore, these effects are multiplicative, with the greatest benefits of low fertility observed when SEP is high. Low fertility and high SEP do not, however, predict increased descendant reproductive success. Our results are therefore consistent with the idea that modern fertility limitation represents a strategic response to the local costs of rearing socioeconomically competitive offspring, but contradict adaptive models suggesting that it maximizes long-term fitness. This indicates a conflict in modern societies between behaviours promoting socioeconomic versus biological success.
The study's lead author, Anna Goodman of the London School of Hygiene & Tropical Medicine, puts the difficulty for Darwinian theory in these terms:
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. 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." However our study found this quantity-quality trade off only applied to descendants' socioeconomic success, not their reproductive success.
Given natural selection, you would expect one thing. What you get is the opposite. That's called a failed prediction and Darwinists have a variety of strategies for dealing with those, as Cornelius Hunter writes at Darwin's Predictions:
Evolutionists argue that evolution is a fact, and that we ought to focus on evolution's successful predictions rather than its false predictions. The tendency to seek confirming evidence over contrary evidence is known as confirmation bias. One consequence of confirmation bias can be that confirming evidence is viewed as correct and typical whereas disconfirming evidence is viewed as anomalous and rare. Not surprisingly the confirming evidence is more often retained and documented. Rarely are the many false predictions found in evolution texts. Confirmation bias can hinder scientific research, particularly when researchers believe they know the truth, as do evolutionists. They view the important predictions of evolution as predominantly true. False predictions, on the other hand, are usually not viewed as legitimate falsifications. Instead, these are interpreted, more positively, as open research questions which are yet to be resolved. Indeed, evolutionists often make the remarkable claim that there is no evidence that is contrary to evolution.
Those British and Swedish researchers ought to have a chat with our friend Eugenie Scott of the National Center for Science Education, who advises Darwin advocates to avoid giving the impression that evolutionary theory has any serious weaknesses at all.

Photo: terrellcwoods/Flickr.