Money Matters for Big Families in the Big City

Every month, there seems to be more and more media coverage about population decline, even in countries traditionally considered to be expanding rapidly. This commentary is often accompanied by concerns over where these populations are headed, towards an ever-aging population and society without the resources of youth to sustain itself.

While these trends are modern phenomena, the key to understanding them may lie in our ancestral past. In our latest study recently published in the Archives of Sexual Behavior, we used a massive, multi-national dataset to examine what influences the number of children people have. We focused on two factors known to have evolutionary significance in mating not just in humans but animals more broadly, population density and resources.

To understand why these matter, we first need to understand an important theory from evolutionary biology that’s often applied to humans: Life history theory.

Life History Theory: A Quick Primer

Life history theory examines how organisms allocate resources to growth, reproduction, and survival. Simply put, it's about the strategic decisions species make to ensure their genes are passed on.

At the species level, life history theory helps explain why some animals mature quickly and reproduce prolifically (for example, flies and fish), while others take their time to grow and have fewer offspring (for example, elephants and great apes). Both strategies can lead to successful reproduction and the passing on of genes; it’s just a matter of quantity versus quality.

As a species, humans sit firmly down the “quality” end of the spectrum. Compared with other animals, we have a much smaller number of offspring, mostly one at a time, and invest heavily in them. That being said, even though humans are “anchored” down the quality end there is still some individual flexibility around it, which is why we have examples of tyrannical despots siring dozens of offspring and many more examples of people never having children at all or being “one and done.”

Thus, some scientists extend life history theory to understand individual differences. Factors like environment, socio-economic status, and personal health might nudge our life history strategy slightly towards either end of the spectrum leaning us towards having more children earlier or fewer children later. This individual-level application is not without controversy and isn’t evolutionary psychological “gospel.” But I find it compelling, nonetheless.

Our latest paper

In our latest research, we used over 4.4 million dating profiles of men and women from 317 states in 23 countries to test predictions from life history theory. Readers of my blog might have seen me write about this data before when we explored how competency impacts how much attention dating profiles receive.

This time we were interested in a different aspect of our daters, their number of children. And because we had data on where they lived and their background it allowed us to reveal some interesting predictive patterns that were universal rather than specific to a single country.

Source: Benoit Dujardin / Pexels

Population density, a driver of small families

The first thing we found was essentially a ‘no-brainer’ from a life history perspective. We found that as local population density goes up, the number of children people have goes down. We’re talking about density here and not size, though they often go hand in hand. When humans become more packed together, there is more competition for resources, space, and opportunities. And this pressure drives parents to invest more heavily in fewer offspring. It’s easier to help one or two children thrive than five or six.

This might also explain why populations are in decline, compared even to a few hundred years ago the density of modern living in cities has changed dramatically. In the footprint of a farmstead, we now build tower blocks that can house hundreds and the daily long queue of strangers in a busy London coffee shop sends the daily signal “society is full” to a Stone Age brain evolved in tight communities with fewer than 150 members.

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The Money Moderator

However, the effects of population density might tweak the life history strategy of some more than others. An exciting finding from our study is that we identified a key variable that acts as a buffer worldwide: Money.

While a coin in your hand or a card you can tap is an evolutionary novel phenomenon, confidence in being able to access resources to overcome obstacles is not. Money acts as a modern signal that we have access to the things we need to cope with life’s challenges.

And so, we found that while higher population density generally leads to smaller families, higher income levels buffer this effect. Wealthier individuals in dense areas tend to have more children than their less affluent counterparts because financial resources can mitigate the competitive pressures of a crowded environment, allowing for larger family sizes.

What’s more, we found this buffering effect was larger for men than women. One only has to look at wealthy celebrities (like Elon Musk) or politicians (like Jacob Rees-Mogg or Boris Johnson) to see examples of men, embedded into city life, with much larger than average families. Our new research shows that they are not odd exceptions but part of a general trend in human behaviour across the globe, something which I’m calling “The Musk Effect.”

Why “The Musk Effect” Matters

Our findings reinforce the applicability of life history theory to humans, showing consistent patterns that cut across cultures. But more importantly, it opens new avenues for understanding how socio-economic factors interplay with biological and environmental influences on family planning.

For policymakers and social scientists concerned about population decline, these insights can inform strategies around urban planning (to reduce perceived population density) and economic support (to enhance resources). Recognizing that income can influence family size in densely populated areas might lead to more targeted support for families, helping them balance the pressures of modern living with their reproductive goals.

Finally, to the individual, it gives insight into what might be nudging our relationship and family choices. And with that self-awareness comes agency. Those searching for a partner with large family might have more options beyond the city limits.