Quality or quantity? How selection pressure shapes our love lives

In 1967, two biologists devised a theory. They were studying the remarkable diversity of life in isolated habitats, such as islands and mountain peaks, and noticed something interesting. 

The plant and animal life forms they examined seemed to fit into two broad categories: those that invested all their time and energy to produce as many offspring as possible, and those that invested the same time and energy to produce only a few. 

The biologists, Robert MacArthur and E. O. Wilson, devised the term “r/K selection” to name this difference. r-selected lifeforms are those like mice, dandelions, frogs, seahorses, and fungi: they produce as many offspring as possible in each generation, and often try to disperse them widely to ensure maximum rates of survival. 

K-selected organisms, such as whales, elephants, parrots, and humans, do the opposite. They have far fewer offspring – but each one receives far more resources. As a result, K-selected animals are generally bigger, longer-lived, and slower to reach maturity.

In the decades since MacArthur and Wilson devised their theory, most biologists have agreed that life on Earth isn’t really so binary. Most organisms don’t fit neatly into the “r” or “K” selection pattern: trees, for example, grow very slowly and live for hundreds of years (K traits), but they produce thousands of seeds each year and disperse them very widely (r traits). Sharks, too, can live for hundreds of years, but many species lay (and then abandon) dozens of eggs at a time. In fact, life-forms that don’t map neatly to “r” or “K” strategies are far more numerous than those that do. 

In modern science, these differences are referred to as fast or slow life histories. The terms are almost always applied to whole species, genera, or families of plant and animal life, rather than differences between individual members of a species. Nonetheless, when we look at different people or different groups of people, different life histories do become apparent.

This has been referred to as phenotypic plasticity, which just means that organisms of the same kind can respond differently to different environments: for example, if resources (food, water, shelter) are scarce, a group of humans or animals is more likely to survive if they can reproduce early on in life, because they may not have the chance to do so in the future. Under conditions of luxury, meanwhile, reproduction can come later, and more resources can be invested into each child. 

We see this played out on an international scale. Globally, poor countries are several times as fertile as rich countries. Note that this doesn’t mean that people in rich countries are “infertile” in the sense of being biologically unable to conceive or give birth; it just means that in reality, they have considerably fewer children than their counterparts in less well-off nations.

But even within rich countries there are differences: for both the UK and the US, there’s a decade-long gap in lifespan between the least and most well-off. And, of course, those who have few children are able to invest proportionally more resources in each. This sometimes means expensive educations or hobbies, but it can also mean more parental love and affection, which we know results in happier, calmer, kinder children and eventually adults.

In recent years, however, some of the world’s very richest people have been pursuing a different path. Elon Musk, aged 51, has nine children by at least three different women, and has spoken several times about his fear of a "fertility crisis" in developed nations. And people are listening: for many, pronatalism is a natural outgrowth of any worldview whose focus is the future of humanity. In places with powerful tech scenes, that worldview is common.

We can see similar patterns when it comes to dating. Some people like to maximise their options by dating widely, optimising for as many new opportunities as possible in the hopes of finding a good match (think Tinder). Others prefer to invest heavily in one person at a time, aiming to uncover deeper insight about each person before making a decision (think Swan).

Of course, the only “correct” approach is the one that works, and a great deal depends on what you want to get out of a relationship in the first place. If you’re looking for something casual, a “quantity over quality” approach to matches will likely serve your needs better. If you’re looking for something serious, the reverse is likely to be true. And just like with other animals and with plants, which of those things we’re seeking is very often a response to our own environment: our age, income, health, family background, education, and genes all play a role in determining whether we’re likely to pursue an “r” or “K” selection strategy.

And because we know that similar people are more likely to be compatible, it's worth trying to establish which of these your relationship partner(s) favour(s), too. If your strategies match, you're more likely to have similar relationship goals. Although as first date questions go, we’ve heard better...

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