Why Blood is Thicker than Water

People tend to favour relatives over non-relatives. But is this a product of culture or a part of human nature?
06 December 2018

Siblings by the sea

Siblings by the sea


People tend to favour relatives over non-relatives. But is this a product of culture or a part of human nature? 

Parts of this article were excerpted, with changes, from the book The Ape That Understood the Universe: How the Mind and Culture Evolve (2018. Cambridge University Press).

In 2011, the Australian state of Queensland suffered extreme flooding. As with any disaster, this one left many tales of heroism in its wake. Among the most poignant is the story of thirteen-year-old Jordan Rice. Jordan had been out shopping with his mum, Donna, and his younger brother, Blake. They were in the car heading home when, out of the blue, they found themselves caught in the middle of a flash flood. Unable to drive any further, and unable to get to dry land, the three scrambled onto the roof of the car and then sat there, stranded in the middle of a violent torrent of water.

Fortunately, some bystanders saw what had happened. One man – Warren McErlean – tied one end of a rope to a post, and the other around his waist, and then pushed his way through the rapidly rising waters to the car. He reached for Jordan, but Jordan pulled away, begging him to save his little brother first. McErlean complied: He picked Blake up and carried him quickly to safety. Before he had time to rescue the others, however, a sudden surge of water flipped the car. Jordan and his mum were swept away and killed.

By putting his brother ahead of himself, Jordan lost his life. From an ethical point of view, this represents the height of moral action. From a Darwinian point of view, on the other hand, it’s initially quite perplexing. Evolutionary theory seems to imply that the only organisms that prevail in the harsh Darwinian struggle for existence are those that look out for number one, and thus that the whole world will be populated with self-interested, self-serving organisms. But Jordan was no such organism; he risked his own life to save someone else, and died in the process. And although his was an extreme and extraordinary case, Jordan was no freak of nature – no moral equivalent of a two-headed gorilla. Self-sacrifice of one sort or another is nearly as common as breathing.

Evolutionary biologists call this the problem of altruism, and it’s occupied many of the greatest thinkers of the field. At one time, the problem looked intractable; today, however, we’ve got a powerful arsenal of theories aimed at explaining the evolution of altruistic behaviour. Arguably the most important – and certainly the one most applicable to the case of Jordan Rice – was the brainchild of the British biologist William D. Hamilton. Hamilton’s breakthrough is now known as kin selection theory, and it offers an explanation for one important type of altruism: altruism toward one’s relatives.

The details of Hamilton’s theory are complex, but the basic idea is fairly simple. The starting point is the observation that organisms share a larger fraction of their genes with relatives than they do with unrelated individuals. This has an important implication, namely that any gene that contributes to the development of a tendency to help one’s relatives has a better than average chance of being located as well in the recipients of that help. As a result, by helping one’s relatives to survive and reproduce, one can indirectly help to spread the genes that gave rise to that very tendency.

To be clear, people aren’t thinking about their genes when they help their relatives; they just help because they want to help. But why do they want to help? Why did evolution bequeath us this inclination? That’s the question that kin selection theory aims to answer.

Needless to say, humans can’t peer into one another’s genomes and directly perceive who their kin are. Strictly speaking, then, people couldn’t have evolved to favour kin. What could have happened, though, is that we evolved to follow certain implicit rules that, in our ancestral past, led us to favour kin over non-kin nine times out of ten. Several such rules have been proposed, among them: “Help people you were raised with or those you helped to raise,” and “Help people who resemble you more closely than the average person in your social environment.” Dispositions such as these could generate kin altruism even in the absence of any conception of kinship or desire to pass on one’s genes.

Consistent with Hamilton’s theory, various lines of evidence suggest that people do indeed favour their relatives over non-relatives in a wide variety of ways. Parents tend to be closer to biological children than stepchildren; will-makers tend to leave more money to relatives than non-relatives; and people of all stripes are more likely to risk life-and-limb to save relatives than anyone else (Jordan Rice is a case in point).

Why, though, should we favour the kin-selection explanation for these tendencies over a rather obvious competitor: the idea that we simply learn to favour our kin? I’ll give you three reasons.

First, the importance of kinship appears to be a human universal. Wherever in the world we look, people’s affinity for kin stands out like a sore thumb – or, more to the point, like an opposable thumb: a species-typical attribute with a clear adaptive rationale. In a wide-ranging survey of the anthropological literature, Essock-Vitale and McGuire found that, across cultures, people are more likely to help kin than non-kin in situations where there’s no expectation of payback. Similarly, Oliver Curry and colleagues surveyed the ethnographies of sixty historically independent cultures, tallying every passage that indicated either a positive or a negative view of kin altruism. The result? Kin altruism was seen as a moral good in 100% of cases.

Admittedly, the fact that a trait is universal doesn’t necessarily mean it has an evolutionary origin; if it did, we’d have to conclude that drinking Coca Cola and using mobile phones are products of natural selection. Still, cross-cultural universality sits a lot more easily with an evolutionary explanation than it does with a Nurture Only one. We’ve got a good understanding of how Coke and mobile phones came to be universal despite not being innate. Until advocates of the Nurture Only view can offer an equally good explanation for the universality of our kinship bias, the default prediction from the Nurture Only perspective is that, across cultures, people’s behaviour toward kin and non-kin will vary randomly with respect to kin selection theory: In some cultures, people will favour kin, in others they’ll favour non-kin, and in others still they’ll show no bias in either direction. That’s not what we see, though; people everywhere favour kin. This represents a major predictive failure for the Nurture Only view.

A second argument for an evolutionary explanation is that not only is our affinity for kin a cross-cultural universal, but it persists even when we try to eliminate it. An example comes from the Israeli communes known as kibbutzim. The kibbutzim were founded on radical socialist principles, and the founders sought to eliminate supposedly “bourgeois” traditions such as parents caring exclusively for their own biological children. To that end, kibbutz children were housed together in large communal quarters, rather than shacking up with their parents. But although this looked fine on paper, in practice it rapidly disintegrated. Parents hated it, and before too long started insisting that their children live with them. Some of the kibbutz men resisted for a while, but eventually they had to give in. Thus, rather than being a product of social pressure, the tendency to favour one’s kin survived even in spite of significant social pressure against it.

And this isn’t an isolated case. Another example comes from a polygynous Mormon community studied by William Jankowiak and Monique Diderich. Men in this community often had several wives, and thus siblings and half-siblings commonly ended up living together in the same home. Despite living together, however, and despite a community ethos which downplayed differences in relatedness among siblings, people were still closer to their full-siblings than their half-siblings. The relatedness bias runs deep – deeper than non-evolutionary theories can readily explain.

A third and final nail in the Nurture Only coffin is the fact that kin altruism is not unique to human beings. On the contrary, among group-living species, it’s virtually universal. Parent birds in some species fake injuries to lure hungry predators away from their nests and their baby chicks. Belding’s ground squirrels issue alarm calls to warn family members of encroaching predators. And worker bees sting any individual that ventures too close to the family hive, which protects the hive but also usually kills the worker. More generally, kin altruism is found throughout the length and the breadth of the animal kingdom.

And that’s just scratching the surface! Kin altruism is even found in plants. When American sea rocket shares soil with sibling plants, for instance, it grows its roots less aggressively and less competitively than it does when it shares soil with non-relatives. More remarkable still, kin altruism can even be found in bacteria: Bacteria cooperate more with closely related bacteria than with more remotely related bacteria. This means that a lot of kin altruism – indeed, the vast majority of the kin altruism taking place on the planet – is invisible to us. But it’s there, and it shows us that kin selection is profoundly important, not only in complex multicellular organisms such as ourselves and plants, but right across the living world.

The fact that kin altruism is so ubiquitous – vastly more so than Coke or mobile phones – tells us something important. It tells us that William Hamilton, in positing his kin selection theory, identified an extremely deep principle in nature: the biological equivalent of the laws of thermodynamics.

More than that, the ubiquity of kin altruism tells us something about the origins of our own nepotistic inclinations. No one would dream of explaining kin-directed altruism in Belding’s ground squirrels, American sea rocket, or bacterial moulds in terms of socialization or cultural norms. We explain it, without hesitation, in evolutionary terms. Is it plausible, then, when we find exactly the same pattern in our own species, to explain it in entirely different terms, as a pure product of learning or culture?

The short answer is no, it’s not plausible. It’s possible, I suppose. But in the absence of strong evidence that it’s actually the case, the default assumption should be that humans are continuous with the rest of nature and thus that our nepotistic streak has an evolutionary origin, just as it does for every other organism. The burden of proof falls squarely on the shoulders of the Nurture Only theorist.

And what a burden it is! Anyone who wants to deny that human kinship behaviour has anything to do with kin selection has an arduous task ahead of them. Here’s what they’ve got to do at a minimum:

1. They’ve got to explain why human beings are mysteriously exempt from a selection pressure that runs like an unbroken thread throughout the animal kingdom and all the way down to plants and bacteria.
2. They’ve got to explain why, at some point in our species’ past, kin altruism ceased to be adaptive for our species, and thus why selection wiped out the nepotistic streak that presumably existed in our pre-human ancestors.
3. They’ve got to explain why, despite this, people in every culture still manage to act in ways consistent with kin selection theory. Why would culture recapitulate biology in this way, given that kin altruism supposedly ceased to be adaptive in our lineage?

Kin selection theory is one of the great theories in the history of science. It places human beings within an explanatory framework that embraces the rest of the biological world – a framework that links suicidal nest-defence in bees and patterns of root growth in plants with the loving bonds and quiet self-sacrifice found in human families. I’m willing to go out on a limb and say that no psychologist or social scientist, who lacks a detailed understanding of kin selection theory, and an awareness of the importance of relatedness throughout the living world, can claim to be an expert on behaviour. Furthermore, any student of psychology or the social sciences who isn’t taught Hamilton’s ideas in some depth has been short-changed: They’ve missed out on one of the most profound theories of social behaviour yet to be proposed.


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