The evolution of kindness towards strangers

Researchers have suggested humans cooperate because we’re used to living in family groups. What do studies of animal sociality and hunter-gatherer tribes say about this idea?

Lisa Shepherd
7 min readFeb 26, 2023
Photo by Kelly Sikkema on Unsplash

Have you ever donated to charity, stopped to help a stranger on the street or showed kindness towards a child who wasn’t your own? If so, then in the context of the rest of the animal kingdom, you’ve done something very odd.

Humans can be described as a hypersocial species. We’re willing to help each other out — often by giving away resources like food, effort and money — with no direct reward. From an evolutionary perspective, it’s hard to understand why natural selection has favoured people who are willing to set themselves back for others.

Helping out family

When an organism’s actions benefits other organisms at a cost to itself, evolutionary biologists call its behaviour altruistic. Costs and benefits are measured in terms of fitness , i.e. how likely an individual is to survive and pass its genes on to the next generation.

Although we typically think of altruistic acts as charity towards people we don’t know so well, the evolutionary definition of altruism encompasses kindness towards family members too — something that’s very common among social species.

Squirrels, which live in family groups, will call out an alarm signal upon seeing predators, putting themselves at risk to warn their extended family. A study of scrub jays found that of 74 cases where one bird was seen assisting another, 73 birds were the offspring or siblings of the individual they were assisting. Finally, as a less heart-warming example, cannibalistic species like ladybirds and tiger salamanders prefer to eat nonrelatives.

If you’re looking to pass on your genes, it makes sense to support family members. Imagine a species in which altruism is conferred by a single gene. Individuals that don’t have the gene are selfish — they never share food with their siblings, they let their parents get eaten by foxes, they’ve cannibalised a cousin or two. Conversely, individuals who do have the gene behave altruistically, but only towards relatives.

Because the altruist shares genetic material with the organism it’s helping out, there’s a good chance that individual also has a copy of the altruism gene. The altruists end up with a fitness advantage over the selfish members of their species. They pass their genes on to more offspring, and a few generations later, everyone is behaving kindly towards their family. Biologists call this form of altruism “kin selection”.

But here’s the issue — kin selection might explain the altruistic behaviour of squirrels, birds and tiger salamanders, but it doesn’t fully explain your actions. You’re not just nice to your family. You buy gifts for your friends. You do favours for your colleagues. You smile at babies. You’re pretty great like that.

Imagine a different altruism gene now — one that makes an organism willing to sacrifice their own needs for anyone, regardless of relatedness. This time, selfish individuals are clearly at an advantage, benefitting from the help of altruists while refusing to provide any aid of their own. Altruists and their genes are doomed to die out.

In light of this thought experiment, how can we explain human hypersociality?

Let’s talk about meerkats.

Meerkat babysitters

It takes a village to raise a meerkat pup.

Meerkats live in groups (or clans) of up to fifty individuals. Just two meerkats in each clan — the dominant male and the dominant female — produce 80% of pups. The other female meerkats help to look after the alpha couple’s offspring. They’ll fetch the pups food when they cry and stay behind to look after the tiniest members of the family while everyone else is out foraging, even if it means the babysitter misses out on a meal.

Almost all meerkats in a clan are related. Helper meerkats are likely to be the daughters, sisters or nieces of the dominant female, so it benefits their genetic legacy to ensure her pups survive. At first glance, this looks like a simple example of kin selection.

But a study by Duncan and colleagues suggests meerkat altruism may be harder to explain than previously assumed.

Photo by Anggit Rizkianto on Unsplash

The researchers kept track of two pup care behaviours in meerkats of the southern Kalahari Desert: feeding and babysitting. They quantified the relatedness of each meerkat to the dominant female, to the whole clan and to any pups they were caring for.

Surprisingly, the relatedness of a meerkat to a pup or to the clan had no impact on how likely that meerkat was to babysit or feed a pup. Meerkats that were more closely related to the dominant female fed pups slightly more often, but they didn’t use relatedness to discriminate between the actual pups.

This behaviour is very sweet, but raises an obvious question: Why wouldn’t meerkats offer preferential treatment to pups who shared more of their genes?

Well, that relies on the assumption that meerkats know which pups are more closely related to them.

Using computational modelling, the researchers worked out which altruism strategy would offer the greatest fitness benefit under different circumstances. Three different strategies were considered: selfishness, kin discriminate altruism (only helping family) and indiscriminate altruism (helping everyone).

For animals like meerkats, whose social groups are composed of close relatives, kin discrimination is the best strategy when kin recognition error is low. But when individuals struggle to identify relatives, the best strategy is just to help everyone.

Circumstances are different for social animals who live in less closely related groups. Indiscriminate altruism is never beneficial for Old World primates, whose social groups consist of approximately 50% relatives and 50% unrelated individuals. Kin discrimination is the best strategy when recognition error is low, and plain selfishness reigns supreme when more mistakes come into play. Ungulates, whose social groups are about 10% kin, benefit from being selfish under an even wider range of conditions.

The social groups of hunter-gatherers

Since humans exhibit indiscriminate altruism, you’d expect our social groups to be overwhelmingly composed of kin, like meerkat clans.

Obviously, this isn’t true in modern society, where we socialise with plenty of unrelated individuals. But for a long time, anthropologists assumed that hunter-gatherer social groups — known as “bands” — were collections of close kin. Evolutionary psychologists hypothesised that the indiscriminate altruism we display today evolved when we almost exclusively interacted with our relatives.

We can’t travel back to prehistoric times and take notes about how our ancestors interacted, but there are still a few hunter-gatherer tribes around that could offer some insight into how we used to live.

Photo by Beth Macdonald on Unsplash

Hill and colleagues analysed the social structures of 32 hunter-gatherer societies. They found that most adult members in a single band were not closely related. For the average adult, less than 7% of individuals in their band were primary kin (parents, siblings or offspring). Even when distant relatives like cousins were included, only 25% of individuals were kin.

Perhaps unsurprisingly, our social groups are far more similar to those of Old World primates than those of meerkats. We can quantify relatedness using a statistic called Hamilton’s r. Higher relatedness corresponds to a higher value of r. r is about 0.5 for siblings, 0.25 for nephews or nieces and 0.125 for first cousins. For your average meerkat clan, the average r is 0.34. For the Aché people of eastern Paraguay, r is only 0.05.

These results suggest that kin selection doesn’t explain human altruism after all.

The rewards of cooperation

So why do humans exhibit indiscriminate altruism?

A possible explanation is that cooperation has direct benefits. Some actions are mutually beneficial, like working together to move aside a heavy obstacle. There are also tit-for-tat situations — you come to my aid when I’m injured, and I’ll do the same if something happens to you.

If both people — or animals — obtain immediate benefits from the interaction, or if the helper derives a future benefit more valuable than their initial sacrifice, we’d expect natural selection to favour cooperative behaviour. Individuals don’t need to be related for altruism to increase fitness.

Given this sound logic, you might wonder why reciprocal interactions are so uncommon outside our species. Thanks to our exceptional intelligence and communication skills, humans possess a few unusual qualities that may have enabled extreme cooperativity to flourish.

Gossip and altruism

Perhaps the most fascinating trait that’s allowed altruism to flourish among humans is our ability to gossip.

Photo by Thriday on Unsplash

The biggest threat to an indiscriminately altruistic species is a freeloader. As previously described, if every member of a species is willing to help each other out except for a small subset of the population, those organisms derive all the fitness benefits of cooperation with no cost, and selfishness becomes the dominant strategy.

Earlier, we discussed animals and their ability to deduce which individuals were close relatives. What if you could instead deduce which individuals were trustworthy?

Language allows humans to gather information about a potential social partner’s behaviour from another party. We’re far better equipped than other animals to work out who’ll scratch our backs if we scratch theirs.

We might think of gossip as a frivolous and immoral thing, but used responsibly, it helps us decide who to trust. Without it, humans may not be able to cooperate as closely as we do.

Is gossip the mother of altruism? I wouldn’t go that far. It’s more like the aunt that watched over altruism as a pup.

This article was inspired by a seminar by Dr Mark Dyble, who was involved in the meerkat study. I’d encourage you to look further into his work if you’re interested in this topic.

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Lisa Shepherd

Science writer with a passion for all things biology.