Kith and Kin

How nature and nurture shape relationships

Born to Share?

Why all humans co-operate – but some are more WEIRD than others.

It may not always seem like it, but humans are an unusually co-operative species. We like to do favors and give people stuff.  We remember favors and return them—or very often we just expect that what goes around will come around: if I let a stranger into heavy traffic then later another stranger will do the same for me.  We punish people who don’t co-operate even if it costs us something: we don’t like to be taken for a ride. Are we born to co-operate or is it something we learn?

There is a long-running debate about how and why people co-operate. To co-operate means doing something that benefits others but has some personal cost. For example, if I share food with you there will be less for me. Our co-operative behaviour is strongly developed compared to other primates and evolutionary anthropologists argue that we have an evolved, innate drive to cooperate, to reciprocate favours tit for tat and to punish nonco-operators and free-riders. Economists, on the other hand, point to the irrational and culturally diverse ways that people actually behave. If this kind of behaviour is an evolved universal trait then why does it vary so much around the world—and even within different groups within a single society? 

Co-operative behaviours are often tested using simple games where the player has to decide how to split the rewards between themselves and another player. In the Ultimatum Game, a player is given an amount of money and offers some (or none) to another player. If the second player accepts, they both keep the money—but if the second player rejects the offer they both get nothing. A rational economist would advise offering a very small amount that should be accepted since something is better than nothing and then both players ‘win’. But that isn’t what usually happens. People don’t respond rationally. In most studies of the Ultimatum Game players reject offers under 20% of the pot and players typically offer close to 50%. Although this suggests a universal strategy the problem is that most of these studies are done in WEIRD populations: Western, Educated, Industrialized, Rich and Democratic. WEIRD students from the US to Japan are all well-acquainted with the kind of market exchange typical of developed economies: they’re all singing from the same hymn sheet. 

When Henrich et al compared fifteen non-WEIRD small-scale societies playing the Ultimatum Game they found different cultures behaving differently. Some peoples, such as the Tsimane of Bolivia, never rejected offers, no matter how small—and didn’t offer as much either, typically just 25% of the pot. Others, like the Au and Gnau peoples of New Guinea actually rejected high offers while the Lamelara in Indonesia generally offered over 50% despite offers of any sort generally being accepted.  It’s hard to argue that co-operative behaviour is an innate human trait with this level of variation—so perhaps co-operative behaviour is entirely cultural. But in that case we have a new question—what is it about these cultures that make them play the game differently?

Different peoples, with different kinds of economies and lifestyles have different trade-offs between co-operative behaviours that are good for the group but probably costly for the individual and behaving more independently. The Tsimane, for example, who gave little and never rejected offers, lived as independent family groups with little, if any, dependence on exchange with non-family members. The Au and the Gnau, who rejected high offers, have a strong tradition of gift giving. But it’s understood that the recipient is then under an obligation to the giver and such obligations are not always welcome. The Lamelaru who make high offers and rarely reject anything are whale hunters who co-operate routinely in the hunting of the biggest game on Earth and share the proceeds.  And so the everyday experiences of co-operation in these different cultures is reflected in the strategies used to play new co-operative games. Maybe culture shapes co-operative behaviour and there is nothing innate about it at all.

House et al’s recent paper tested the development of co-operative behaviour in six very different kinds of societies (including one WEIRD group from LA) to see how people vary their game-playing depending on age and culture. Players had two versions of a simple food-sharing game—a costly version, where they paid a cost when they rewarded another player, and a version with no costs. Across all cultures children under five gave rewards without regard for personal cost or even meanness from their gaming partners. (This echoes another recent study that found Californian 2.5 year-olds gave rewards indiscriminately—even to those who never returned the favor.) But approaching middle-childhood around 7-8, children’s behaviour changed becoming more like that of adults from the same cultural group - and differences between the cultural groups became marked. In particular, the way children and adults played the costly version of the game changed while responses to the no-cost version were generally unchanged. This is exactly what you might expect if co-operation has both an innate, genetic component and a local, cultural influence—an example of gene-culture coevolution. Co-operation that comes at a cost differs between cultures because the costs are not the same across cultures.

Children take on cultural norms from their parents and society in general. From the same relatively simple, co-operative beginnings, our particular society then teaches us when and how much to give, to accept—and to punish.

Gillian Ragsdale, Ph.D. is an Associate Lecturer in biological psychology with the Open University, in the U.K.

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