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Sharing is contagious, according to a new study

In the modern world, people cooperate with other people including strangers all the time.

We give blood, tip providers of various services, and donate to charity despite the fact that there is seemingly nothing in it for us. Now, researchers who studied Hadza hunter-gatherer people in Tanzania over a six-year period have new and surprising insight into why people work together.

Their new evidence shows that the Hadza are generally willing to share. But, that doesn't mean that they always do. In fact, whether a particular individual will share generously depends less on the individual and more on the group they happened to live with at the time.

  • They found that year after year, willingness to share with others clustered within residence groups or what are called 'camps'.

  • People were living with other people who were similar to them in levels of generosity.

  • They also found individual willingness to share changed from year to year to match their current camp-mates and found no evidence that people preferred living with more cooperative people.

The findings show that it isn't that people who like to share choose to live with others who also like to share. Rather, people adapt their own sharing tendencies so as to match that of the group they currently live in. In other words, sharing is driven by local group norms and behaviour.

The researchers were particularly interested in exploring cooperation in the Hadza. That's because the Hadza still obtain a majority of their calories by hunting and gathering, a subsistence lifestyle that more closely approximates most of human evolution.

The Hadza are one of the last populations left on the planet who live a similar lifestyle to how our ancestors lived for millions of years. They offer insight into how cooperation evolved

Food is unreliable and people often worry about there being enough food to feed themselves and their families. To counteract this, the Hadza share their food with their campmates. High levels of cooperation help to ensure survival in this unpredictable setting.

To find out what drives the Hadza's cooperative behavior, the researchers visited 56 camps in Tanzania over six years. During those visits, nearly 400 Hadza adults of all ages were asked to play a public goods game. While the game is typically played by asking people to decide whether to share money with the group or keep it for themselves, the researchers instead asked the Hadza to consider sharing straws of honey, their favorite food.

Each person started with four straws, which they could put toward the whole group or not. The honey straws contributed to the group got tripled.

The data showed that Hadza individuals living in certain camps were consistently more generous than others were. Moreover, individuals behaved differently over time, modifying their behavior to match the norms of the camp they were currently living in.

The findings highlight the flexible nature of human cooperation, the researchers say. More generally, they show that generosity can be contagious.

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