KentuckyFC writes "The Tsimane tribe are hunter-gatherers living in the forested region between the foothills of the Andes and the wetland-savannas of the Llanos de Moxos in Bolivia. They drink beer made from boiled manioc (a type of sweet potato) which they chew and spit into the mix to trigger fermentation. After a week or so, the resultant brew is about 4 per cent alcohol. Now anthropologists studying this tribe say the way they host beer drinking events for each other offers important clues into their culture. At issue is the question of altruism: why people spend considerable time and effort doing favors for others that don't directly benefit them. The answer from studying these beer drinking events is that the favor is quickly returned by the guests in the form of another beer drinking event. This helps to build good relations with neighbors and family. And when the beer drinking invite is not returned, the researchers speculate that this is probably because there is some other favor involved, such as helping to gather or prepare food, suggesting mates or political co-operation."