To begin with, they taught capuchin monkeys to trade pebbles for slices of cucumber. Then they gave one of the monkeys a grape — viewed by the monkeys as more valuable — for the very same pebble. “You can do it twenty-five times in a row, and they are perfectly happy getting cucumber slices,” said de Waal. But if a grape was substituted — thus unfairly giving one monkey a better pay packet for work of equal value — the cucumber-receivers got upset, began throwing pebbles out of the cage, and eventually refused to co-operate. And the majority of the monkeys got so angry if one of them was given
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