In a study of primate behavior, researchers observed that in groups of capuchin monkeys, individuals would often refuse a piece of cucumber—a perfectly acceptable food—if they saw a neighbor receiving a grape, which they prefer. This 'inequity aversion' was thought to show a primitive sense of fairness. However, later experiments showed that the monkeys also refused the cucumber if the grape was simply placed in an empty cage nearby, even when no other monkey was present to receive it.