Do rhesus monkeys (Macaca mulatta) attribute knowledge and ignorance to others?

The ability of 4 rhesus monkeys (Macaca mulatta) to understand the causal connection between seeing and knowing was investigated. The subjects were tested to determine if they could discriminate between information provided by experimenters who randomly alternated between roles of guesser and knower. In a series of tests, the knower either hid food under 1 of 3 cups or watched as someone else hid the food. The guesser waited outside the room or covered her or his head until the food was hidden. The subjects watched this procedure occur but could not see which cup the food was hidden under. The knower pointed to the correct cup while the guesser pointed to an incorrect one. None of the macaques provided any evidence that they realized the different states of knowledge possessed by the guesser and knower. The results are consistent with the hypothesis that rhesus macaques are incapable of making inferences about the mental states of others.