When trained Macaques were given a choice of three answers in a computer game; one of which was correct, one incorrect, and an additional option to pass – macaques where shown to choose the latter option to pass rather than risk being incorrect. The macaques were rewarded for a correct answer, but an incorrect answer initiated a pause in the game until the next question.
The “pass“ option was used in an identical fashion to that of human participants, and the macaques were observed to show self-doubting behaviour – a trait which was previously thought to be unique to us. When capuchins were faced with an identical challenge, they failed to take this third option, and seemed unaware when they are likely to make an error.
More information and Video at; BBC – Earth News