Implicit preference for human trustworthy faces in macaque monkeys.

It has been shown that human judgements of trustworthiness are based on subtle processing of specific facial features. However, it is not known if this ability is a specifically human function, or whether it is shared among primates. Here we report that macaque monkeys (Macaca Mulatta and Macaca Fascicularis), like humans, display a preferential attention to trustworthiness-associated facial cues in computer-generated human faces. Monkeys looked significantly longer at faces categorized a priori as trustworthy compared to untrustworthy. In addition, spatial sequential analysis of monkeys' initial saccades revealed an upward shift with attention moving to the eye region for trustworthy faces while no change was observed for the untrustworthy ones. Finally, we found significant correlations between facial width-to-height ratio- a morphometric feature that predicts trustworthiness' judgments in humans - and looking time in both species. These findings suggest the presence of common mechanisms among primates for first impression of trustworthiness.