Infant brain responses are sensitive to natural variance in facial trustworthiness

When adults see someone for the first time, they automatically form a first impression of that person as more or less trustworthy. Recent studies suggest that already infants show a similar sensitivity to variance in facial trustworthiness. However, prior studies on infant trustworthiness perception have always used artificially manipulated faces. The question therefore arises how infants respond to a larger variety of naturalistic faces, which are perceived by adults to vary in trustworthiness. To investigate this question, we presented 7-month-old infants with 15 different male faces which adults had previously rated in trustworthiness and measured ERP responses and looking behavior. When grouping faces categorically according to trustworthiness, no differences in ERP response or looking behavior emerged. However, when predicting the Nc amplitude from exact adult trustworthiness ratings, we found that higher trustworthiness ratings corresponded to larger Nc responses in infants. Our findings therefore suggest that, on a neural level, infants discriminate faces according to features adults associate with trustworthiness from naturally varying faces, and that their neural response tracks the naturally variance between faces. Furthermore, ecologically valid faces yield a different neural response pattern compared to artificially manipulated faces, highlighting the importance of using naturalistic stimuli to study social processing.