The commitment function of angry facial expressions

What function do facial expressions have? We tested the hypothesis that some expressions serve as honest signals of subjective commitments-in particular, that angry faces increase the effectiveness of threats. In an ultimatum game, proposers decided how much money to offer a responder while seeing a film clip depicting an angry or a neutral facial expression, together with a written threat that was either inherently credible (a 50-50 split) or less credible (a demand for 70% of the money). Proposers offered greater amounts in response to the less credible threat when it was accompanied by an angry expression than when it was accompanied by a neutral expression, but were unaffected by the expression when dealing with the credible threat. This finding supports the hypothesis that angry expressions are honest signals that enhance the credibility of threats.