The human anger face evolved to enhance cues of strength

Animals typically deploy their morphology during conflict to enhance competitors' assessments of their fighting ability (e.g. bared fangs, piloerection, dewlap inflation). Recent research has shown that humans assess others' fighting ability by monitoring cues of strength, and that the face itself contains such cues. We propose that the muscle movements that constitute the human facial expression of anger were selected because they increased others' assessments of the angry individual's strength, thereby increasing bargaining power. This runs contrary to the traditional theory that the anger face is an arbitrary set of features that evolved simply to signal aggressive intent. To test between these theories, the seven key muscle movements constituting the anger face were systematically manipulated one by one and in the absence of the others. Raters assessed faces containing any one of these muscle movements as physically stronger, supporting the hypothesis that the anger face evolved to enhance cues of strength.