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.
The human anger face is an early (Stenberg, Campos, & Emde, 1983), reliably-developing (Galati, Sini, Schmidt, & Tinti, 2003), species-typical (Ekman, 1973) expression consisting of a stereotyped array of coordinated muscle contractions (Ekman & Friesen, 1978) (see Fig. 1). However, why did evolution give the human facial expression of anger the particular form that it has? The most common position is that the anger face is a universal but arbitrary signal of aggressive intent (Blair, 2003, Matsumoto et al., 2010 and Schmidt and Cohn, 2001). According to this widespread view, the expression would have evolved to be salient and distinguishable from other emotional expressions (Darwin, 1872), but there need be nothing functional about the particular pattern of muscle movements per se. We agree that the anger expression functions as a signal ( Reed, DeScioli, & Pinker, 2014), but we propose that the specific array of muscle contractions that constitute the anger face was in fact tailored by selection to be functional rather than arbitrary. Specifically, during conflicts of interest, natural selection favored displaying those configurations of muscle activation that amplified others' assessments of the sender's fighting ability—in the human case, those configurations that amplified cues of strength. This hypothesis is made plausible by recent work showing the existence of cues of strength in the face—cues that are rapidly and spontaneously assessed when estimating another's fighting ability ( Sell et al., 2009a, Trebicky et al., 2013 and Zilioli et al., 2014).
