Elsevier

Evolution and Human Behavior

Volume 35, Issue 5, September 2014, Pages 425-429
Evolution and Human Behavior

Original Article
The human anger face evolved to enhance cues of strength

https://doi.org/10.1016/j.evolhumbehav.2014.05.008Get rights and content

Abstract

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.

Section snippets

Theoretical background

An animal's fitness is crucially dependent on the outcomes of conflicts of interest. Accordingly, selection should have organized behavioral systems in animals that bargain for improved outcomes in these conflicts. In particular, aggression is a type of bargaining behavior that deploys the threat or actuality of cost-infliction as a tool to incentivize others to reduce their resistance to the aggressor's realization of its interests (Huntingford & Turner, 1987). In humans, these tactics are

Experiment 1: do the components of the anger face increase perceived strength?

To test whether each feature that constitutes the anger expression increases perceived strength, we used a program calibrated with a large number of statistical composites of real faces to generate seven pairs of faces. Each pair contrasted a single feature of the face as modified by anger (e.g. lowered brow) with the opposite modification (e.g. raised brow). Raters then chose the stronger of the pair to determine the effect of each component of the anger face.

Experiment 2: do the components of the anger face increase perceived maturity?

There is another theory, put forward by Marsh, Adams, and Kleck (2005), that argues that the anger face evolved to enhance the maturity of the face, distinguishing it from immature, neotenous faces, i.e. “the origins of the appearances of anger and fear facial expressions…might lie in the expression's resemblance to, respectively, mature and babyish faces.” This position predicts that the constituent parts of the anger face should each—independently— increase the perceived age of the face.

Experiment 3: do the components of the anger face increase perceived maturity or strength in older faces?

The fact that four out of seven of the constituent features of the anger face increased perceived age could be due to the positive relationship that exists between fighting ability and age at young ages. In other words, because male strength peaks in the 20s (Walker, Hill, Kaplan, & McMillan, 2002) and fighting ability near 30 (von Rueden, Gurven, & Kaplan, 2008), making a young face look stronger may lead to the inference that the target is older. In order to decouple increasing age from

General discussion

Taken together, these experiments show that the constellation of features that comprise the anger face was selected for over evolutionary time to enhance cues of physical strength during agonistic bargaining. These results are consistent with previous literature showing that several of the components of the anger face are more prominent in males—the sex that shows evidence of combat design (Becker, Kenrick, Neuberg, Blackwell, & Smith, 2007; see Sell, Hone, & Pound, 2012 for evidence of combat

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