“If you think of anger as a signal, then any old signal will do,” says Aaron Sell, a psychology professor at Griffith University in Australia. “You could raise one eyebrow, you could stick your tongue out—it could be anything. But what natural selection appears to have done is tailored a pretty complicated display.”
In a study recently published in the journal Evolution and Human Behavior, Sell and colleagues from the University of California, Santa Barbara tested the effects of each individual feature of the anger face on a person’s overall appearance, using seven previously identified components of anger: Along with changes to the nostrils, lips, and chin, the brow and brow ridge both lower and the cheekbones and mouth both raise.
Starting from a computer-simulated image of a 20-year-old man, the researchers created pairs of faces for each of the seven features—one face neutral, one face with the anger-related change—and asked volunteers to assess each one for physical strength. Across the board, the faces with a single feature activated—neutral except for flared nostrils, for example—were rated as belonging to stronger men.
One reason for this link, Sell says, may be because of the increased leverage that fighting ability afforded our ancestors in resolving conflicts of interest: The more physically threatening a person looked, the more bargaining power they had to influence the outcome of a given situation. “The reason natural selection designed [the anger face] is that the individuals who made that face out-reproduced the other ones,” he explains. “And they out-reproduced them because the people who made that face won their conflicts. The other people backed down because they looked at them and thought, ‘Wow, he looks really tough.’”
The concept of aggression as an assertion of the upper hand has been well-documented in scientific literature, and anger faces are thought to be more easily identified than other expressions of emotion, allowing for more efficient responses to perceived threats. Previous research from Sell has also found that both genders can more readily identify expressions of anger on men—who are more likely to be aggressors, evolutionarily speaking—than on women, and that men with greater upper-body strength and more attractive women—two groups that, in the early days of humanity, would have had increased bargaining power—may also be quicker to anger than their weaker or less attractive peers.
But a separate study, published in 2005 in Personality and Social Psychology Bulletin, argues that the anger face evolved to enhance the wearer’s threatening nature by creating an older appearance, rather than a stronger one: “The origins of the appearance of anger and fear facial expressions … might lie in the expression’s resemblance to, respectively, mature and babyish faces.”