The universal anger face is believed to be part of basic human biology, now researchers have identified the functional advantages behind it. 

The anger expression requires seven distinct muscle groups, researchers at UC Santa Barbara and at Griffith University in Australia looked at why evolution chose those specific contractions to express the emotion. 

"The expression is cross-culturally universal, and even congenitally blind children make this same face without ever having seen one," said lead author Aaron Sell, a lecturer at the School of Criminology at Griffith University in Australia, who was formerly a postdoctoral scholar at UCSB's Center for Evolutionary Psychology.

Past research showed that anger evolved to suit effective bargaining behavior in the face of conflicts. 

"This general bargaining-through-menace principle applies to humans as well. In earlier work we were able to confirm the predictions that stronger men anger more easily, fight more often, feel entitled to more unequal treatment, resolve conflicts more in their own favor and are even more in favor of military solutions than are physically weak men," said John Tooby, UCSB professor of anthropology. 

The team believes the first step in the bargaining process is communicating to the other party involved that their behavior is not acceptable and the conflict will not end until an agreement is reached. 

"Each element is designed to help intimidate others by making the angry individual appear more capable of delivering harm if not appeased," Sell said. 

For our ancestors more upper body strength made a person appear to have the ability to impose more harm, so the anger face may make one appear stronger. To test this the team took a computerize image of an average human face and morphed it in two ways, one with a lowered brow (which is characteristic of an anger face) and one with a raised brow. 

"With just this one difference, neither face appeared 'angry,' " Sell said. "But when these two faces were shown to subjects, they reported the lowered brow face as looking like it belonged to a physically stronger man."

The experiment was repeated with different anger face characteristics such as "raised cheekbones (as in a snarl), lips thinned and pushed out, the mouth raised (as in defiance), the nose flared and the chin pushed out and up." The presence of these changes led the viewer to perceive the faces as stronger.

"Our previous research showed that humans are exceptionally good at assessing fighting ability just by looking at someone's face," Sell said. "Since people who are judged to be stronger tend to get their way more often, other things being equal, the researchers concluded that the explanation for evolution of the form of the human anger face is surprisingly simple -- it is a threat display."

The finding was published in the journal Evolution and Human Behavior.