The facial expressions people take on when they're mad are apparently the result of human biological characteristics that grew from the need for social power, say researchers from California and Australia.

The new findings about everybody's "anger face" have been published in the online edition of the journal Evolution and Human Behavior.

Scientific teams from the University of California, Santa Barbara and Griffith University in Australia have examined the lowered brow, thinned lips and flared nostrils of people who are angry and determined they are cross-culturally universal expressions that "even congenitally blind children make ... without ever having seen one," lead study author Aaron Sell, a lecturer at the School of Criminology at Griffith University in Australia and a former postdoctoral scholar at UC Santa Barbara's Center for Evolutionary Psychology, said in a news release.

The anger expression, explains the research, includes seven distinct muscle groups that contract in a highly-exaggerated, stereotyped manner.

The researchers explored why those particular muscle movements evolved to signal the emotional state of anger and found out they help those wearing their "anger faces" in positions of strength.

"Our earlier research showed that anger evolved to motivate effective bargaining behavior during conflicts of interest," said Sell. As such, the greater the harm an individual could inflict, or at least, convince others he or she could inflict, the more bargaining power an individual could claim.

"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 study co-author John Tooby, a UCSB professor of anthropology and co-director of the school's Center for Evolutionary Psychology.

Therefore, building on the idea anger is a bargaining emotion, the researchers figured the first step is communicating to the other party that the anger-triggering event is not acceptable -- and will not end until an implicit agreement is reached.

But, "the anger face not only signals the onset of a conflict," said Sell. "Any distinctive facial display could do that. We hypothesized that the anger face evolved its specific form because it delivers something more for the expresser. Each element is designed to help intimidate others by making the angry individual appear more capable of delivering harm if not appeased."

For humankind's predecessors, the research asserts, greater upper body strength led to a greater ability to inflict harm.

So, the analysis goes, the anger face evolved to make a person appear stronger.

The research team used computer-generated faces to demonstrated that each of the individual components of the anger face made people appear physically stronger.

For instance, researchers took a computerized image of an average human face and then digitally changed it in two ways, one edited image showing a lowered brow, the other with a raised brow.

"With just this one difference, neither face appeared 'angry,' " said Sell. "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 one-by-one with each of the other major components of the classic anger face: raised cheekbones -- as in a snarl, lips thinned and pushed out, the mouth raised, the nose flared and the chin pushed out and up.

As expected, the presence by itself of any one of the muscle contractions prompted observers to judge the person making the face as physically stronger.

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

These threat displays -- like those of other animals -- consist of exaggerations of cues of fighting ability, Sell continued. "So a man will puff up his chest, stand tall and morph his face to make himself appear stronger."

Said Tooby: "This makes sense of why evolution selected this particular facial display to co-occur with the onset of anger. Anger is triggered by the refusal to accept the situation, and the face immediately organizes itself to advertise to the other party the costs of not making the situation more acceptable. What is most pleasing about these results is that no feature of the anger face appears to be arbitrary. They all deliver the same message."