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Santa Barbara scientists say that the 'anger face' evolved to increase humans' bargaining power, Nature World News reports.

During a conflict, the odds are in favour of the individual with highest power during a conflict. The researchers say that the anger face helps individuals pretend that they are in power and so, in a better position to bargain.

"This general bargaining-through-menace principle applies to humans as well," John Tooby, UCSB professor of anthropology, said in a news release. "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."

According to the team, the anger face also helps intimidate the other party in an argument and helps the expresser appear stronger.

For the study, the researchers used computer-generated faces to demonstrate that people with angry faces appear physically stronger than their peers.

Scientists say that a lower brow isn't inherently an expression for anger. However, participants are more likely to choose an image with a low brow as belonging to a person with more strength than an image with a high brow.

"Our previous research showed that humans are exceptionally good at assessing fighting ability just by looking at someone's face," said Aaron Sell, a lecturer at the School of Criminology at Griffith University in Australia. "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 study is published in the journal Evolution and Human Behavior.