A man's aggressive tendencies are written in his facial features, according to a study that has found a link between bad temper and the shape of a man's face.
The distance between a man's cheekbones compared to the height of his face is a good indicator of how likely he is to explode with rage when provoked and this may be an evolutionary feature going back thousands of years, scientists said.
Research on ice hockey stars in Canada has found a significant correlation between the ratio of width-to-height distances of faces and the time spent by the players in the "sin bin" of the dugout as a result of committing fouls.
The scientists failed to find a similar link between aggression and female facial characteristics, which led them to believe that the findings could help to explain why there is such a difference between masculine and feminine faces.
"We chose ice hockey players because there is a lot of aggressive behaviour in the games and it's usually an acceptable way of performing successfully. We could also measure this aggression in terms of the number of penalty minutes," said Justin Carre of Brock University in Ontario.
The study looked at three groups of men taken from either the general population, from university hockey teams or from national hockey teams. In all three groups, the researchers found a statistically significant correlation between facial shapes and aggressive tendencies.
"Together, these findings suggest that the sexually dimorphic facial width-to-height ratio may be an 'honest signal' of propensity for aggressive behaviour," the scientists say in the journal Proceedings of the Royal Society B.
Facial shape was determined by measuring the distance between the outer edges of a person's two cheekbones and dividing this by the distance between the eyebrows and the top of the upper lip.
Previous work had found differences between the sexes in terms of facial shape, with men generally having wider faces relative to facial height than women, irrespective of body size.
The sexual "dimorphism" in the face suggests a long evolutionary history based on mating preferences stemming back many generations. Some studies have also indicated that a more masculine face is used by women to predict certain features about that man, such as whether he is likely to make a good father or indeed whether he is considered to be sexually more attractive than other men.
"Together, these findings suggest that people can make accurate inferences about others' personality traits and behavioural dispositions based on certain signals conveyed by the face," the scientists said.
Mr Carre said that the differences in facial characteristics between the sexes came about at the onset of puberty as a result of the surge in sex hormones, particularly testosterone in boys. It is also possible that testosterone influences aggressive tendencies, he said.
"We propose that the physical features that become evident around puberty are a result of differences in testosterone and that testosterone may be responsible for determining the width-to-height ratio," Mr Carre said.
The next stage is to see whether people can actually judge a person's facial shape with any accuracy, and use that information to form a judgement about their personality. "The next thing is to see if people are sensitive to these differences in width-to-height ratio," Mr Carre added.