PHYSIOGNOMY, the art or science of predicting inward character from outward form, has had its ups and downs over the years. A century ago, the idea that a person's character could be seen in his face was more or less taken as given. It then fell out of favour, along with the idea that behaviour is genetically determined, as Marxist ideas of the pliability and perfectibility of mankind became fashionable. Now, it is undergoing something of a revival. It has been found, for example, that women can predict a man's interest in infant children from his face. Trustworthiness also shows up, as does social dominance. The latest example comes from a paper just published in the Proceedings of the Royal Society by Justin Carré and Cheryl McCormick, of Brock University in Ontario, Canada. This suggests that in men, at least, it is also possible to look at someone's face and read his predisposition to aggression.

The thesis developed by Mr Carré and Dr McCormick is that aggressiveness is predictable from the ratio between the width of a person's face and its height. Their reason for suspecting this is that this ratio differs systematically between men and women (men have wider faces) and that the difference arises during puberty, when sex hormones are reshaping people's bodies. The cause seems to be exposure to testosterone, which is also known to make people aggressive. It seems reasonable, therefore, to predict a correlation between aggression and face shape.

To test their thesis, Mr Carré and Dr McCormick looked at the fine, old Canadian sport of ice hockey. This is, famously, not a gentle game. It is also a game in which the rules provide a plausible proxy for aggressiveness, namely the amount of time a player spends off the ice in the penalty box for such infringements as knocking his opponent's teeth out with a well-aimed stick.

The two researchers obtained photographs of several university and professional ice-hockey teams, and measured the facial ratios of the players. They also obtained those players' penalty records. Just as they expected, the wider a player's face, the more time he spent in the cooler.

Ice hockey, though, is mostly a man's game (women might argue that they are too sensible to get involved, although the Canadian ladies did win a gold medal at the last winter Olympics). To find out whether the theory was true for females as well, Mr Carré and Dr McCormick turned to that stock experimental subject, the university undergraduate. They recruited several dozen of both sexes and got them to play a game against what they thought was a person in another room but was actually a computer. Various measures of aggression taken during this game suggest that men are the same everywhere, be they students or sportsmen. Aggression was not, however, predictable in women students—or, at least, not from the shapes of their faces.

It seems, therefore, that facial ratio in men is a biologically honest signal of aggressiveness. Honest signals are those, such as luxuriantly feathered tails, that cannot be mimicked by individuals who would like the benefits without the costs. In the case of aggressiveness, the benefit to the aggressive individual is, paradoxically, that he will not have to get into fights in order to prove the point. The fear induced by his face should be enough by itself. At least, that is the hypothesis. The experiment to prove it has yet to be done.