Women do not avoid fighting because they are dainty or scared, but because they have a greater stake than men in staying alive to rear their offspring. Women compete with each other just as tenaciously as men, but with a stealth and subtlety that reduces their chances of being killed or injured, says Anne Campbell of the department of psychology at the University of Durham.

Across almost all cultures and nationalities, men have a much smaller role than women in rearing children. "Males go for quantity of children rather than quality of care for offspring, which means that the parental investment of women is much greater," says Campbell. And unlike men, who can't be sure that their children have not been fathered on the sly by other men, women can always be certain that half an offspring's genes are theirs.

Women have therefore evolved a strong impulse than men to see their children grow up into adults. Men's psychological approach is geared to fathering as many children as possible.

To make this strategy work and to attract partners, men need to establish and advertise their dominance over rival males. Throughout evolution this has translated into displays of male aggression, ranging in scale from playground fights to world wars.

Men can afford to take more risks because as parents they are more expendable. Women, meanwhile, can only ensure reproductive success by overseeing the development of their children, which means avoiding death.

"The scale of parental investment drives everything," says Campbell. "It's not that women are too scared to fight," she says. "It's more to do with the positive value of staying alive, and women have an awfully big stake not just in offspring themselves but in offspring they might have in the future," she says.

This means that if women do need to compete—perhaps for a partner—they choose low-risk rules of engagement. They use indirect tactics, such as discrediting rivals by spreading malicious rumors. And unlike men who glory in feats of dominance, women do better by concealing their actions and their "victories".

But there is no doubt, says Campbell, that the universal domination of culture by males has exaggerated these differences in attitudes to physical aggression. "The story we've always been told is that females are not aggressive," says Campbell. And when they are aggressive; women are told that their behavior is "odd or abnormal".