From the heroic 300 Spartans of Thermopylae to the Charge of the Light Brigade, history is littered with tales of the bravery of men who knew that death was as likely an outcome as glory.

Such courage has always been recognised as a supreme asset by military strategists — Carl von Clausewitz, the 19th-century Prussian theorist, described it as “above all things . . . the first quality of a warrior”. For biologists, however, it poses a problem: humans simply should not have evolved to be heroic: the dangers to life and limb are too great.

Now, it appears, the solution to this evolutionary puzzle may lie in sex. New research suggests that braver soldiers may ultimately win more sexual partners as well as more battles, and that the extra chances to spread their genes can outweigh the risk of dying in combat.

Natural selection deals brutally with qualities that hurt organisms’ chances of survival and reproduction, and few ways of harming these prospects are quite as blatant as a heroic charge on enemy lines. American scientists have now shown how such courage could have evolved in the small tribal societies of human prehistory.

Times Archive, 1954: Charge of the Light Brigade

We can have no reason to regret the melancholoy loss which we sustained in a contest with a savage and barbarian enemy

• London, Monday, November 13, 1854

• The Battle of Balaclava

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The study, by Laurent Lehmann and Marcus Feldman, of Stanford University in California, suggests that great bravery can have evolutionary benefits under certain circumstances, despite its obvious dangers.

If courage makes it significantly more likely that small bands of tribes-men will win military confrontations with their neighbours, its overall advantages can easily outweigh its risks, a mathematical model has shown.

Some men who carry genetic variants that promote bravery might perish because of them, but the ones who survive may win more battles through their greater daring. The resulting opportunities for rape and pillage can create a net evolutionary benefit.

By having sex with their vanquished enemies’ wives and children, and by taking land on which their own womenfolk could grow or gather more food, particularly courageous and successful warriors would have more offspring who share their genes. “This has consequences for our understanding of the evolution of intertribal interactions, as hunter-gatherer societies are well known to have frequently raided neighbouring groups from whom they appropriated territory, goods and women,” the scientists said.

In the research, details of which are published in the journal Proceedings of the Royal Society, Dr Lehmann and Dr Feldman concentrated on two traits that they imagined might affect societies’ capacity and aptitude for war: bravery and belligerence.

They assumed that tribes with a high proportion of belligerent men would be more likely to attack rival groups, while those with a high proportion of brave men were more likely to win such battles. Both traits, however, also increased the chances of death.

While neither of these qualities is controlled by a single gene, the scientists imagined the emergence of single genetic variants that promoted one trait or the other. The multiple genes that influence bravery or belligerence can be assumed to have evolved in a similar way.

The scientists concentrated on the likely effects among small bands of hunter-gatherers, living in an environment in which rival groups competed intensely for food and shelter.

It is thought that people have lived in such groups for most of our evolutionary history, and that these conditions are thus the main ones that have influenced the development of the human brain and temperament.

The model demonstrated that belligerence or bravery genes could spread quite rapidly, despite the increased risk of death, if the conquest of neighbouring tribes brought a group one of two significant advantages. The first was increased opportunities for men to have sex and father offspring, in this case through capturing the women of a defeated tribe. The second was the capture of extra territory, or other material resources.

While the findings do not explain the emergence of belligerence or bravery, or shed any light on what the genes that might affect these traits might be, they do show a mechanism by which they could have evolved.

“We show that the selective pressure on these two traits can be substantial even in groups of large size, and that they may be driven by two independent, reproduction-enhancing resources: additional mates for males and additional territory (or resources) for females,” the scientists said.

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