Women and property are the cause of male aggression
By Roger Highfield, Science Editor
Last Updated: 12:01am BST 27/08/2008



The reason why aggressive men tend to get the girl has been found by scientists.

The hunger for women, along with property, are blamed for the evolution of belligerence and bravery by a study of the mathematics of warfare.

Today`s research chimes with the findings of earlier genetic studies that revealed an estimated 16 million men today, plus an uncounted number of women, are the direct descendants of Genghis Khan, the 13th century Mongolian warlord who carved out a vast empire.

Other work has shown Viking genetic signature from the frontiers of their empire, such as the Scottish Western Isles, the Isle of Skye and Iceland, which fits the popular image of male invaders who took local wives. And studies of chimpanzees suggest that the urge to go to war is lies in our genes.

Now a Californian team has studied the evolutionary forces that shaped the need for males to be belligerent, which raises their probability of trying to conquer neighbours, and of bravery, which increases the probability of succeeding in conquest.

The mathematical analysis of the evolution war by Laurent Lehmann and Prof Marc Feldman of Stanford University focused on small-scale, pre-state societies, for instance hunter-gatherers societies.

In the Proceedings of the Royal Society, Biological Sciences, the study shows that the `selective pressure` on genes linked with belligerence and bravery can be substantial even in groups of large size, so that evolution has smiled on the most aggressive and audacious group.

When scientists refer to selective pressure they mean to the extent to which a population of individuals (or ancestor for instance) may acquire either a beneficial genetic or cultural trait that gave them an advantage and therefore made that group more likely to survive and multiply or lost a trait that makes that group do something that makes them more likely to become extinct.

This pressure is substantially driven by the benefits of conquest that also accrue on the relatives of the belligerent and/or brave males within their group, showing how the spoils of war are shared by those who are related.

They add that bravery and belligerence may be driven by two independent type of `resources` that may boost the ability of a gene to be passed to future generations: additional mates for males to have more children by and additional territory (or material resources) for females, so they are more successful at bringing up the next generation of warriors.

The team points out that `hunter-gatherer societies are well-known to have frequently raided neighbouring groups from whom they appropriated territory, goods and women.`

The study builds on pioneering work to understand why people help each other, in this case to wage war, by Prof Bill Hamilton (1936-2000) of Oxford University, one of the greatest evolutionary theorists of the 20th century, and the leading British geneticist, J. B. S Haldane (1892-1964).

The latter is reputed to have been in a pub one day when conversation turned to the subject of the lengths one would go to save another`s life. Would he risk his life to save a drowning man? Haldane, after a few moments consideration, including some scribbling on the back of a napkin, or envelope (depending on which account you believe), replied `No, but I would do it for two brothers or eight cousins.`

Haldane had an intoxicating insight into cooperation: if genes were the entities vying to get to the next generation, then it made sense for individuals to incur a cost at their own expense if at the same time they conferred a benefit on relatives who carried the same genes.

Or to put it at its most basic, blood is thicker than water and we will put ourselves out more, the closer the relative and the `thicker` the blood tie.

So brothers who share half their genes are four times more likely to put themselves out for each other than cousins, who share one eighth. In turn, what is now called Hamiton`s rule helps develop coalitions with relatives and hostility against strangers.

To study how blood ties shape warfare, the Stanford team came up with an idealised mathematical model consisting of an infinite number of groups, each with a finite number of adult males and of adult females.

They assumed random movements, and warfare, between the groups, and worked out how each group would have children as a result of the influence of genes linked with belligerence and bravery.