The fact that prehistoric armies were made up almost exclusively of men has puzzled archaeologists for decades.

But now researchers have a theory – all-male armies formed because of a fear of sexual betrayal by their wives.

If all men did not go off to fight, those remaining behind would be able to mate with others' partners, the study says.

A new study has revealed why traditionally conflicts were fought by male armies. File image

Once a few more men were fighting than women, social pressure created a 'sexual feedback loop' that incentivised ever more to take part, researchers said.

The team at the University of St Andrews said the rarity of mixed-sex armies from prehistoric times to the present day was 'puzzling' because excluding women would effectively halve your fighting force.

Using a computer model, they found that once a few men had left for battle, the departure of each extra male created a greater incentive for others to take part – making them all less likely to be cuckolded.

The study appears in the Proceedings of the Royal Society B journal.