Feminists have debated it for decades, but scientists have finally got to the bottom of why men still exist.
Biologists have always puzzled over why males have survived given that their only contribution to reproduction is sperm.
It makes far more sense in evolutionary terms to have an all-female asexual population which creates daughters who can reproduce rather than sons who cannot, such as the Mexican whiptail lizard.
But research suggests that sexual competition for mates keeps populations healthy, free of disease and genetically diverse.
"Almost all multicellular species on earth reproduce using sex, but its existence isn't easy to explain because sex carries big burdens, the most obvious of which is that only half of your offspring - daughters - will actually produce offspring," said lead researcher Professor Matt Gage, from the University of East Anglia School of Biological Sciences.
"Why should any species waste all that effort on sons? An all-female asexual population would be a far more effective route to reproduce greater numbers of offspring.
"Our research shows that competition among males for reproduction provides a really important benefit, because it improves the genetic health of populations."
Charles Darwin first suggested the idea of "sexual selection" in which males compete for reproduction and females choose. It is why in the animal kingdom males are often far more brightly coloured than females, and partake in elaborate courtship rituals.
But until now nobody realised how great a role it played in the health and success of societies.
To find out how important sexual selection was to populations the scientists allowed a colony of Tribolium flour beetles to evolve over 10 years in the laboratory.
In some groups, 90 beetles had to compete for the affections of just 10 females, while in others females far outnumbered the males.
After seven years, or 50 generations, the researchers found that the males who had competed the most for females were fitter and more resistant to disease and inbreeding. In contrast, beetles without sexual selection became extinct after 10 generations.
Gage said: "These results show that sexual selection is important for population health and persistence, because it helps to purge negative and maintain positive genetic variation in a population.
"To be good at out-competing rivals and attracting partners in the struggle to reproduce, an individual has to be good at most things, so sexual selection provides an important and effective filter to maintain and improve population genetic health."
The research was published in the journal Nature.