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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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New study: Man flu not a myth

A new study reiterates a long-standing evolutionary conundrum

So Man flu is not a myth, because testosterone inhibits the immune response.

This has been known to biologists for ages. In The Red Queen, I challenged readers to explain why bodies should be designed that way: why set up an immune system in such a way that it gets hindered by normal hormonal action? I still find it baffling. Over the years readers took up my challenge and wrote to me. They still do. Their answers nearly always boil down to a version of this: to weed out weedy males. That is to say, if males cannot both keep their testosterone levels up and resist disease they don't deserve to contribute to posterity's genes.

Trouble is, like all group selectionist arguments, it's vulnerable to the evolutionary free rider. Along comes a mutant animal that breaks the link between testosterone and illness and hey presto it can breed away to its gonads' content, propagating its subprime genes as if they were triple A.

Then there's the old 'constraint' argument - that the system's built that way because it's the only way mother Nature knows how to build it. Not very satisfying. Mutation finds a way round most obstacles.

In the new study, Olivier Restiff says it's all about reinfection. If males, which are the 'live hard die young' sex, are only going to get reinfected quickly, then there's no point wasting energy clearing previous infections. Divert the energy into fighting instead.