Men are often said to think not with their brains, but with another body part between their legs. We might be castigated for this, but according to new research, it might not be such a bad thing: apparently a man's sperm quality turns out to be a decent indicator of his brain power.

Men who scored high on a battery of intelligence tests boasted high counts of healthy sperm, while low scorers tended to have fewer and more sickly little guys.

This suggests that intelligence might tip off a man's overall health to women looking for a mate with healthy genes, explained University of New Mexico evolutionary psychologist Geoffrey Miller at a recent Harvard University talk.

"It's not necessarily that the same genes are influencing sperm quality and intelligence," he said. Rather, the two traits could be linked through a tangled web of biological and environmental interactions that has evolved to help women pick a mate.

Miller and his colleagues uncovered the apparent sperm-intelligence connection after reanalysing data gathered in 1985 to assess the after-effects of the Vietnam War, particularly exposures to Agent Orange. Of the 4,402 veterans who participated in three days of physical and mental testing, 425 provided sperm samples.

After accounting for factors that could skew the results, such as age, drug use, and abstinence before providing a sample, Miller's team looked for a statistical link between men's sperm counts and motility and their scores on several tests of verbal and arithmetic intelligence.

Though the connections between brains and sperm were "not awesome, they're there and highly significant," Miller said. All things held equal, good sperm and good brains go together.

Exactly why smart men would have healthy sperm is unclear, but Miller has one idea: "I'm thinking of intelligence as being quite closely related to individual fitness."

We wrote about a related study, which found that women favour intelligent men both for marriages and one-night stands. It makes sense to have a smart husband and father who can help provide for his family, but why go for a brains when you just want a one-off encounter?

Good genes, Miller argued. Nearly all animals have evolved traits that give them a leg up on competition for a mate. When choosy females get their pick, male birds sing better songs, peacocks boast larger tails and lions bigger manes.

In humans, physical features still convey a lot of information about a potential mate's genetic health, with symmetric faces scoring high for both sexes. One study even found that handsome men have the best sperm.

But in two million years of evolution, women may have homed in on other traits that offer an honest assessment of a man's worth, and intelligence could be one of them.

About half of our genes are switched on in the brain, so intelligence might provide women with a rough but handy read-out of mutations in our genomes, Miller said.

And just as an elaborate birdsong does little to put worms in the nest or fend off predators, human traits like intelligence, creativity and even humour might have evolved only because of choosy women, Miller told the crowd. "You can't get the sabre-tooth not to eat you by telling a joke."

Ewen Callaway, online reporter, NEW SCIENTIST, 9 Oct 2008