Women tend to like smart men because they're usually more successful and better providers. But here's another reason: Their sperm is better, a new study says.

Researchers at King's College London, the University of Delaware and the University of New Mexico recently compared results from five intelligence tests given to 425 Vietnam War vets in 1985 as part of the U.S. Centers For Disease Control and Prevention's Vietnam Experience Study. These vets, aged 31 to 44, also provided sperm samples, so the researchers analyzed the sperm per milliliter of semen, plus how many of the sperm swam normally, and other measures of sperm health.

The smarter the men were, the more sperm they produced and the better their wee ones swam — and it didn't matter how old the men were or whether they smoked, drank or were obese.

But why might these two seemingly unrelated traits be linked? Why would calculus aces or business consultants make better sperm?

Turns out that intelligent people are generally healthier than their less-clever peers — studies have shown that brainiacs are, for instance, less likely to suffer from heart disease and Alzheimer's. Scientists have suggested that smart people may score less stressful jobs in safer places and that they may make better lifestyle choices, for instance by exercising more and eating better. In other words, maybe bright people actually listen to the Surgeon General.

But these newest findings, to be published in an upcoming issue of the journal Intelligence, found that negative habits had little effect on sperm quality, so they don't support that theory.

The researchers instead speculate that intelligence might be passed down as part of a larger package of good attributes. One gene can influence multiple traits, so the genes involved in smarts may somehow improve sperm quality — and perhaps other characteristics as well.

This could help explain, then, why intelligence can be so sexy: It could simply be an indicator that a person has a lot of good genes and traits, says study co-author Geoffrey Miller, a psychologist at the University of New Mexico.