The smarter the man, the higher the quality of his sperm, new research published in the journal Intelligence shows.

But women having difficulty conceiving shouldn't question their partner's intelligence and men who aren't quite geniuses should not worry about their ability to father children, lead researcher Rosalind Arden of Kings College London told Reuters Health. "This is scientifically interesting, but unimportant in terms of people's likelihood of conception or fertility," she said in an interview.

Scientists have wondered whether there might be a human "fitness factor," a correlation between several characteristics of the mind and body that could help determine a person's likelihood of passing on their genes to future generations. Candidate characteristics include intelligence, height, attractiveness, resistance to disease, and mental stability.

Several studies have shown that taller people tend to be smarter, providing some support to this idea.

Arden and her colleagues set out to see if sperm quality and intelligence might be similarly related. "We were particularly interested in sperm quality because sperm quality seems to be such a commonsense measure of what evolutionary people call fitness, which means the likelihood of surviving and having children," she explained.

They looked at a sample of 425 Vietnam era veterans who underwent intelligence testing and sperm quality measurements between the ages of 31 and 44. The researchers calculated intelligence by incorporating the men's results on five different tests. They then compared this "intelligence factor" to the men's sperm concentration, sperm count and sperm motility.

Intelligence rose in tandem with each of the three measures of sperm quality, the researchers found. The relationship remained statistically significant even after accounting for the potential effects of age, cigarette smoking, body mass index, alcohol consumption and sexual abstinence.

The size of the relationship between sperm quality and intelligence was similar to that previously observed between height and intelligence, Arden said, although it may be even larger given that height is more reliably measured than sperm quality.

The idea of a "fitness factor" should not be interpreted to mean that only the genes of the very handsomest, smartest, and tallest will be passed along, according to Arden.

"Throughout history, as far as we know it, most men and women have ended up having some children," she noted. "It's the people who are at least average and a little bit above average who tend to have more children.

"It's not necessarily that women are going to favor men who are going to be rocket scientists, fantastically intelligent, fantastically cool," Arden added. "They might just say, 'I'd rather not have the guy who has a whole suite of unpleasant characteristics, I'd rather have the guy who is at least average."