Looking for Mr. Right to start a big family with? Science says you should start by counting how many brothers he has.

Scientists say that the more brothers a man has, the greater his baby-making potential, after discovering a link between the swimming speed of a man's sperm with the number of male siblings in his family.

The latest findings, published in the Asian Journal of Andrology, add to a previous theory that parents with genes for good male fertility are more likely to have boys. If the theory is correct, it seems Americans have excellent male fertility genes. According to the latest report from the U.S. Centers for Disease Control and Prevention, more boys than girls are being born in the United States, and there were exactly 94,232 more male births than female births in the U.S. in 2004.

Researchers at the University of Sheffield compared the traveling speed of 500 men with their family make-up.

The study found that the greater number of brothers rather than sisters a man has, the faster his sperm, and faster sperm is associated with greater fertility. Researchers noted that having mostly brothers can also indicate that the man's parents have strong male fertility genes and that they could have passed it on to him.

"The results are very surprising and could provide genetic insights into why some men are more fertile than others but at the moment have no clinical relevance to how we might manage and treat male infertility," researcher Dr. Allan Pacey, of the University of Sheffield, said in a statement.

"It does, however, give food for thought about the importance of genetics for sperm motility and may open the way to more studies in this area," he said, adding that the latest research did not offer any clues to treat male infertility.

Sperm motility or activity is believed to be a major factor in determining a man's fertility.

"We are very intrigued by this finding and hope other researchers examine their datasets in a similar fashion," Professor Jon Slate, an expert in evolutionary genetics at the University of Sheffield, said in a statement.

"If our results can be replicated we think it provides some evidence that humans have experienced what evolutionary biologists like to call 'sexual conflict'. The idea behind this is that genes that make males reproductively successful make females reproductively unsuccessful and vice versa," he said.

Researchers noted that the discovery does not suggest that men with mostly sisters should worry. They added that the study did not look into whether women with many sisters are more fertile, as it would be significantly more difficult to determine.

However, lead researcher Dr. Jim Mossman, postdoctoral researcher at Brown University said that the latest findings suggest that women with many sisters are possibly more fertile.

"This is certainly not a smoking gun as a reason for infertility in men. However, it would be interesting to test whether the same relationships are observed in other human populations as well as in other species," Mossman said.

"Likewise, would we observe similar associations when looking at female fertility?" Mossman said.

"If the relationship between sex-bias in the number of children and fertility is a more universal phenomenon, then we may expect female fertility to follow a similar pattern, albeit in the opposite direction," he concluded.

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