Men with more brothers than sisters are likely to have faster swimming sperm and greater fertility, a study suggests.
Scientists assessing the fertility of 500 men correlated sperm swimming speed with family make-up.
They found that men who mostly had brothers had faster sperm than those with mostly sisters.
The finding, published in the Asian Journal of Andrology, supports the theory that parents with genes for good male fertility are more likely to have boys.
Dr Allan Pacey, one of the researchers from the University of Sheffield, said: "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.
"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."
Sperm activity or "motility" is known to be a major factor influencing male fertility.
Jon Slate, Professor of Evolutionary Genetics at the University of Sheffield, said: "We are very intrigued by this finding and hope other researchers examine their data sets in a similar fashion.
"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."