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The findings, reported online in the Asian Journal of Andrology, support previous theory that parents with genes for good male fertility are more likely to have boys.

"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," says Allan Pacey, senior lecturer in andrology at the University of Sheffield.


In a study of 500 men attending semen analysis as part of infertility investigations, the team used sophisticated computer technology to accurately measure how fast the men's sperm swam and then correlated this information with how many brothers or sisters each had in his family.



"This is certainly not a smoking gun as a reason for infertility in men," says Jim Mossman, a postdoctoral researcher at Brown University who collected the data during his PhD studies in Sheffield and who led the research.



Jon Slate, professor of evolutionary genetics at the University of Sheffield adds: "We are very intrigued by this finding and hope other researchers examine their datasets 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."

Source: University of Sheffield