Men with mostly brothers are likely to be more fertile, states study. The findings also support the theory that parents with genes for good male fertility are more likely to have boys. Normally, in Britain 105 boys are born for every 100 girls.
Experts from the University of Sheffield assessed 500 men and compared the travelling speed of their sperm with their family make-up.
Dr Allan Pacey, one of the researchers from the University of Sheffield, said: "We found the greater number of brothers rather than sisters a man has, the faster his sperm is, increasing the likelihood of fertility. Lots of brothers is also an indication that the man's parents have strong male fertility genes, and they would then be passed on to the son."
"The results are very surprising and could provide genetic insights into why some men are more fertile than others. The research did not give any clues as to how to treat male infertility," Dr Pacey said.
"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 added.
Sperm activity or "motility" is known to be a major factor influencing male fertility.
The findings are published in the Asian Journal of Andrology.