A new study suggests that the more brothers a man has, the faster his sperm swim, meaning greater fertility.
Scientists from the University of Sheffield in the UK and Brown University in the US assessed the fertility of 500 men, and found that sperm swimming speed correlated with family makeup.
The findings, published in the Asian Journal of Andrology, supports the theory that men with strong fertility are more likely to father boys.
"We found the greater number of brothers rather than sisters a man has, the faster his sperm is, increasing the likelihood of fertility," says Dr. Allan Pacey, one of the researchers. "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."
Still, men with a lot of sisters needn't worry, and the scientists also didn't measure whether women with sisters were linked to enhanced fertility.
"This is certainly not a smoking gun as a reason for infertility in men," said Dr. Jim Mossman, study leader and postdoctoral researcher at Brown University, according to the UK's Daily Mail on Wednesday. "However, it would be interesting to test whether the same relationships are observed in other human populations as well as in other species. Likewise, would we observe similar associations when looking at female fertility?"
Access the study: http://www.nature.com/aja/journal/vaop/ncurrent/abs/aja2012109a.html