Men with more brothers than sisters may have faster swimming sperm – and are more likely to have increased fertility, according to new research by Sheffield University experts.
Scientists found a correlation between the swimming speed of a man’s sperm and the number of brothers he has.
The findings support previous theories that parents with genes for good male fertility are more likely to have boys.
Dr Allan Pacey said: “The results are very surprising and could provide genetic insights into why some men are more fertile than others.
“However, at the moment they have no clinical relevance to how we might both manage and treat male infertility.”