If you are seeking a man who will give you a big family, first make sure you count how many brothers he has.
The greater the number of male siblings your husband has, the more fertile he is likely to be, scientists say.
They have discovered that men with mostly brothers are likely to be more productive.
The findings, published in the Asian Journal of Andrology, 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. 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.
Sperm activity is known to be a major factor influencing male fertility
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.'
The researchers said it did not mean men with a large number of sisters should worry, and they did not look at whether women with lots of sisters are likely to be more fertile, as it is much more difficult to measure.
But Dr Jim Mossman, postdoctoral researcher at Brown University in the US, who led the study, said the findings may indicate that women with lots of female siblings are likely to be more fertile.
'This is certainly not a smoking gun as a reason for infertility in men,' he said. '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?
'If the relationship between sex-bias in the number of children and fertility is a more universal phenomenon, then we may expect female fertility to follow a similar pattern, albeit in the opposite direction.'