Researchers from Sheffield University have come to the conclusion that the number of brothers for a man talks about how much he's fertile. The more he has brothers, the more children can conceive himself.
The current view is that parents who are endowed with the genes of fertility, often give birth to male children, became the basis of this theory. Scientists have made the analysis of fertility almost 500 men, during which compared the motility of their sperm with their families.
The results showed that those men who had more brothers than sisters, the speed of movement of spermatozoa was higher. As you know, sperm motility increases the chance of conceiving a child. Thus, this analysis also revealed a high degree of fertility and their parents that are genetically passed on to them. That is why not all men can conceive a child.