Men with many brothers are more likely to have sons, while those with many sisters are more likely to have daughters, according to a new study.
Corry Gellatly, who led the study at the University of Newcastle, came to this conclusion after studying family trees dating back more than 400 years.
He and his colleagues say that their study points to the existence of a fatherhood gene that determines whether men are likely to father boys or girls.
The researchers have revealed that the gene is heritable, and that it comes in one of three variations.
They claim that it affects the number of sperm carrying male or female chromosomes.
According to them, men with a version of the gene known as "mm" produce more sperm with the Y chromosome, and are more likely to have sons than daughters.
The team further revealed that men with another variant, known as "mf", produce roughly equally numbers of sperm with the female X and male Y chromosomes, and thus have 50/50 chances of having sons or daughters.
As regards the third version, called "ff", the researchers say that it produces more X sperm, and increases the likelihood of having daughters.
Describing their study in the journal Evolutionary Biology, the researchers revealed that they involved a study of 927 family trees containing information on 556,387 people from North America and Europe going back as far as 1600.
While the researchers have suggested the existence of three types of fatherhood gene, they have yet not discovered the gene itself.
Gellatly, however, says that the system would tend to balance out gender numbers.
"The gene that is passed on from both parents, which causes some men to have more sons and some to have more daughters, may explain why we see the number of men and women roughly balanced in a population," the Telegraph quoted him as saying.
It may also be helpful in understanding the reason behind sudden increases in the number of boys born after the two world wars, he added.
He says that it perhaps might have happened that men with more sons saw a son returning from the war, and the survivor sons had inherited the "mm" gene variant that increased their likelihood of having sons.
In contrast, men with more daughters might have lost their only sons in the war, who would also have bene more likely to father girls, said the researcher.
"We now know that men are more likely to have sons if they have more brothers but are more likely to have daughters if they have more sisters," said Gellatly.
"However, in women, you just can't predict it," he added.