New research hints that whether a man has sons or daughters is influenced by his genes. The study, by Mr Correy Gellatly from Newcastle University, was published in the journal Evolutionary Biology last week.
Mr Gellatly looked at 927 family trees from Europe and North America, detailing over 556,387 people, dating back to 1600. He observed that a man with many brothers is more likely to have sons, while a man with many sisters is likely to have more daughters. This effect was not seen in women.
The sex of a baby is determined by its father's sperm, an 'X' sperm (after the version of the sex chromosome it carries) makes a girl and a 'Y' sperm a boy. Mr Gellatly hypothesises that there is a gene, only active in males but a version of which is inherited from both parents, which determines the ratio of X and Y sperm a man produces.
He also suggests such a gene could explain the increase in baby boys being born after World War I. Mr Gellatly explains that the odds were in favour of men with more sons seeing a son return from the war. This would mean such men were likely to have sons, a trait inherited from their father. In contrast men with more daughters may have lost their only sons in the war, and those sons would have been more likely to father girls.
Other explanations have been proposed as to why the birth rate is not 50:50 in certain couples. It has been suggested that the sex of a baby could be influenced by differences in the time in a woman's monthly cycle sex happens, or the amount of time sperm spends in the testicles. Mr Galletly's study indicates there is a genetic component.
He says the net effect of such a gene is to balance out the population: 'If there are too many males in the population, for example, females will more easily find a mate, so men who have more daughters will pass on more of their genes, causing more females to be born in subsequent generations.'