Having a boy or girl doesn't run in the family - it's truly random, study finds

Longing for a baby girl after siring a rugby team's worth of boys? Keep on trying - the next one's just as likely to be a girl as it is a boy, new research has found.

It turns out there's no truth to the idea boys or girls "run" in families, scientists in Australia have discovered.

They looked at data recording every single birth from Sweden until now, covering more than 3.5 million parents and 4.7 million children. They tested whether the sex of a person's children was linked with the sex of their brother or sister's children, and found no correlation at all.

"We found individuals don't have an innate tendency to have offspring of one or the other gender," said Dr Brendan Zietsch of the University of Queensland, who led the study - the largest ever done on the subject.

"The chances are more like 51 to 49 of having a boy, but the genes of the mother and father don't play any role."

He said the study also busted myths around what might influence a baby's sex.

"It was thought that rich or tall parents should have more boys and beautiful parents should have more girls. It was also thought that parents' hormone levels at the time of conception were important.

"Our results rule out all these possibilities and suggest a rethink of offspring sex ratio theory is necessary to properly understand why offspring sex ratios appear to vary, for example, across countries."

Previous research has suggested there are slightly more males born than females because females are more likely to die before they're born. But males have a higher mortality rate once they're out of the womb, largely correcting the imbalance.

The latest research was published in journal Proceedings of the Royal Society B.