If you’ve ever imagined your future family and wondered whether you’d have sons, daughters, or both, there’s one new clue that may just tip you off to what lies ahead. A recent study published by a research team from Newcastle University in England has determined that whether you have sons or daughters is the result of the father’s genes—those with many brothers are more likely to have a son, and those with many sisters are more likely to have a daughter.
The Newcastle team analyzed 927 family trees dating back to the year 1600 to build on what scientists have long understood about how the sex of a baby is determined: that sperm carries either an X or Y chromosome, and combines with the mother’s X chromosome to make either a baby girl (XX) or a baby boy (XY). The new research suggests that an unnamed gene controls a man’s ratio of X to Y sperm, making them marginally more likely to have a child of one sex or the other.
“The family tree study showed that whether you’re likely to have a boy or a girl is inherited. 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. However, in women, you just can’t predict it,” said Corry Gellatly, a research scientist on the Newcastle team.
This theory also helps to explain why we see a roughly balanced ratio of male to female births. “If there are too many males in the population… 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 later generations,” Gellatly explained.
Of course, in most present day North American families, sibling sets are limited to smaller pools—meaning it would be difficult to glean any definitive information from the gender of a man’s one or two siblings. And even when you know that a person has a statistically higher likelihood of a son or daughter, there’s always room for variance.
Still, this method of family tree tracing is your best chance at an accurate gender prediction, giving you a good guess at what to expect even before you’re expecting. And for more on surprising family traits, check out You May Have Your In-Laws to Blame for This Bad Habit.