Girls with older brothers get their periods at a comparatively later age, researchers in Australia have discovered.
"What we found was that there was an older age at menarche [when a girl first gets her period] for girls who had older brothers," ABC Science quoted human behavioural ecologist Dr Debra Judge, of the University of Western Australia in Perth, as saying.
Judge and PhD student Fritha Milne investigated the effect of siblings among 273 Australian men and women between the ages of 18 and 75.
They gave participants a questionnaire that asked about the number, age and gender of all siblings.
They also asked when participants had their first sexual experience, when girls got their first period, when participants had their first child and how many children they had.
Judge said the study, which took account of the fact that menarche has been starting earlier in successive generations of girls, found some curious trends.
The most puzzling finding was that the more older brothers a woman has the older she was when she reached menarche.
The results showed that women who have older brothers (but no older sisters) got their periods at 13.6 years (mean age). And those who have no older brothers or sisters, or just older sisters, got their period at 12.7 years.
And those who had both older brothers and sisters got their period at 13.3 years.
Judge said that previous research has found girls with absent fathers start their periods earlier, possibly due to some sort of stress effect, compared to girls whose fathers are around at the time they are growing up.
To investigate whether the latest findings were a result of brothers having some sort of 'father figure' effet, Judge and Milne checked to see if the age of the older brother affected the onset of menarche, but found it didn't at all.
So why would older brothers affect the age of menarche in their sisters?
"We don't know what's delaying menarche. It could be a physiological effect in terms of food or some other sort of resource, but I doubt it. Or it could be some sort of psychosocial process that's slowing down the girls' maturity," Judge said.
She said that it may have something to do with hormones or the different way girls are treated when they have an older brother.
The findings have been reported in this week's Proceedings of the Royal Society B.