Girls with younger brothers tend to get into a sexual relationship late by two years because of their perceived responsibility of care-giving in the family, concludes a new research study.
Researchers said that elder girls were more likely to take on the role of helping with family chores and as a result were less likely to form sexual relationships.
"Elder daughters may provide more help to their parents in raising their younger brothers than do elder brothers," the Telegraph quoted study authors Fritha Milne and Debra Judge, of Western Australia University, as saying.
"Associated with this prolonged help is a delay in sexual activity and thus a delay in potential for starting their own family."
The researchers studied 273 people aged 18 to 75 and found girls with exclusively elder brothers were almost a year older when they had their first period than those with no older siblings or those with only elder sisters.
And girls with only younger brothers were on average almost two years older when they first had sex than those with no younger siblings.
The findings were published in Proceedings of the Royal Society B.