GIRLS with younger brothers become sexually active later than those who have only younger sisters or older siblings, research suggests.
Scientists at the University of Western Australia found girls with a younger brother have sex an average 14 months later than those without. Girls with both younger brothers and sisters become sexually active 23 months later.
The explanation could be that older sisters are preoccupied with looking after their younger brothers because sons require more care from their families than daughters do – a theory supported by evidence from other mammals.
Boys tend to be born later than girls, weigh more at birth, and be weaned at a later age. Other evidence suggests that girls tend to help to care for their younger siblings more than boys do. The study also found that girls with older brothers first menstruated 11 months later than those who did not.
“Elder brothers delay physiological maturation, while younger brothers delay behavioral maturation,” the team said. The study, which questioned 273 adults about the age when they had become sexually active, is published in the Proceedings of the Royal Society.
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