By LiveScience contributor
updated 8/18/2010 4:17:52 PM ET 2010-08-18T20:17:52

Brothers may delay their sisters from becoming women and from having sex, new findings from Australia suggest.

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The presence of older brothers seemed to delay the onset of menstruation of girls by nearly a year on average, and having younger brothers seemed to postpone the beginning of sexual activity in women by slightly more than a year, scientists found after interviewing 273 Australians.

The presence of brothers was already known to have other influences on girls. To learn whether they also had an effect on their sisters' reproductive success, researchers investigated 197 women and 76 men. They ranged in age from 18 to 75, and all but 10 — seven of them women — had siblings.

"This research helps us to better understand how family dynamics influence development," said researcher Fritha Milne, a behavioral ecologist at the University of Western Australia.

In attempting to explain the delayed starts of menstruation and sexual activity, the researchers ruled out factors such as socioeconomic class. Instead, they conjectured that older brothersmay have delayed the physiological maturation of sisters by absorbing more parental resources or by applying psychological stresses. Meanwhile, younger brothers may have delayed girls' behavioral maturation by demanding that their older sistersassume care-taking roles.

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Other statistics, such as the number of pregnancies sisters had, or their ages at first pregnancy and first birth, did not seem affected by the presence of brothers. This lack of effect may be due to the prolonged independence contemporary women often get to experience prior to child-bearing, the researchers suggested. During this pre-motherhood stint, women could earn resources on their own to help provide for any future families, overcoming the negative effects of having brothers.

Prior studies had suggested that boys and girls who were born after one or two brothers weighed significantly less at birth than infants with no older brothers. It was also found that bearing sons is more costlyto a mother than bearing daughters is: Women pregnant with a boy require 10 percent more food and carry male offspring longer in the womb, and boys on average weigh more at birth than girls do.

"Future research might focus on measures for potential mechanisms,and thus determine why and how brothers delay the onset of sexual maturity and sexual activity of their sisters," Milne said.

The scientists detailed their findings online Aug. 18 in the Proceedings of the Royal Society B.

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