By Charles Q. Choi, LiveScience Contributor
posted: 17 August 2010 07:08 pm ET
Boys may delay their sisters from becoming women and from having sex, new findings from Australia suggest.
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 brothers may 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 sisters assume care-taking roles.
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.