Boys Hamper Their Sisters' Development

In a new series of studies, researchers were puzzled to discover that the presence of brothers in households delayed the time when girls got their periods, and also the time of first intercourse.

The correlation primarily held true if the boy was an older brother, Australian researcher say, especially when it came to delays in the start of menstruation.

According to the data derived from their study, it would appear that menstruation is delayed by a full year, on average, when the sisters are compared to girls with no siblings, or with sisters.

If the girls had a younger brother, then they were very likely to delay having their first intercourse by about a year as well, say experts from the University of Western Australia, who led the study.

The investigation was conducted on a study sample of 273 Australians, of which 197 were women, LiveScience reports. The group was aged 18 through 75.

“His research helps us to better understand how family dynamics influence development,” explains UWA researcher and behavioral ecologist Fritha Milne.

One of the conclusions of the research is that having smaller brother took a lot of girls' energy, in the sense that they had to take on more care-taking roles from their mothers.

This in turn prevented from starting their own intimate life with partners.

In the case of sisters with older brothers, the correlation was a bit more hidden. Researchers say that menstruation is delayed because the girls feel more pressure, and also get less parental resources than they normally would.

“Future research might focus on measures for potential mechanisms, and thus determine why and how brothers delay the onset of […] maturity and [intercourse] of their sisters,” Milne explains.

Full details of the new work were published today, in the August 18 online issue of the esteemed scientific journal Proceedings of the Royal Society B.

Researchers add that the presence of sisters in the household, or socioeconomic factors, had no bearing on the girls' development.

The study found that even if little girls had been in contact with older, pregnant sisters, their evolution was not influenced.