Brothers and Periods
Hmmm, this is a strange one...
It's been known for a while that girls from families with absent
fathers begin periods earlier than those whose fathers are present
before and around puberty, but Australian researchers have discovered that having older brothers delays the onset of a girl's menstrual period. Dr.Deborah Judge and Phd student Fritha Milne, from the University of Western Australia recently released the relevent study in Proceedings of the Royal Society (Biological Sciences)
According to Judge "What we found was that there was an older age at menarche [when a girl first gets her period] for girls who had older brothers,". However it's not clear why this should be occurring.
The study reveals that the greater the number of older brothers, the later the occurance of menarche. Girls with older brothers and no sisters started menstruating art 13.6 years. Girls who had no older brothers or sisters or only older sisters began menstruating at 12.7 years, while girls who had both older brothers and sisters began menstruationg at 13.3 years.
The Western Australian research is an interesting addition to previous, related studies about reproduction and family relationships, where anthropologists have noted that in traditional societies the more brothers a family has, the fewer children their siblings have. However Judge notes that in societies where resources are more plentiful, one would expect siblings not to have this kind of effect on each other...and indeed in the Western Australian study Judge and Milne did not find that the number of brothers affected the number of children siblings had, or the age at which they had their first child.
Interestingly though, a further finding of the study revealed that girls with younger brothers started having sexual activity later than those
without younger brothers. Judge posited that the impact on sexual activity could be because older girls tended to help parents look after younger children....thus older sisters are too busy looking after their younger brothers to go out and potentially explore sexual experiences.
In order to test whether or not the latest findings were a result of brothers having a kind of *father substitute* effect, Judge and Milne checked to see if the age of the older brother had any effect on the age a girl starts her period, but found it did not.
Why older brothers should affect the age of menarche in their sisters is something of a mystery:
"We don't know what's delaying menarche.It could be a physiological effect in terms of food or some other sort of resource, but I doubt it. Or it could be some sort of psychosocial process that's slowing down the girls' maturity." Judge speculated and added " It may have something to do with hormones or the different way girls are treated when they have an older brother".
Other researchers have speculated that when the father is absent, girls menstruate earlier in order to unconsciously attract a mate to have a greater chance of securing resources but with older males in the house the biological imperative is not so great. However this would contradict Judges conclusion that in plentiful societies resources would not be an issue.
Judge was keen to emphasise that the findings do not indicate that there is any adverse effect from having later periods.
Catalyst, ABC Science
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