In a new study Australian researchers have found that having an older brother delays the age at which girls get their periods. The study conducted by Human behavioral ecologist Dr Debra Judge and PhD student Fritha Milne, of the University of Western Australia in Perth was published in this week's Proceedings of the Royal Society B.
Dr. Judge said, “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.” There have been studies that noticed that in traditional societies the more brothers a family has, the fewer children their siblings have. Dr. Judge said that some believed this was due to the fact that brothers generally need more resources, leaving less to go around.
For their study the researchers investigated the effect of siblings among 273 Australian men and women between the ages of 18 and 75. They gave participants a questionnaire that asked about the number, age and gender of all siblings. There were questions related to first sexual experience, when girls got their first period, when participants had their first child and how many children they had. They found that although menarche has been starting earlier in successive generations of girls, the study showed different trends.
Results showed that the more number of older brothers a woman has, the older she was when she reached menarche. Women who have older brothers but no older sisters got their periods at a mean age of 13.6 years. Those who have no older brothers or sisters, or just older sisters, got their period at 12.7 years. And those who had both older brothers and sisters got their period at 13.3 years.
Dr, Judge also explained that previous studies show girls with absent fathers start their periods earlier, possibly due to some sort of stress effect, compared to girls whose fathers are around at the time they are growing up. Some have explained the older brother thing as being the “father figure” influence of younger sisters. Dr Judge and Mr Milne checked to see if the age of the older brother affected the onset of menarche, but found it did not at all.
The explanation for this phenomenon is elusive. “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,” Dr Judge said. She says it may have something to do with hormones or the different way girls are treated when they have an older brother. She warned, “We don't want to speculate too much because I think it's very easy to over-interpret what is a very basic straightforward study.”
Other findings of the study show that girls who have younger brothers start having sexual activity later than those without younger brothers. She says the impact on sexual activity could be because older girls tend to help parents look after younger children. Researcher Milne said, “Elder daughters may provide more help to their parents in raising younger brothers than do elder brothers…Associated with this prolonged help is a delay in sexual activity.” However number of brothers did not affect number of children siblings had, or the age at which they had their first child. Dr Judge says in affluent societies one would expect siblings not to have this kind of effect on each other. She called for future studies to test the 'babysitting hypothesis' to explain the delay in age of first sex in older sisters.
Explaining the delay in periods or sexual activity in girls Dr. Judge said, “It's not like this is any sort of pathology.”
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