Hey girls, your brother will not only pull out your Barbie's head but he will also delay your sexual maturation as well ... allegedly. A interesting study by two behavioral ecologists (Milne & Judge, 2010) on contemporary western society of Australian adults found that:
Girls with older brothers experience a delay in menstruation.
Girls with older brothers (but with no older sisters) started their menstruation at a mean age of 13.6 years while girls with older brothers and older sisters started their menstruation at a mean age of 13.3 years. Girls with no older brothers (with or without older sisters) started their menstruation at a mean age of 12.7 years. The more older brothers the girls had, the older they were when they started their periods. However, the age of the brothers does not effect the results.
Girls with younger brothers experienced a delay in first sexual activity.
Girls with one or more younger brothers experienced their first sexual activity up to two years later than girls without any younger brothers.
Neither younger nor older brothers influenced the fitness of their sisters.
Brothers do not affects number of pregnancies, number of children, age at first pregnancy or the firstborn of their sisters.
It is thought that having boys in subsistence and preindustrial societies require more resources than girls, therefore imposing a constraint on lifetime reproductive success. Results from Milne & Judge (2010) were unusual for a society that has plenty of resources because it shows evidence that having brothers negatively affect their sister's sexual maturation. However, having brothers do not influence the reproductive fitness of their sisters. The authors posit that brothers in contemporary Australian are not associated with reproductive fitness due to their sisters having a long period of independence before child bearing. Dr. Judge said that she prefer not to speculate too much of the study as the study was very basic and straightforward. She emphasized that the delay in sexual maturation should not be interpreted as something negative.
Possible reason why younger sisters are experiencing a delay in first sexual activity.
Important to note that the study interviewed only 273 (n) adults between the age of 18 - 75. Out of the total 197 are females and 76 are men. It would be interesting to see results of the study from a bigger sample size because the study essentially represents the western society of Australia (N) as a whole, so the sample size (n) should be bigger. I don't know off hand the population size of western society of Australia, but the Australian population is currently 22,434,156 (current as of August 24, 2010 from Australian Bureau of Statistics). So I would be interested to see if the trend holds up to a bigger sample size, where (n) is at least over 1000. Also, would sisters of twins (with younger twin brother or older twin brother) exhibit the same delay in sexual maturation as well?
Milne, F.H. Judge, D.S. 2010. Brothers delay menarche and the onset of sexual activity in their sisters. Proceedings of The Royal Society B DOI: 10.1098/rspb.2010.1377
Edwards, L. 2010. Having brothers delays sexual maturation in women. Physorg.com Retrieved August 23, 2010 http://www.physorg.com/news201419637.html