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Girls with an older brother are more likely to begin menstruating at a later age, and younger brothers can delay the onset of their older sisters' sexual activity, according to researchers at The University of Western Australia.

PhD student Fritha Milne and Assistant Professor Debra Judge from UWA's School of Anatomy and Human Biology have co-authored a paper on this subject, to be published online in the prestigious international biological research journal Proceedings of the Royal Society B.

Their study of 273 Australian respondents builds on previous studies into the effects of brothers on their siblings' reproductive success.  They found that in subsistence and pre-industrial societies, brothers had a negative impact on the age at which their sisters first gave birth and on the number of children they had.

But the researchers found that in contemporary Australia, these later life events were not affected even though early markers (menarche and onset of sexual activity) were delayed.  It is hypothesised that the impact of having brothers was offset by the length of time the woman was independent from her birth family, before starting her own family.

"What we're suggesting is that the period of independence for a woman between leaving home and starting her own family is what moderates the effect of the childhood environment," Ms Milne said.  "That is, in populations with a younger average age of first birth, the childhood environment does influence the age at which a woman gives birth to her first child and the number of children she has.

"However, in populations with an older age at first birth, such as Australia, there is a longer period of independence before starting a family, so the influence of siblings has a more limited impact on adult reproductive behaviour."

The results of the study were important in helping understand how family dynamics influenced development, Ms Milne said.

The Royal Society is the world's oldest scientific academy in continuous existence, and has been at the forefront of enquiry and discovery since its foundation in 1660.

Media references

Fritha Milne (School of Anatomy and Human Biology)  (+61 8)  6488 3647
Assistant Professor Debra Judge (School of Anatomy and Human Biology)  (+61 8)  6488 3304
Janine MacDonald (UWA Public Affairs)  (+61 8)  6488 5563  /  (+61 4) 32 637 716