New study shows that big brothers reduce fertility (9/28/2007)

Researchers at the University of Sheffield have shown that having an older brother can affect an individual's fertility. The research, published in Proceedings of the Royal Society, shows that people who have an older brother produce fewer children than those born after a sister.

The findings add to existing evidence that sons are more costly to produce than daughters. Previous research by the group also found that giving birth to sons, but not daughters, was associated with reduced maternal lifespan. However, the current study is the first to show important long-term consequences to those born after their mothers produced sons.

Researcher Ian Rickard, together with Dr Virpi Lummaa and Dr Andrew Russell, all from the University's Department of Animal and Plant Sciences, studied Finnish church records from the 18th and 19th centuries and traced the reproductive histories of 521 Finnish women, their 1721 children and 2789 grandchildren.

They found that both men and women, whose mother had previously produced a son, produced and raised fewer children than those born to mothers who had previously produced a daughter.

Ian Rickard said: "There is evidence from many species that sons are more costly for mothers to give birth to and care for than daughters. It now appears that after producing a son, a mother may be less able to invest in her next child. This reduced investment may compromise the early development of the body's physiological systems, including those important for reproduction."

Note: This story has been adapted from a news release issued by The University of Sheffield

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