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Last Updated: Friday, 5 October 2007, 10:21 GMT 11:21 UK
Older brother 'dampens fertility'
Baby
Could brothers make these less common?
Having an older brother may reduce your fertility, research suggests.

The University of Sheffield found people who had an older brother had fewer children than those who had an older sister.

Experts believe that having a male child takes more out of mother - and may have a direct impact on the care she can give to further offspring.

However, an expert said modern high-quality healthcare probably neutralised the impact.

Our modern healthcare infrastructure, longer life spans and use of contraception may overcome whatever mechanism is at play to generate this effect
Dr Allan Pacey
British Fertility Society

The Proceedings of the Royal Society study focused on historical data from the 18th and 19th centuries.

The researchers conceed that the circumstances faced by modern day mothers in developed nations are very different - but argue that their findings may still be very relevant to developing countries, where food may be scarse, and the risk of infection high.

Previous research has shown women that giving birth to sons, but not daughters, was associated with reduced maternal lifespan.

But the Sheffield study is the first to show important long-term consequences to those born after their mothers produced sons.

The researchers found the effect held good even if the older child failed to live to six months - suggesting that something which takes place before birth may be key.

Hormone impact

One theory is that the male sex hormone testosterone and the stress hormone cortisol, both produced in higher concentration by male foetuses, may dampen down a mother's immune system, making her more vulnerable to disease.

The researchers studied Finnish church records from the 18th and 19th centuries and traced the reproductive histories of 521 Finnish women, their 1,721 children and 2,789 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. Children with an older sister had 12% more children.

Lead researcher 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."

Better healthcare

A separate study by the University of Sheffield, published earlier this year, found a twin brother can reduce his female twin's chances of having children.

Women were 25% less likely to have children if their twin was male, the study found.

Dr Allan Pacey, secretary of the British Fertility Society, who is also based at Sheffield University, said the study gave an interesting insight into the evolutionary biology of human reproduction.

But he said: "It is unlikely that we would see such effects in our modern industrialised world.

"Our modern healthcare infrastructure, longer life spans and use of contraception may overcome whatever mechanism is at play to generate this effect.

"There is no evidence I am aware of to suggest that infertility is more common in the younger siblings of older brothers."



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