Bearing a son takes so much out of a woman that subsequent children suffer from reduced fertility, a study has shown (Lewis Smith writes).

Fertility levels among siblings with an older brother were found to be significantly lower than among those who were preceded by sisters.

Researchers found that children with an older sister had 12 per cent more children themselves than those with an older brother. They were also 27 per cent more likely to bring up their own children to adulthood. The research was carried out by scientists from the University of Sheffield, who analysed detailed records of births and deaths in Finland between 1734 and 1861. Finns were used for the study because from the 17th century the Church in Finland kept detailed records of births, marriages, deaths and movements between parishes.