Baby boys are a quarter more likely to die in their first year of life

The analysis of centuries of birth and death records from 16 countries including England and Wales concluded that baby boys are 24 per cent more likely to die in their first year than baby girls.

Although this difference is down from a peak of 31 per cent in 1970, it is still more than double the rate it was in the 1750s, the U.S. study said.

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Writing in the journal Proceedings of the National Academy of Sciences, the researchers said baby boys are more vulnerable because their bigger size raises the risk of a difficult birth, they are more likely to be born prematurely and they also have weaker immune systems.

The study of three centuries of birth and death data in countries spanning three continents clearly showed boys to be more vulnerable in the early months of life than girls.

In the 1750s, baby boys were ten per cent more likely to die than girls, but by the 1970s the gap had widened to over 30 per cent, despite major advances in public health.

The University of Southern California researchers said that while both sexes had benefited from modern healthcare, girls had benefited more than boys.

Before 1950 poor hygiene and nutrition weakened all babies and mothers, making the gender gap less visible because death rates were high for both girls and boys.

But by the 1970s vaccination, antibiotics and better hygiene had cut deaths from infection, which made birth complications and premature babies the leading cause of death, and these potentially fatal problems are more common in baby boys.

Since then improved treatment of premature babies and increasing use of Caesarean sections for risky births have narrowed the gap to 24 per cent in 2000.

Nature also works against the trend of male infant mortality by selecting more males. In western nations 105 boys are born for every 100 girls.

However, stress can affect the gender ratio, with studies showing fewer boys are born after stressful events, for example in New York after the September 11 attacks, or to mothers with stressful lifestyles.

It has been suggested that higher levels of stress hormones may make it more difficult for male embryos to implant in the womb, or somehow increase the likelihood that male foetuses are miscarried.