Researchers in the United States have revealed that baby boys are much more likely to die in the first year of life than baby girls.
Three centuries of records have revealed that in the 1750s baby boys were 10 per cent more likely to die than girls, but by the 1970s the gap had widened to over 30 per cent, regardless of major advances in public health.
According to researcher Dr. Eileen Crimmins in the past three decades, the gap has closed a bit, with boys this decade having roughly a 20 percent higher chance of death by age one than girls.
The researchers from University of Southern California reached this conclusion after analysing three hundred years of birth and death records from 16 countries, eleven in Europe, Canada, the United States, Japan and Australia.
They say though 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 researchers suggest baby boys are more vulnerable because of their bigger size which raises the risk of a difficult birth and they are also more likely to be born prematurely; they also have weaker immune systems.
The birth and death data clearly showed boys to be more vulnerable in the early months of life than girls.
The researchers say while both sexes have benefited from modern healthcare, girls had benefited more than boys.
Prior to 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.
The researchers say this made birth complications and premature babies the leading cause of death, and these potentially fatal problems are more common in baby boys.
The researchers say boys are 60 percent more likely than girls to be born prematurely and therefore to have conditions such as neonatal respiratory distress syndrome, which makes it difficult for a baby to breathe.
The improved treatment of premature babies and the increasing use of Caesarean sections for risky births have narrowed the gap to 24 per cent in 2000.
The researchers also say nature itself also works against the trend of male infant mortality by selecting more males - in western nations 105 boys are born for every 100 girls.
It also appears that stress can affect the gender ratio, with some research indicating that fewer boys are born after stressful events or to mothers with stressful lifestyles.
The suggestion is that higher levels of stress hormones may make it more difficult for male embryos to implant in the womb, or possibly increase the likelihood that male foetuses are miscarried.
The findings are published in the journal Proceedings of the National Academy of Sciences.