For the first time in history, women now outlive men all around the globe. The milestone is as significant as the eradication of smallpox 30 years ago but is unlikely to be celebrated with the same fanfare, researchers say.
In the West, women have been outliving men for more than a century. Deaths from cancer, heart disease, suicide, accidents and Aids are all higher among men. In England and Wales life expectancy for men last exceeded that for women in 1841.
But in the developing world, women have been held back by the high risk of bearing children.
The biggest killer of women of childbearing age in the developing world is childbirth.
Professor Danny Dorling, of the University of Sheffield, and colleagues writing in the British Medical Journal, say: "We will never know with certainty the exact year in which women everywhere can expect to live on average longer than men, but this year - 2006 - is as likely as any." The authors describe it as "a remarkable achievement".
Professor Dorling said: "The reason for the increase in female life expectancy has got to be improvements in care during pregnancy and the presence of a midwife or other skilled attendant at birth.
"There is still a long way to go but things have improved enough to put women ahead everywhere."
The explanation for the female advantage in life expectancy has taxed health experts for years. Sir Donald Acheson, the former chief medical officer, argued that it was due to hormones. Testosterone-fuelled young men died in accidents and violence while their less driven female peers lived quieter and safer lives, he said. Later in life, men died of heart disease from which women were protected by the female hormone, oestrogen.
But Professor Dorling warns in the BMJ that the gender gap is narrowing as women have become more emancipated and adopted lifestyles more like men. In England and Wales, the gap was widest in 1969 - with women living 6.3 years longer on average.
Smoking is the main reason why it is now narrowing again, which was taken up by women later than men. Deaths from smoking in men peaked in the late 1960s, reflecting smoking habits 20 or 30 years earlier, but were still rising in women until the late 1980s.
The authors warn this year's milestone may not be a permanent achievement. "The largest remaining untapped market for cigarettes is made up of women in poorer countries," they note.
Women versus men: the statistics
* In developed countries, average life expectancy is 79 years for women and 72 years for men.
* Among centenarians worldwide, women outnumber males nine to one.
* Zimbabwe is one of the few countries where men outlive women. The average life expectancy for men is 40 whereas for women it is only 38.
* The mortality gap is widest in Russia where females outlive men by an average of 13 years.
* Men are more likely to smoke and drink than women.
* Oestrogen in women combats heart disease by helping to reduce levels of harmful cholesterol.
* Men are four times more likely to die during the "testosterone storm" between the ages of 16-24. This is often as a result of reckless and/or violent behaviour, such as dangerous driving or fighting.
* About 125 men are conceived for every 100 women but problems that develop in the womb - cerebral palsy, premature birth, stillbirth, deformities - are more likely to afflict males.
* Women have stronger immune systems than men, which scientists think may be designed to combat the rigours of childbirth.
* Men are twice as likely as women to opt for unhealthy snacks during work, such as crisps.
* Of the 72 major causes of death, only five affect more women than men. These include breast cancer and pregnancy.
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