Last updated January 18, 2018 at 9:13 am
A study of crises of different types across the ages show that women are the life expectancy champions. The same is true for other animals where females routinely live longer than males.
The highest mortality ever registered in recorded human history took place when returned American slaves returned to Africa in the 19th century. Between 1820 and 1843, freed slaves from America were encouraged to return to Africa.
Many of those that undertook this dangerous and risky trip returned to Liberia, but their arrival was a “mortality shock”. Exposed to a raft of new diseases they had not encountered before, about 43% died in their first year of arrival, and life expectancy at birth was just 20 months for men and a little over two years for women.
This crisis is just one of several analysed in a new study released today asking why women live longer than men.
“Women are the life expectancy champions: they can expect to live longer than men almost anywhere in the world today”, writes Virgina Zarulli from the University of Southern Denmark. Researchers wanted to find out whether this advantage holds in situations of crises like famine, slavery or epidemics.
Newborn girls survive extreme events better than boys
By analysing population records from several tragic parts of human history, the researchers found that even in extremely dire conditions, women outlive men. The researchers examined seven crises during which life expectancy dropped to 20 years or less, which might seem unrealistically low but has actually occurred several times in recent centuries.
Three periods of famine, including the Ukrainian famine of 1933 during which seven to 10 million people died, the Swedish famine of 1772-73 following crop failures due to poor weather and the Irish famine of 1845-49 caused when potato mold Phytophthora infestans devestated potato crops, the staple food of much of the population. The resulting crop failures over three years caused mass starvation.
Slavery, to no one’s surprise, is associated with high mortality, and one of the populations analysed included plantation slaves in Trinidad between 1813-16, where a detailed register exists. Other records included were from the Icelandic measles epidemics of 1846 and 1882, which were particularly devastating because measles is not endemic to the island. The populations there were especially vulnerable to infection when Danish boats visited and presumably spread the disease.
In all the populations, men had an equal or higher mortality than women across almost all ages. The key distinction was the type of stress that a population had been exposed to: in famines and epidemics, women survived at higher rates than men at all ages. In slave populations that are, or were, under human control to some extent, men could have higher life expectancy and lower mortality than females at some ages. The researchers suggest that this is because slave owners placed a premium on survival of young men over young women.
The biggest contributor to the gap between life expectancy of women over men in these conditions, which ranged from six months to almost four years, was the difference in survival of girl babies. Newborn girls were able to survive these extreme events better than newborn boys.
There are not many behavioural differences between newborn boys and girls, although there are some differences in attitudes of parents towards boys and girls which may effect how likely they are to survive, although this has previously shown a penalty for girls not an advantage.
Biological factors at play
This suggests that there are biological factors at play. There are hormonal and chromosomal differences which could explain some of the difference. Estrogens are known to have anti-inflammatory effects, and testosterone is had been associated with increased risk of dying from some diseases and having an immunosuppressive effect.
It’s not just humans that see females living longer, this effect is also seen across wild and captive populations of apes and monkeys. Males are more likely to be infected with parasites and have more severe infections that females, whether because of the immunosuppressive effects of testosterone, or because testosterone influences behaviour in a way that makes males more likely to get an infection.
Similarly in humans, the difference in risk-taking behaviours of men and women like drinking, smoking, driving less carefully and eating more poorly (all more prelevant in men) are known to contribute to the gap in life expectancy.
But even in populations where these behavioural differences might be minimised, like monks or nuns which have been studied, the survival of women is still higher. This new research looking at times of crises suggests that there are strong biological roots to this survival gap that are also influenced by social and environmental factors.