Women may have the reputation of being the fairer sex, but when it comes to life expectancy, we are also the stronger. In almost all modern populations, women can expect to outlive men – in some cases, by as much as five or six years.

This is despite the fact that women experience higher stress, are more likely to suffer a chronic disease, depression, and anxiety, and are more often victims of violence, according to the World Economic Forum.

But what about in times of crisis? While there has been a great deal of research into the gender gap, more generally, there is little science looking at what happens during times of high mortality. Think famine, disease outbreak, and slavery. 

New research, published in the journal Proceedings of the National Academy of Sciences, reveals that even in times of high stress, females have a higher life expectancy than males. A team of scientists led by the University of Southern Demark analyzed demographic data from seven periods of intense hardship and found that in virtually all populations and age groups women were the better survivors. 

There were just two exceptions to this rule: plantation slaves in 19th-century Trinidad and freed Liberian slaves, again in the early 19th century.

For the former, the researchers trawled through the death rates and birth rates of slaves between 1813 and 1816, finding that life expectancy may have been as low as 15.18 years for men and 13.21 years for women. They suggest this anomaly comes down to slave owners and the higher premium they placed on male slaves.

In the case of the latter, life expectancy was higher for male freedmen aged 35 to 49. Yet, in all other age groups, including infancy, women outlived men. Overall, 43 percent of ex-slaves who were encouraged by the US government to migrate to Liberia died within their first year in Africa because their immune systems were exposed to new diseases. Life expectancy at birth was just 1.68 years for boys and 2.23 years for girls.

In each of the remaining high-mortality populations – the Ukrainian famine of 1933, the Swedish famine of 1772-73, the Icelandic measle epidemics of 1846 and 1882, and the Irish famine of 1845-1849 – women consistently out-survived men, providing further evidence that there is more than environmental or social factors to blame for the gender gap.

Traditionally, women's higher life expectancy is the result of a healthier and less violent lifestyle. Men are more likely to be heavy smokers and drinkers, for example. But as lifestyle differences shrink, the gender gap persists, strengthening the case for a biological explanation. Research examining life expectancy in Mormon populations and cloistered monks and nuns, where men and women share similar lifestyles but women still outlive men, again suggests there is something biological at play.