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Michael: According to the World Health Organization, a human male born in the USA in 2015 has a life expectancy of 76.9 years, while a female’s life expectancy is 81.6 years. Women tend to outlive men worldwide, and we’re really not sure why. There are a few theories, and they rely on social and biological differences to distinguish between biosex females and males – those who have XX or XY chromosomes. But there’s still a lot we don’t understand about this trend.

Now, the evidence isn’t new: for example, Sweden has collected demographic data since before 1800, and even then, women had a life expectancy of 33 years, while men had 31. Plus, it’s a global trend. And as scientists learn more about the human body and how we age, they’re revising their theories about why women seem to live longer than men. Some theories, from the 1970s or earlier, suggest that men are more likely to have work-related injuries and stress, which can lead to heart disease.

Or, that men tend to do more unhealthy things, like smoke cigarettes or drink heavily. But these social trends vary across cultures, and don’t explain everything – we’re working towards greater gender equality in workplaces, and women have unhealthy habits, too. So scientists think it might have more to do with biological differences.

It turns out that it’s kinda hard to study male and female longevity in animal models, because other mammals – like primates or mice – don’t always show the same trends as humans. So most of the theories just come from observing human populations, rather than controlled research studies. One possibility is that genes on the X chromosome may affect longevity, but they have so many other biological effects that it’s hard to tell. Another option is that since sex hormones are known to affect the immune system, they might influence longevity, too – female hormones might give some sort of protection against aging and DNA damage, while male hormones could give a disadvantage.

This is kinda supported by two small studies – one that looked at longevity records in 16th to 19th century Korean royal courts, and one that examined records from a 20th century U.S. mental hospital. They both found evidence that people who were born male and had their testicles removed – and therefore weren’t producing as much testosterone – had longer lifespans than other males. One weird puzzle, though, is that even though adult women live longer, they tend to have worse health as they age – especially more bone and joint issues.

So maybe those later-in-life health issues, combined with different hormones, give a sort of paradoxical advantage – the health problems might activate immune responses, which protect those joint tissues, and could ultimately lead to a longer life. Whatever the reason, this trend does seem to exist. But scientists still need to do a lot more research on people of all sexes and ages to completely solve this biological mystery.

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