Why do women tend to outlive men?

In the United States, the average life expectancy for women is 81 years, according to the Centers for Disease Control and Prevention (CDC). For men, it's 76 years. Around the world, women live longer, on average. So why do women tend to outlive men?

Two of the main causes are biological, said Virginia Zarulli, an associate professor of demography at the University of Southern Denmark. The first cause relates to differences in sex hormones, at least in cisgender people (people whose gender identity matches the biological sex they were assigned at birth). Cisgender women produce more estrogen and less testosterone than cisgender men do. Estrogen provides protection against a range of diseases, such as cardiovascular disease, according to a 2017 study in the journal Biology of Sex Differences. 

High levels of testosterone, on the other hand, have been linked to an increased risk of some diseases, such as endometrial and breast cancer in women and prostate cancer in men, according to a 2020 study in the journal Nature Medicine. Testosterone has also been linked to risky behavior and higher levels of aggression, which can increase the risk of dying at a younger age, Zarulli said.

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There's also a genetic component at play. Humans have two sex chromosomes: X and Y. Cisgender women have two X chromosomes, and cis men have an X and a Y. "If you think about that, the Y chromosome is an X chromosome with a missing leg. It's missing genetic material," Zarulli said. "Women have this double X chromosome — extra genetic material — which allows them to, for instance, have a backup plan if there is a bad mutation on one of the two X chromosomes. The other X can let them live anyway." This is the case for diseases such as hemophilia, a type of bleeding disorder, and Duchenne muscular dystrophy, which causes the muscles to progressively weaken.

This biological advantage gives women, on average, just under a year of longer life expectancy when they are young adults compared with men, according to a 2003 study published in the journal Population and Development Review on more than 11,000 Bavarian Catholic nuns and monks who lived between 1890 and 1995. In strict religious settings, men and women have similar lifestyles, and both sexes avoid risky behaviors; therefore, their difference in longevity is probably biological, Zarulli said. However, the study doesn't report on life expectancy from birth but from young adulthood, so the difference in total life expectancy is probably more. Zarulli said that biology gives women about two additional years of life, on average.

Additionally, when infants are in settings with particularly high mortality rates, such as during severe famines and epidemics and when they are enslaved, baby girls have higher survival rates than baby boys, according to a 2018 study led by Zarulli and published in the journal Proceedings of the National Academy of Sciences.

But, on average, women live four or five more years than men, Zarulli said. So what accounts for the rest of their survival advantage?

Social factors play a large role, she said. Men tend to smoke cigarettes and drink alcohol more often than women; men are nearly twice as likely to binge drink and are more likely to have had alcohol in the past 30 days, according to the CDC, and 35% of men in the world smoke compared with 6% of women, according to data from the World Bank. Women are more likely to trust in healthy nutrition and men are more likely to prefer fatty meals and eat fast food, according to a 2020 review study in the journal Advances in Clinical and Experimental Medicine. And women are 33% more likely to visit a doctor, excluding pregnancy-related care, than men are, according to a 2001 study by the CDC.

But it's impossible to completely tease apart biology and social effects to explain phenomena such as why men engage in riskier behaviors. "Both tend to influence the sex gap in life expectancy," Zarulli said. The interaction between the two is "impossible to divide," she said.

The life expectancy gap hasn't always been as wide as it is now. Detailed mortality records show that women didn't consistently live longer than men until about the start of the 20th century, according to a report from the National Bureau of Economic Research. Before then, infectious diseases ran rampant and hit both sexes fairly equally. In addition, women often died during childbirth.

Since then, women's life expectancy hasn't always risen as much as it could. Starting in the mid-1970s, the gap between potential and observed life expectancy started to increase for women due to cigarette smoking, according to a 2011 report by the National Research Council. By 2005, women were living, on average, 2.3 years less than expected because so many women had started smoking.

Originally published on Live Science.