The difference is there from the very moment a child is born.
‘When we were there on the neonatal unit and a boy came out, you were taught that, statistically, the boy is more likely to die,’ explains Joy Lawn. Besides her academic research into child health, she has worked in neonatal medicine in the United Kingdom and as a paediatrician in Ghana. The first month following birth is the time at which humans are at their greatest risk of death. Worldwide, a million babies die on the day of their birth every year. But if they receive exactly the same level of care, females are statistically less likely to die than males. Lawn’s research encompasses data from across the globe, giving the broadest picture possible of infant mortality. And having researched the issue in such depth, she concludes that boys are at around a 10 per cent greater risk than girls in that first month – and this is at least partly, if not wholly, for biological reasons.
Thus, in South Asia, as elsewhere in the world, the mortality figures should be in favour of girls. The fact that they’re not even equal, but are skewed in favour of boys, means that girls’ natural power to survive is being forcibly degraded by the societies they are born into. ‘If you have parity in your survival rates, it means you aren’t looking after girls,’ says Lawn. ‘The biological risk is against the boy, but the social risk is against the girl.’
Elsewhere, child mortality statistics bear this out. For every thousand live births in sub-Saharan Africa, ninety-eight boys compared with eighty-six girls die by the age of five. Research Lawn and her colleagues published in the journal Pediatric Research in 2013 confirmed that a boy is 14 per cent more likely to be born prematurely than a girl, and is more likely to suffer disabilities ranging from blindness and deafness to cerebral palsy when he’s at the same stage of prematurity as a girl. In the same journal in 2012 a team from King’s College London reported that male babies born very prematurely are more likely to stay longer in hospital, to die, or to suffer brain and breathing problems.
‘I always thought that it was physically mediated, because boys are slightly bigger, but I think it’s also biological susceptibility to injury,’ says Lawn. One explanation for more boys being born preterm is that mothers expecting boys are, for reasons unknown, more likely to have placental problems and high blood pressure. Research published by scientists from the University of Adelaide in the journal Molecular Human Reproduction in 2014 showed that newborn girls may be healthier on average because a mother’s placenta behaves differently depending on the sex of the baby. With female foetuses, the placenta does more to maintain the pregnancy and increase immunity against infections. Why this is, nobody understands. It could be because, before birth, the normal human sex ratio is slightly skewed towards boys. The difference after birth might simply be nature’s way of correcting the balance.
But the reasons could also be more complicated. After all, a baby girl’s natural survival edge stays with her throughout her entire life. Girls aren’t just born survivors, they grow up to be better survivors too.
‘Pretty much at every age, women seem to survive better than men,’ confirms Steven Austad, chair of the biology department at the University of Alabama at Birmingham, who is an international expert on ageing. He describes women as being more ‘robust’. It’s a phenomenon so clear and undeniable that some scientists believe understanding it may hold the key to human longevity.
At the turn of the millennium, Austad began to investigate exactly what it is that helps women outlive men at all stages of life. ‘I wondered if this is a recent phenomenon. Is this something that’s only true in industrialised countries in the twentieth century and twenty-first century?’ Digging through the Human Mortality Database, a collection of longevity records from around the world founded by German and American researchers in 2000, he was surprised to discover that the phenomenon really does transcend time and place.
The database now covers thirty-eight countries and regions. Austad’s favourite example is Sweden, which has kept some of the most thorough and reliable demographic data of any country. In 1800 life expectancy at birth in Sweden stood at thirty-three years for women and thirty-one for men. In 2015 it was around eighty-three for women and around seventy-nine for men. ‘Women are more robust than men. I think there’s little doubt about that,’ Austad says. ‘It was true in the eighteenth century in Sweden, and it’s true in the twenty-first century in Bangladesh, and in Europe, and in the US.’
I ask Austad whether women might be naturally outliving men for social reasons. It’s reasonable to think, for instance, that boys are generally handled more roughly than girls are. Or that more men than women take on risky jobs, such as construction and mining, which also expose them to toxic environments. And we know that in total across the world, far more men than women smoke, which dramatically pushes up mortality rates. But Austad is convinced that the difference is so pronounced, ubiquitous and timeless that it must mean there are features in a woman’s body that underlie the difference. ‘It’s hard for me to imagine that it is environmental, to tell you the truth,’ he says.
The picture of this survival advantage is starkest at the end of life. The Gerontology Research Group in the United States keeps a list online of all the people in the world that it has confirmed are living past the age of 110. I last checked the site in July 2016. Of all these ‘supercentenarians’ in their catalogue, just two were men. Forty-six were women.
Yet we don’t know why.
‘I’m absolutely puzzled by it,’ says Austad. ‘When I first started looking into this, I expected to find a huge literature on it, and I found virtually nothing. There’s a big literature on “Is this a difference between men and women?”, but the underlying biology of the survival difference, there’s very little on that. It’s one of the most robust features of human biology that we know about, and yet it’s had so little investigation.’
For more than a century, scientists have painstakingly studied our anatomy, even collected thousands of litres of horse urine in their attempts to isolate the chemicals that make men more masculine and women more feminine. Their search for sex differences has had no boundaries. But when it comes to why women might be more physically robust than men – why they are better survivors – research has been scarce. Even now, only scraps of work here and there point to answers.
‘It’s a basic fact of biology,’ observes Kathryn Sandberg, director of the Center for the Study of Sex Differences in Health, Aging and Disease at Georgetown University in Washington, DC, who has explored how much of a role disease has to play in why women survive. ‘Women live about five or six years longer than men across almost every society, and that’s been true for centuries. First of all, you have differences in the age of onset of disease. So, for example, cardiovascular disease occurs much earlier in men than women. The age of onset of hypertension, which is high blood pressure, also occurs much earlier in men than women. There’s also a sex difference in the rate of progression of disease. If you take chronic kidney disease, the rate of progression is more rapid in men than in women.’ Even in laboratory studies on animals, including mice and dogs, females have done better than males, she adds.
By picking through the data, researchers like her, Joy Lawn and Steven Austad have come to understand just how widespread these gaps are. ‘I assumed that these sex differences were just a product of modern Westernised society, or largely driven by the differences in cardiovascular diseases,’ says Austad. ‘Once I started investigating, I found that women had resistance to almost all the major causes of death.’ One of his papers shows that in the United States in 2010, women died at lower rates than men from twelve of the fifteen most common causes of death, including cancer and heart disease, when adjusted for age. Of the three exceptions, their likelihood of dying from Parkinson’s or stroke was about the same. And they were more likely than men to die of Alzheimer’s Disease.
When it comes to fighting off infections from viruses and bacteria, women also seem to be tougher. ‘If there’s a really bad infection, they survive