As the average human life grows longer, fewer people die at very young or very old ages.
By Nathan Collins
(Photo: Matthias Wiemann/Flickr)
Whereas kids were once lucky to live past 50, a baby born in the United States today is expected to live about three decades longer. More encouraging still, while the average life expectancy has grown, the range of typical lifespans has narrowed, according to a new study, though some inequalities remain.
“The conquest of early death through collective human efforts to avert mortality from disease and accidents has yielded lifespans that are both longer and more [quantitatively] equal in modern industrial humans than at any other time,” write Fernando Colchero and his colleagues this week in Proceedings of the National Academy of Sciences. Yet “considerable inequality in lifespans still exists in industrial human populations, in part because of disparities among socio-economic groups.”
Colchero and his team arrived at these conclusions based, in part, on data from six populations: two hunter-gatherer societies, the group of early 19th-century freed slaves who founded Liberia, 18th-century Swedes, modern Swedes, and modern Japanese. The distribution of lifespans for each of those groups followed the same basic pattern—there’s relatively high mortality among young children and an “old-age mortality hump,” reflecting the range of ages at which most people die.
That general pattern, however, conceals considerable variation over time and space. Modern Japanese life expectancy, for example, is over 80 years, with the bulk of individuals living between roughly 70 and 90 years. Meanwhile, the average Liberian lived to be just two years old, reflecting an extremely high infant mortality rate and a very wide range of lifespans among those who make it past their first years.
Such observations led the researchers to wonder whether there was a connection between life expectancy and what they called “lifespan equality”—roughly, the range of lifespans within a population. They found that to indeed be the case.
Colchero and his colleagues discovered lifespan equality to reliably increase alongside average lifespan in the six populations they began with. That held true for another 16 more populations surveyed in 2013, including the United States, India, Nigeria, and Russia; and more distressed populations, such as Ukraine during the country’s 1932–33 famine.
Despite finding regularities across a wide range of societies, there was one notable gap: Men died younger than women on average, and had less equal lifespans.
Still, the study’s real value to science may not reside specifically in those findings—which, the team argues, aren’t actually all that surprising given our enormous gains in reducing infant mortality over the last several centuries. In addition to humans, the team also looked at six non-human primates, including gorillas and lemurs, and found a weaker link between lifespan and lifespan inequality—hinting, the authors argue, that there’s something special about human social structures underlying the connection.