For most of human history, the average human life span wasn't great. But we've made some extraordinary leaps lately that make an average span of 100 seem less like science fiction and more like an inevitability: A little more than a century ago in the United States, the average life expectancy was 49.24. In 2012 it was 78.8, a record high [source: Arias].
If our biology fixes a maximum life span for humans, we haven't hit it. But most of our progress hasn't resulted from adults' improved behavior or even medical advances. While many of us assume life expectancies were so much shorter before the 20th century because everyone was running around hitting one another with axes and contracting tuberculosis, the truth is life expectancy dramatically increases if you make childhood less dangerous. We've done a good job of that.
In 1900 in the United States, 165 infants died for every 1,000 born [source: PBS]. That was more than a 1 in 10 chance of dying before your first birthday, and all those zeros really dragged down the average life expectancy. Even today's highest infant mortality rate — 117.23 deaths for every 1,000 births in Afghanistan — is significantly lower. The U.S. rate is 6.17 deaths per 1,000, which is high for a developed country [source: CIA]. Also, if you grew up in the developed world, it is unlikely you'll die of a lung disease you got while working in a knife factory at age 8.
So dodging early dangers helps increase the average human life span. But other factors contribute, too. While most people might be tempted to credit medical advances (e.g., antibiotics, chemotherapy) for our longer lives, historians are more likely to credit public health efforts: clean water, hand-washing, better food-safety practices and better housing with occupants less susceptible to the spread of germs [source: Helmuth]. They all go a long way toward keeping us alive longer. And in our hypothetical 100-year-life span world, fewer people are making dangerous health decisions, or they're at least postponing their risky behavior until their 90s. People aren't smoking. People aren't driving drunk. People aren't eating nachos at every meal. Throw in moderate exercise and access to health care, and you have a good shot at lasting a century. Even now the average life expectancy in Monaco is a little higher than 89.
That sounds nice, right? Long, healthy lives? But what would the consequences of a 100-year life span be for human society?
Well, it might make us smarter. Most primates have relatively long juvenile periods, because it takes primates so long to learn social, language and other skills necessary for survival. Humans already have a longer juvenile period than other primates. But a life span of a century would allow us to extend that period culturally, which we've already been doing by implementing child labor laws and an education system that extends well after humans reach sexual maturity. Maybe we'd redefine childhood and become wiser adults by spending more time in the stage of development devoted solely to learning.
But would extended lives — more old people living longer, even as new people are born — doom us to overpopulation? Not necessarily. In fact, there seems to be a strong correlation between having a lot of old people around and having fewer babies. Hong Kong, for example, has one of the highest life expectancies, at 82.8 in 2014. It also has one of the lowest birth rates, with only 1.1 babies born for every woman. Generally, the rule of thumb to achieve a stable population is to average 2.1 babies for every woman. Among 20 nations with the world's longest life spans, just one – Israel – has a fertility rate greater than 2.1 babies per woman [source: World Bank]. In 2015, nearly half of the world's population lives in a country with subreplacement fertility — when a generation does not bear enough children to replace its population — and that was expected to increase to 82 percent by the end of the century. It seems we're safe on that front.
That doesn't mean a 100-year life span won't cause population problems, though, especially if it means fewer babies. Economies are driven by growth and rely on a steady supply of new workers. If birth rates decline for long enough, national economies will begin to stagnate and shrink. Compounding the problem, a large percentage of the population will spend a third of their lives as retirees. Even if the retirement age is raised to, say, 85 (which sounds impossibly sad), caring for the elderly will require more energy and resources.
The U.S. is already feeling the tension between a declining birth rate and a large aged population, as more and more Americans enter retirement. By and large, the government will bear the burden; providing for the elderly has been a goal of government programs such as Social Security since the Great Depression. That could necessitate tax increases in a faltering economy, an outcome that would excite few. However, a declining fertility rate is more difficult to deal with than a low fertility rate — with the latter, population growth will eventually stabilize. Governments and economies will adjust. And, as the oldest among us can testify, life will go on.