We've had plenty of reasons to discuss decreasing birth rates and declining population trends recently, particularly among more westernized and industrialized nations. Several plausible explanations have been offered, including a reluctance among younger generations to take on the responsibilities of parenthood or simply adulthood in that fashion. Some societies such as China even mandated radically smaller families for a period of time, only to reverse those policies later. But even in countries without such restrictions, similar contractions were observed. Why is that? One new study from researchers at the University of Illinois-Chicago suggests that we're not actually seeing some results driven by social factors but by inherent physical limitations. They suggest that humans may be running into a type of evolutionary firewall, where humans are reaching our capacity to age past some preprogrammed plateau.  (Associated Press)

Humanity is hitting the upper limit of life expectancy, according to a new study.

Advances in medical technology and genetic research — not to mention larger numbers of people making it to age 100 — are not not translating into marked jumps in lifespan overall, according to researchers who found shrinking longevity increases in countries with the longest-living populations.

“We have to recognize there’s a limit” and perhaps reassess assumptions about when people should retire and how much money they’ll need to live out their lives, said S. Jay Olshansky, a University of Illinois-Chicago researcher who was lead author of the study published Monday by the journal Nature Aging.

The study focused on life expectancies in nations with the longest average life expectancy. These included Australia, France, Hong Kong, Italy, Japan, South Korea, Spain and Switzerland. They also tossed in the United States for good measure. All have advanced healthcare systems and high levels of medical technology. Yet even in the countries with the greatest life expectancy, the number of children has fallen well below the established replacement rate for each nation's population.

What explains this in countries where children come into the world under what appear to be ideal circumstances for them to thrive? As it turns out, that generation really isn't having that many fewer children these days. It's just that the elderly population is passing away too quickly to keep up with them. 

The researchers concluded that "there’s a limit to how long most people live, and we’ve about hit it." Understanding what sort of mechanism is causing this effect is the challenge, but there are no easy answers. At some point, the body simply appears to conclude that it has run into a barrier and there isn't much to be done about it. But does that really come as a surprise to anyone who has been studying these trends?

There was no doubt a reason that Leonidas' last words to Ephialtes on the battlefield were reportedly, "I hope you live forever." It was meant as a curse. Granted, that was rooted in the Spartan belief that one should return from battle carrying one's shield in victory or upon it as one of the fallen, but the reasons likely go deeper than that. There may be an evolutionary element to this line of thinking as well. Beyond a certain point, the species needs to clear out the dead wood (if you'll pardon the phrase) and make room for the next generation. The current math seems to back up that idea. The only areas still experiencing significant gains in average life expectancy are from African populations and technologically lagging remote groups. They live the shortest lives, but now they are catching up. The advanced societies with advanced technologies and longer lifespans are the ones dropping off of the map the most quickly. We may not grasp the underlying mechanism yet, but it does seem to make sense.