Highly Cited: Turning Thoughts on Aging and Mortality on End

September 07, 2018

Evolutionary biologist James Curtsinger’s finding about mortality rates continues to influence research on mortality and population heterogeneity.

James Curtsinger

Editor’s note: In this new series we’ll showcase highly cited research by College of Biological Sciences faculty.

For decades, if not centuries, demographers believed that the risk of dying goes up as a person gets older. But College of Biological Sciences faculty member James Curtsinger had his doubts. In the late 1990s an editor of the journal Science heard a lecture of his and invited him to submit a review paper on the topic. Assembling a diverse team that included a demographer, an entomologist, several statisticians, and CBS colleague Aziz Arabkhazaeli, Curtsinger took a closer look at population data from Chinese and Swedish population records as well as from Drosophila, medflies, and nematodes, and discovered that what we thought was true might not be. The resulting paper, published in Science in 1998, “Biodemographic Trajectories of Longevity,” has been cited more than 1,000 times.

In a nutshell, what did this paper show?
In the 1980s the view of old age in humans and experimental organisms was that the risk of death goes up and just keeps going up as the animal gets older. There was this feeling, “old age, nothing you can do about that.” In fact, there were very specific predictions that in future society almost everybody would survive to about 85, and then, over a brief period, they would collapse. What this paper did was summarize some 10 years of research and present some new data using very large datasets that challenged that and led to a pretty big change in the way people think about old age. We found that at some point mortality rates level off. The risk of death isn’t increasing anymore as a result of getting older.

What was the reaction?
The immediate response in Science was that there was some kind of error. Then, gradually people came around. It went through the classic cycle of, “That can’t possibly be true” to “Wait a minute, maybe it works” and then after that, “Oh well, we all knew that anyway.”

What’s your explanation for your findings?
In humans, we see lots of variation in frailty and lots of death before people get to their 90s. That means the people who make it to their 90s are selected in a certain sense as the most vigorous individuals. So probably the reason we see leveling mortality rates and very healthy nonagenarians is that they are a very select group. In retrospect it makes a lot of sense. But that was not previously recognized.

What new lines of research did this catalyze?
There’s now a whole lot of interest in studying the well-being and the mortality survival prospects of old people and in collecting very accurate data on the oldest individuals.

Who is citing this paper now?
People in evolutionary biology who think about evolution of senescence. It’s a really interesting problem from an evolutionary point of view. You think of evolution as producing adaptations --but if that’s the case, why do we end up with organisms that fall apart in old age? Anytime somebody thinks of that, this subject of mortality plateaus comes up. Also, within demography, there’s a general interest in the subject of population heterogeneity: To what extent do the differences between individuals influence the dynamics that you see in the population? In both of those areas the paper continues to be cited.

How has your own research evolved?
The main thing that’s happened since this paper for me is that we’ve started to think about fecundity at the same time we’re thinking about survival. That makes it an order of magnitude more complicated. I now understand that the mortality plateau we see in Drosophila is fundamentally different from humans. In Drosophila, it’s now clear in individual females reach a point where they’re mostly done with reproduction, then they experience this flat mortality.  In humans, most studies tell you there’s no direct connection. I’m working with [co-author] Jim Carey to answer that question to see if medflies retire too. I think they do.

Do you think the interdisciplinary nature of this work contributed to its success?
Absolutely. It was unusual to have both human data and data on experimental animals in the same paper, and the vitality of that research group had a lot to do with the fact that it was interdisciplinary. The guy who worked on medflies is an entomologist. The head of the project is a demographer. I’m in evolutionary genetics. There were other people in it who were from statistics. I remember my father telling me -- he was an English professor -- if you go into science, the really exciting stuff is between fields. He was right.

Was there something special about the University of Minnesota that made this kind of interdisciplinary high-impact research possible?
This is a place that fosters interdisciplinary work. We’re big, we’re diverse, we’ve got one of anything you can think of, and we have the freedom to pursue our intellectual interests. All of that contributed.