U.S. Department of Defense


Date of this Version



Annals of Global Health, VOL. 81, NO. 5 , 2015, September - October 2015 : 587 - 588.


Published by Elsevier Inc. on behalf of Icahn School of Medicine at Mount Sinai. This is an open access article under the CC BY-NC-ND license.


Much of what happens in our world is linear and predictable, like clockwork in some cases. In other areas, nonlinear and startling trajectories best describe the behavior of economies, political movements, or complex health systems. In global health, progress has been surprising and dramatic in some areas, but discouraging and slow in others. Its future might be more plodding slowly forward, or it might behave as other complex open systems do, in spectacular and nonlinear ways.

Many complex physical processes, such as chemistry “saturation” curves, the growth curve of populations, and improvements in national longevity follow a sigmoid (S-shaped) curve. Input in the early stages does not create much response, until reaching a tipping point and breaking into an exponential rise, when return on investment vastly outruns input. After the dramatic rise, output stabilizes and reaches a plateau, when saturation or carrying capacity is achieved, and additional input produces diminishing returns. I believe that 21stcentury growth in most global health indices will describe a sigmoid curve.

Samuel Preston demonstrated this behavior in his description of the relationship of longevity to per capita gross national product (GNP).1 At relatively low GNP values ($2000), average national longevity rises dramatically with small incremental increases in economic activity. Many of the developing world nations are poised today at the base of the steep up-sloping segment of Preston’s curve. At about $5000 GNP per capita, the remarkable longevity gains flatten out, and slowly inch up as GNP increases to about $20,000 per capita. Preston’s curve has been plotted for each decade since his original report in 1975, and the sigmoid shape of the curve has not changed. Swedish epidemiologist and YouTube star Hans Rosling tells us this story in four minutes in his unforgettable and energetic style.