Populations exhibit phenomena that are difficult to deduce from the characteristics of an isolated member. For example, counterintuitively, the proportion of elderly in a human population is more strongly affected by the population’s birthrate than by its life expectancy. Dr. Cohen studies groups of living beings and their interactions, including people, assassin bugs, parasites of Chagas disease in Argentina and prey and predators in a food web, to develop concepts helpful for understanding populations.
Dr. Cohen and his colleagues combine mathematical tools with observations of concrete problems in demography, epidemiology and ecology to understand populations better. His research spans studies of human population growth, infectious diseases and food webs.
A major challenge in the coming decades is to understand how demographic, economic and cultural changes will interact with Earth’s physical, chemical and biological environments. Dr. Cohen’s laboratory has analyzed the spatial distribution of Earth’s human population in relation to geophysical factors such as elevation, distance from coasts or navigable rivers, temperature and precipitation. As neutral experts for federal courts, Dr. Cohen and his colleagues developed and used methods to predict future claimants of asbestos-related diseases and injuries. Their methods may provide models for future mass-tort litigation as well as for long-term assessments of the impact of other environmental contaminants.
Dr. Cohen also studies Chagas disease, an insect-borne infectious disease that is a New World relative of African sleeping sickness. Chronic Chagas disease afflicts millions of people in Latin America and has no cure. Dr. Cohen collaborates with Argentine colleagues in a field study of the control of Chagas disease in rural northwest Argentina. They have developed a mathematical model of the risk of household transmission to humans, which enables the indigenous population to reduce risks of infection by better household management.
Because ecological communities strongly affect human wellbeing, Dr. Cohen’s research extends to nonhuman species as well. One approach focuses on a food web, a flowchart of who eats whom that describes the major pathways of food energy and of chemical and biological toxins. Dr. Cohen and his colleagues developed a new food web graph that plots species and feeding links in the plane spanned by species’ average body mass and numerical abundance. They analyzed unique data on soil food webs at 146 sites in the Netherlands to understand how environmental variables, human land uses and below-ground food webs interact.
Dr. Cohen’s laboratory also studies international migration in collaboration with colleagues at the United Nations Population Division. Newly developed mathematical and statistical models make it possible to account for more than half of the variability in the annual numbers of migrants among 229 countries or regions from 1960 to 2004. These models can be incorporated in deterministic or probabilistic population projections in combination with standard demographic techniques for projecting births and deaths.
The laboratory seeks to understand how stochasticity — random influences such as changes in weather or resources, or chance events of birth and death — creates novel patterns in nonlinear dynamics of population change. Dr. Cohen and his colleagues studied contained populations of flour beetles, cannibalistic insects that have long been used to study population dynamics. From time series of counts of the numbers of larvae, pupae and adults in jars, the researchers derived “power spectra” to compare the accuracy of predictions about the beetle population generated by their model with that of predictions from the linearization methods of physics, a more traditional means of understanding randomly perturbed dynamical systems. In most cases, predictions from their model were far more successful in describing the experimental data.
Dr. Cohen earned his B.A. from Harvard University in 1965. He holds two doctoral degrees from Harvard, a Ph.D. in applied mathematics (1970) and a doctorate of public health in population sciences and tropical public health (1973). He taught at Harvard from 1971 until his appointment as professor at Rockefeller in 1975.
Dr. Cohen was a John D. and Catherine T. MacArthur Foundation Fellow and is a member of the National Academy of Sciences. He was a member of the board of trustees of the Population Reference Bureau and a member of the board of directors of The Nature Conservancy and cochair of its Science Council. He shared the Tyler Prize for Environmental Achievement from the University of Southern California in 1999 and the 1997 Fred L. Soper Award for Excellence in Public Health Literature from the Pan American Health and Education Foundation for his work on Chagas disease.
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