Biology, Economics, and Models of Humanity's Future: What Have We Learned Since Malthus?

BIOLOGY, ECONOMICS, AND MODELS OF HUMANITY'S FUTURE: WHAT HAVE WE LEARNED SINCE MALTHUS? DONALD B. MARRON" Introduction Two hundred years have passed since Malthus first published his famous vision of humanity's future [I]. Oft quoted, and oft misunderstood, his essay was a pioneering effort to analyze human population growth in a world oflimited natural resources. While subsequent experience has invalidated many specifics ofMalthus' analysis, the issues he raised remain timely and controversial. The bicentennial of his essay thus seems a fruitful time to reflect. What have we learned since Malthus? A complete review of advances in the natural and social sciences would far exceed the limits of this essay (for an excellent overview, see [2]). For that reason, Iwill focus on a single question that lies at the heart ofMalthus' analysis and its modern counterparts: how can we combine biological and economic perspectives into a coherent model of humanity's future? Malthus recognized that the interplay of resources, population, and human behavior depends on both biological and economic factors. Much subsequent research has, inevitably, fallen within narrower disciplinary lines. While the growing field of ecological economics has begun to synthesize key insights across disciplinary boundaries [3], biologists and economists still analyze resource and population issues with too little regard for each other's work. My goal, then, is to compare contemporary biological and economic perspectives and to suggest how they might be reconciled. The key, I believe, is to understand the differing assumptions on which these sciences base Correspondence: 5108 Wilson Lane, Bethesda, MD 20814. e-mail: donald_marron@ yahoo.com. The author wishes to thank Alex Pfaff and Ted Steck for helpful comments and Kyoung Rim Lee for research assistance. * University of Chicago Graduate School of Business.© 1999 by The University of Chicago. All rights reserved. 0031-5982/99/4202-1089$01.00 Perspectives in Biology and Medicine, 42, 2 ¦ Winter 1999 195 their models of humanity. Biological models assume that humans behave like other species. In particular, they assume that humans behave myopically . As we will see, such models can easily yield pessimistic projections of humanity's future. Economic models, in contrast, emphasize peculiarly human traits: the potential for technological change, the ability to use foresight , and the incentives created by social institutions. Such models often yield more optimistic projections than do biological models, not because they ignore underlying biological forces, but because they assume that humans have sufficient ingenuity, foresight, and social organization to adapt rapidly to such forces. A coherent account of humanity's future must balance this potential for adaptation against the real imperfections of human foresight and institutions. Biological Models The basic assumption behind biological models of humanity is that natural laws should apply equally to humans and other species. Malthus provides an early example of this reasoning. He observed that, unchecked, animal and plant populations tend to grow exponentially. Since populations do not actually grow that fast, there must be natural limitations retarding their growth. There is no reason to suppose that "the physical laws to which [humans are] subjected should be essentially different from those which are observed to prevail in other parts of animated nature" [4, p.225], so similar tendencies and checks must act upon human populations as well. Building on Malthus' analysis, the burgeoning science of ecology has developed a sophisticated framework for understanding the growth tendencies of, and checks upon, natural populations (for a good introduction, see [5] ) . Researchers have developed a variety of mathematical models and empirical case studies that illuminate the population dynamics of different species. An essential finding is that populations display a range of qualitative behaviors. Some tend towards a constant equilibrium, others cycle regularly , and still others cycle irregularly, perhaps chaotically. To understand the basic methodology behind biological models ofpopulation dynamics, it is useful to examine a particular model in some detail. The logistic model, one of the oldest and simplest such models, analyzes population growth as a balance between two Malthusian forces: the natural tendency for exponential growth and the natural restraints that limit such growth. The logistic model formalizes these forces as an interaction between the current population size (N1) and two fixed parameters, the intrinsic growth rate of the...