RESEARCH DESIGNS THAT ADDRESS EVOLUTIONARY QUESTIONS ABOUT MEDICAL DISORDERS

Diseases result usually from webs of interacting causes of enormous complexity, while human minds seek explanatory principles of extraordinary simplicity. This conflict gives rise to a central problem for medicine. Explanations of disease, and most programmes of medical research, tend to emphasize a single cause, while most diseases result from multiple environmental factors interacting with several sources of vulnerability. An evolutionary approach fosters clear thinking about the complex origins of disease. It is, however, easy to underestimate the change in perspective that an evolutionary view of disease offers and requires. Many evolutionary research questions differ qualitatively from those usually pursued in medicine, and the routine application of traditional modes of medical inference may prove inadequate to the task of testing them. Addressing the formulation and testing of evolutionary hypotheses may seem like abstract philosophy, but it is essential if this enterprise is to succeed. New evolutionary perspectives on basic mechanisms are leading to substantial discoveries in established areas of medical research, such as virulence, senescence, and genetic variation, as documented by the chapters in this book. Equally important is the potential for integrating multiple causes of specific diseases provided by an evolutionary approach. The benefits of this approach will be delayed, however, if its initial applications are poorly done. It is easy to make up evolutionary stories about adaptation but harder to test them. This chapter outlines how evolutionary hypotheses about specific diseases can be formulated and tested. It is premature to promote any such system as definitive, and we hope that readers will sympathize with the difficulties of trying to impose some order, however preliminary and arbitrary, on an unruly tangle of questions. This chapter proceeds in two steps. First, we list several distinctions that are essential to defining the objects of explanation and the kinds of explanations proposed, and second, an outline of how hypotheses about vulnerability to diseases can be formulated and the kinds of evidence that can be used to test them. Examples are provided in other chapters; Chapter 23 applies this framework systematically to the major psychiatric disorders.