A “risk factor” is generally defined as a characteristic of an individual that is associated with the subsequent development of a disease. Establishing a causal link between a putative risk factor and a disease is easy if a single factor is associated with the disease and if its removal is curative; myxedema is caused by deficiency of thyroxine, and its supplementation is curative. Sometimes a causal agent, for example, the tubercle bacillus, needs the appropriate circumstances to become clinically manifest. But the atherosclerosis underlying heart attack and stroke is associated with multitudinous factors. Many of these are markers of a developed or “Western” lifestyle, but only some are “causal” in the sense that the likelihood of causality is sufficient to justify a clinical or public health intervention. Defining the criteria for causality and judging when the benefits of intervention outweigh adverse effects has challenged basic science, epidemiology, and clinical and public health medicine for the past century...