The precautionary principle has often been invoked to justify a ban on genetically modified (GM) crops. However, this justification is based upon a selective application of the principle to the potential public health and environmental benefits of such a ban, while ignoring a ban's potential downside. This is due principally to the fact that the precautionary principle itself provides no guidance on its application in situations where actions (such as a ban on GM crops) could simultaneously lead to uncertain benefits and uncertain costs to public health and the environment. Accordingly, a framework for applying the principle in cases where the final outcome is ambiguous because both costs and benefits are uncertain is developed. Then, based on a brief survey of the public health and environmental costs and benefits of GM crops, this framework is applied to the broad range of consequences of a ban on GM crops. This application of the framework indicates that by comparison with conventional crops, GM crops would increase the quantity and nutritional quality of food supplies. Accordingly, GM crops ensure that-despite the expected increases in human population-the world's progress in improving public health, reducing mortality rates, and increasing life expectancies during the twentieth century should be sustained into the twenty-first. Plant and animal genes have always been part and parcel of the human diet, and consumption of these genes has not modified human DNA. The public health benefits from GM crops, therefore, are likely to be larger in magnitude and more certain than the adverse public health effects from the ingestion of any genes that may be transferred from various organisms into GM crops. With respect to environmental effects, cultivation of GM, rather than conventional, crops would be more protective of biological diversity and nature. By increasing productivity, GM crops reduce the amount of land and water that would otherwise have to be converted to mankind's needs. Reductions in land conversion to agriculture would reduce soil erosion, conserve carbon stores and sinks, and improve water quality. GM crops also could help limit environmental damage by reducing reliance on synthetic fertilizers and pesticides, and increasing no-till cultivation, which would further reduce soil erosion, water pollution, and greenhouse gas emissions. A comprehensive application of the precautionary principle indicates that a GM crop ban, contrary to the claims of its advocates, would increase overall risks to public health and to the environment. Thus it would be more prudent to research, develop, and commercialize GM crops than to ban such crops, provided reasonable caution is exercised
