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The regulation of health and environmental risks has generated transatlantic controversy concerning precaution and the precautionary principle (PP). Conventional wisdom sees the European Union endorsing the PP and proactively regulating uncertain risks, while the United States opposes the PP and waits for evidence of harm before regulating. Without favouring either approach, this paper critically analyses the conventional depiction of transatlantic divergence. First, it reviews several different versions of the PP and their different implications. Second, it broadens the transatlantic comparison of precaution beyond the typical focus on single-risk examples, such as genetically modified foods. Through case studies, including hormones in beef and milk production and mad cow disease in beef and in blood donations, as well as reference to a wider array of risks, the paper demonstrates that relative precaution varies enormously. Sometimes the EU is more precautionary than the US (such as regarding hormones in beef), while sometimes the US is more precautionary than the EU (such as regarding mad cow disease in blood). Thus, neither the EU nor the US can claim to be categorically 'more precautionary' than the other. The real pattern is complex and risk-specifc. Third, the paper seeks explanations for this complex pattern in five sets of hypotheses: optimal tailoring on the merits, political systems, risk perceptions, trade protectionism, and legal systems. None of these hypotheses fully explains the observed complex pattern of relative transatlantic precaution. The paper concludes that differences in relative precaution depend more on the context of the particular risk than on broad differences in national regulatory regimes.

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