All over the world, there is increasing interest in a simple idea for the regulation of risk: In case of doubt, follow the Precautionary Principle. Avoid steps that will create a risk of harm. Until safety is established, be cautious; do not require unambiguous evidence. In a catchphrase: Better safe than sorry. In ordinary life, pleas of this kind seem quite sensible, indeed a part of ordinary human rationality. People buy smoke alarms and insurance. They wear seatbelts and motorcycle helmets, even if they are unlikely to be involved in an accident. Shouldn't the same approach be followed by rational regulators as well?

Many people, especially in Europe, seem to think so. In fact it has become standard to say that with respect to risks, Europe and the United States can be distinguished along a single axis: Europe accepts the Precautionary Principle, and the United States does not. On this view, Europeans attempt to build a “margin of safety” into public decisions, taking care to protect citizens against risks that cannot be established with certainty. By contrast, Americans are reluctant to take precautions, requiring clear evidence of harm in order to justify regulation. These claims seem plausible in light of the fact that the United States appears comparatively unconcerned about the risks associated with global warming and genetic modification of food; in those contexts, Europeans favor precautions, whereas Americans seem to require something akin to proof of danger.