Abstract: Because risks are all on sides of social situations, it is not possible to be globally "precautionary." Hence the Precautionary Principle runs into serious conceptual difficulties; any precautions will themselves create hazards of one or another kind. When the principle gives guidance, it is often because of the availability heuristic, which can make some risks stand out as particularly salient, whatever their actual magnitude. The same heuristic helps to explain differences across groups, cultures, and even nations in the perception of risks, especially when linked with such social processes as cascades and group polarization. One difficulty here is that what is available is sometimes a result of predispositions, cultural and otherwise. There are complex links among availability, social processes for the spreading of information, and predispositions.