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The Drake Equation, Take Four In the best of all worlds, a revised equation would include the most mysteri- ous factors of all—the motivations and intentions of intelligent extrater- restrials. Brin may have come closest to making this a factor in a revised Drake equation through what he called contact cross section, a term derived from nuclear physics. This factor would include the probability of approach or avoidance, which may be different for each culture. We need to think more broadly and systematically about the motivations that would cause other civilizations to initiate or avoid contact, whether by signals or by interstellar vehicles. We would have to encompass the possi- bility that a technological civilization might take action against others. Sentient beings can make choices. We cannot assume that those choices will be the ones we prefer. Cultural values and policy decisions, as much as physical factors, may determine the fate of intelligence. Success in a search for others may depend on sustained curiosity; positive consequences of contact may depend on ethics. 324