Researcher: The role of chemicals called pheromones in determining the sexual behavior of some animals is well documented. But, even though humans also produce these chemicals, it is clear that psychological factors have taken over this role in us. Whereas for animals these behaviors are involuntary, which is a clear sign of chemical control, humans, by virtue of their free will, choose how they behave, and thus psychological factors take over. So pheromones are merely a vestige of our evolutionary past. The researcher's argument requires the assumption that ['free will can be found only in humans', 'whatever does not have a chemical explanation must have a purely psychological one', 'voluntary action cannot have a chemical explanation', 'voluntary action cannot have an evolutionary explanation']