Evolutionary psychology is a relatively new field. Scientists like Victor Johnston study the human brain and human behaviors -- why we do the things we do -- in the context of evolution. This video segment outlines the "sweaty T-shirt" experiment, which showed that the sense of smell may have more to do with mate choice than previously thought. Females sniffing the T-shirts recently worn by males favored the scent of those whose immune response genes were different from their own. Meredith Small and Geoffrey Miller are also interviewed. From Evolution: Why Sex?