In a laboratory in Philadelphia, zoologist Nigel Marven discovers that scientists have developed a new weapon of mass repulsion: a universally repellent smell. This is no joke shop stink bomb; the manufacturers are considering how it could be used for crowd control. And it's based on sound scientific principles, exploiting the protective distaste we have acquired for certain smells, over millions of years of evolution. It makes good sense, for example, not to enjoy the odour of rotten meat if you want to avoid food poisoning. Other antipathies, allegedly, are cultural: we are told that babies do not mind the scent of dirty nappies. In coming weeks, Marven will explore why sights, sounds and tastes have such powerful effects on us.