A stock-in-trade of many writers is a scenario where a city gent, a stranger, enters a country pub and is received by the locals with a mixture of suspicion and hostility. It is a scenario that is taken from real life since the intrusion of strangers into an established group is usually resented. It is a special case of a much broader phenomenon not merely in the human, but also the animal world. When ants encounter a conspecific whose smell reveals that it comes from another nest, they will attack. In many animal species, the introduction of a stranger into an established group is liable to evoke aggression. More generally, animals, like people, do not merely distinguish their own group from “others” but respond more negatively to the latter.