In 1997, after the death of Princess Diana, the British popular press declared that Britain was having a "collective nervous breakdown". This paper considers whether it is possible to assert that a collective mental state could exist, such that it could have a "breakdown" (to use a non-technical term). In reality, the concept of the collective mental state has a long history, but it also has a well-known critique called Methodological Individualism, where thoughts and actions can only be attributed to individuals. If the concept of the collective mental state is valid, then it is possible to argue that the normal functioning of a group, community, or whole society, that is, a collectivity, can be disrupted by contagion, and that it can be said to be disordered. Some of the common large-scale disorders that have been recognised are depression, fear, aggression, desensitization and acceptance of genocide. Thus, the concept of the collective mental state can provide a valuable transdisciplinary vehicle for the investigation of some of the great issues confronting humankind.