In February 2010, cleaners working at Dutch railway stations went on strike for several weeks. Their stations quickly fell to dirtiness and disarray, but most people didn’t mind; public support for the strike was high. But two scientists – Diederik Stapel and Siegwart Lindenberg from Tilburg University – were particularly delighted. In the growing chaos of the stations, they saw an opportunity to test an intriguing concept – that disorderly environments promote stereotypes and discrimination.

Their big idea is that stereotypes, being a set of simplified categories and judgements, can help people to cope with chaos. They are “a mental cleaning device in the face of disorder”. When our surroundings are full of chaos – be it dirt or uncertainty – we react by seeking order, structure and predictability. Stereotypes, for all their problems, satisfy that need.

To test that, the duo went to Utrecht station after it hadn’t been cleaned for a few days and asked 40 travellers to fill in a questionnaire. Their task was to say how much Dutch, Muslim and homosexual people conform to different personality traits. When the cleaners returned to work, and the station ...