Living in a messy environment fosters racism and discrimination, according to psychologists.
Chaotic surroundings bring about a desire to restore order, leading people to resort to stereotyping as a "mental cleaning device", a study suggests.
Scientists tested the theory with a series of experiments, one of which took advantage of a strike by cleaners at Utrecht train station in the Netherlands.
The researchers asked 40 white, Caucasian travellers to sit anywhere in a row of chairs and complete a survey about stereotypes.
When the station was strewn with litter, participants put a greater distance between themselves and a black member of the team occupying the first chair than when it was clean.
In another experiment, the researchers carried out a street poll of 47 passers by who were also quizzed about stereotypes.
During the first round of interviews, tiles were removed from the pavement, a car parked on the kerb, and a bicycle abandoned in the street. On the second day, the street was cleaned to make it look neat and orderly.
The scientists found people discriminated against others more in their survey answers when the street was messy. They also donated less money to a fictitious charity collection for ethnic minorities.
Professor Diederik Stapel and Professor Siegwart Lindenberg, from Tilburg University in the Netherlands, wrote in the journal Science: "Our studies show that disorder increases the need for structure and, thus, the goal to create order. The study also shows that stereotyping is an effective mental way to reach this goal.. Stereotyping is a mental cleaning device that helps people to cope with physical chaos..
"Thus the message for policymakers is clear. One way to fight unwanted stereotyping and discrimination is to diagnose environmental disorder early and to intervene immediately by cleaning up and creating physical order. Signs of disorder such as broken windows, graffiti, and scattered litter will not only increase antisocial behaviour, they will also automatically lead to stereotyping and discrimination."