Hostile feelings towards minority groups can be contagious, say scientists who found that people are more likely to be influenced by the prejudiced behaviour of their peers.

Whether in Bosnia, Liberia, or Rwanda, violent conflicts have suddenly broken out between ethnic groups that have lived peacefully together for a long time.

So far, there is no satisfactory scientific explanation as to why aggression can repeatedly develop such a dynamic.

Researchers from Max Planck Institute for Tax Law and Public Finance in Germany, developed a novel experiment to test how social environment influences hostile behaviour towards other ethnic groups.

The study, published in the journal Proceedings of the National Academy of Sciences, examined adolescents from schools in eastern Slovakia and their behaviour towards members of the Roma people - a minority against whom there are substantial prejudices, and in recent years, have been increasing acts of aggression.

The unique element about this research project was that participants were able to act within their social environment.

To examine hostile behaviour, the researchers had the young people play a "Joy of Destruction game": a game in which the participants can live out their willingness towards destruction.

Two players receive two euros each and simultaneously choose whether to pay 20 cents to reduce their counterpart's income by one euro or simply keep the money unchanged. The players remain anonymous and play against each other only once.

The researchers used a list of typical names to inform the participants of whether their counterpart was a member of the Slovak majority population or the Roma minority.

In addition, the scientists designed the course of the game in such a way that three young people from the same school class made their decision shortly after each other. The following players knew the decisions of their classmates.

It turned out that the destructive behaviour of the peers had a significant influence on the players' choices. The willingness to also act aggressively grew significantly, researchers said.

It was striking that this influence more than doubled when hostility was directed against Roma rather than against one of their own social group.

In a second related experiment, the researchers examined the stability of social norms. They asked young people from the same region to assess whether the hostile behaviour shown by their peers in the first experiment was appropriate.

Here, too, it became clear that the social environment makes a significant contribution to whether an action is judged to be socially appropriate or not, researchers said.

In an environment without hostile peers, aggressive behaviour by players towards Roma or the subjects' own social group was rated negatively to a similar extent.

However, knowing that one player showed destructive behaviour after his or her classmates had been hostile to a Roma, they rated this behaviour as more appropriate.

The ratings of hostile behaviour directed against a member of their own social group are more stable - they depend less on whether the players had previously observed destructive behaviour in the environment.

"Our results suggest that fragile social norms can lead to a sudden change in individual behaviour towards other ethnic groups - from good coexistence to aggression," said Jana Cahlikova, a scientist at Max Planck.

It is therefore important to consistently prosecute and punish hate crimes. Researchers also point out that politics and society should react very sensitively when prejudices and hostilities against certain social groups increase.