An Oxford University-led international study found white people develop "passive tolerance" of minorities in mixed areas, even if they have no direct contact with them.
Professor Miles Hewstone, director of the Oxford Centre for the Study of Intergroup Conflict, said governments could create more "harmonious neighbourhoods" by doing more to encourage ethnic groups to mix.
"If two white people with identical views went to live in different postcodes for a year, the person in the neighbourhood with more mixing between ethnic groups would likely leave more tolerant," he said.
"We would see this effect even if they never personally spoke to people from other ethnicities. The size of this 'passive tolerance' effect on people's prejudice is of the same order as the effect of passive smoking on lung cancer risk."
The research, published in the Proceedings of the National Academy of Sciences journal, is based on studies carried out in England, mainland Europe, the United States and South Africa.
It found that even highly prejudiced people who did not mix with those of different ethnicities became more tolerant the longer they lived in mixed areas.
The scientists suggest seeing white strangers "interacting positively" with ethnic minorities is enough to reduce racial prejudice.
Prof Hewstone, the report's senior author, said: " "Governments should do more to encourage different groups to mix with each other, as we know that this reduces prejudice not just in individuals but throughout entire neighbourhoods."