As the fires die out in the troubled suburbs surrounding Stockholm, arguments about the root causes of last week's riots are running rampant. Sweden prides itself on being the most tolerant, inclusive country in the world. So what impelled hundreds of angry young men to run amok, torching cars and setting fire to police stations and even their own schools?

A lot of people were baffled. "We don't know why they are doing this," said a spokesman for the Stockholm police. "There is no answer to it."

Many observers, and most Swedish academics and politicians, deny the riots had anything to do with the country's immigration policy. "This is not a question about immigration, it is a class question," one Social Democratic politician said in parliament. Others blame a lack of jobs, social exclusion, bad schools, poor living conditions, growing inequality, police brutality and racism, and the erosion of the welfare state. "Sweden has become a neo-liberal experiment," Arne Johansson, the head of a social justice group, told The Christian Science Monitor, "and class differences have increased as the wealthy receive more and more subsidies and tax cuts."

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There is an obvious explanation for the recent unrest – but in Sweden, it's almost impossible to discuss. The country has opened its doors to a flood of people from some of the most troubled parts of the world – especially the Muslim-majority countries of Syria, Afghanistan, Somalia and Iraq. Hundreds of thousands have arrived in the past decade alone. Many will not be able to succeed in Swedish society. They are destined to become permanent wards of the state.

Last year, tiny Sweden admitted 103,000 new immigrants. (Canada, with 31/2 times the population, took in almost 260,000.) Today, 15 per cent of its population is foreign-born. That's below the ratio of foreign-born Canadians, but the demographics are much different.

Unlike Canada, Sweden doesn't select for skills, education or the potential to succeed. It has no quotas. Most of its recent immigrants are refugees, family-class relatives and spouses in arranged marriages. Many have few skills and are barely literate. Some come from nomadic cultures with a different sense of time. Yet there are hardly any jobs in Sweden for people with no skills.

The Swedes are extremely progressive. Underlying their open-hearted immigration policy is a broad humanitarian desire to do good, combined with the belief that the right kind of social policies – education, welfare, job training – can transform newcomers into Swedes in short order. After all, it worked with the Chileans.

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But semi-literate people from the tribal cultures of the Middle East or Africa are not the same as the Western middle-class Chileans who fled Pinochet. The culture clash extends from the importance of religion, the rights of women and the proper way to raise children to the benefits of exercise. (Swedes are fitness nuts.) To put it mildly, assimilation is a challenge.

The newest Swedes have settled in suburbs that sound like names from the Ikea catalogue: Tensta, Rinkeby, Husby. These suburbs are not slums. They have expansive parks and recreation centres and free Swedish-language classes. The schools get extra subsidies. Welfare benefits are generous. But these neighbourhoods have become welfare traps. The schools are almost entirely segregated. Graduation rates are low (three-quarters of Somali kids drop out of school, according to the Economist) and unemployment rates are high. Central Stockholm is a short ride away by public transit, but it might as well be on Mars.

The newcomers are vastly better off than they were back home. But their kids don't care about back home. Here, most of them are stuck at the bottom of the social heap. Some of them are angry, and take it out on society. They stone their own fire fighters and burn their own schools. The authorities are not inclined to be too harsh. "Our ambition is really to do as little as possible," Stockholm's police chief told the Swedish newspaper Expressen, during the recent riots.

As riots go, these were rather small. A few dozen cars were burned. Nobody was shot. Still, they are a sign that something has gone very wrong in Sweden. Yet frank discussions of the country's immigration problems are virtually taboo. Anyone who brings them up is likely to be labelled as a xenophobic racist. When the immigration minister, Tobias Billström, mildly suggested that "we need to discuss the volume" of immigration, his own party nearly disowned him.

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What accounts for this excruciating excess of political correctness?

The best explanation I have heard comes from Jonathan Friedman, an American anthropologist who is married to a Swedish woman, and lived in Sweden for several years before moving back to California. He blames a "politics of submission by Swedish elites." Continued large-scale immigration, he told me in an e-mail, is untenable in a situation of economic decline. But Sweden's elite "refuses to see what is really happening and instead holds on to absurd ideologies of immigration as enrichment."

In other words, such outbreaks are bound to happen. And they are bound to create big cracks in Sweden's famous tradition of social cohesion. As Swedes redistribute more and more of their wealth to people whose habits are culturally alien, and who are permanently dependent on the state, the immigration consensus is bound to crack.

We love to envy Sweden. But really, it's Sweden that should envy us.