This week's revelations surrounding the UK terror plot targeting U.S. bound airliners once again focused attention on the phenomenon of "homegrown terrorism." As with last year's 7/7 "Underground Bombers," the Heathrow suspects are virtually all British residents, with most UK citizens and many second-generation Pakistani immigrants. And just as in the aftermath of last November's street riots in France, a flood of analysis seeks to explain the threat of radical Islamic extremism in Europe and its relative absence in the United States.

Commentators of all political stripes, however, are too quick to draw conclusions about the comparative dangers of radical Islam within European and American Muslim communities. Predictably, conservatives use terror plots in England, clashes in France, train bombings in Spain and cartoon outrage in Denmark to attack the economic stagnation and social rigidity of Europe, while lauding the opportunity and equality of American society. In turn, liberals see multi-culturalism, affirmative action and group politics as a safety valve that provides American minorities political expression, electoral muscle and social standing missing in Europe.

The reality is much more complicated than that, defying such facile comparisons and ready morality plays. Simply put, Muslim immigrants have come to Europe and the United States for very different reasons. Whereas the small but diverse American Muslim community came to the U.S. primarily to pursue economic opportunity and escape political oppression, across the Continent the legacy of European colonialism has helped produce large, monolithic and increasingly restive Islamic populations with a multi-generational sense of grievance. Those different motivations and distinct histories, and not the supposed goodness or badness of America or Europe, explain today's gulf in domestic terror threats on either side of the Atlantic.

But the relative diversity of American versus European Muslim populations may more essential to understanding the Continent's fertile breeding ground for homegrown terror. England, France, Germany, Holland and other former colonial powers are natural magnets for immigration from their former territories. Waves of immigrants from Morocco, Tunisia, Algeria and other nations in Francophone North Africa have come to France for generations. Pakistanis have been coming to the UK since before independence in 1947. (69% of all UK Muslims hail from the Indian subcontinent, according to a 2001 census.) Guest worker plans of the 1960's and 1970's - a neo-colonial program if you will - brought hundreds of thousands of Turks to Germany. The result is geographically concentrated, ethnically and religiously homogenous Muslim enclaves often mired at the bottom of societies run by their former colonial masters.

The differences with the United States could not be more stark. Economic opportunity, political oppression and religious freedom helped create different Muslim diasporas across the U.S. While the largest group of American Muslim immigrants from the South Asia (India, Pakistan and Bangladesh), the United States is also home to 600,000 Arabs and 300,000 Iranians, as well as African-American Muslims.

Their motivations for coming to the United States are just as diverse. Following the overthrow of the Shah, hundreds of thousands of Iranians flocked to Los Angeles and Southern California. After the Gulf War, Iraqis, Jordanians and Lebanese boosted the already growing Arab populations of Detroit and Cleveland. And the high-tech waves of the 1980's and 1990's brought Indians and Pakistanis to Silicon Valley, Route 128, and Austin. Arab and non-Arab Muslims flocking to American universities has helped expand, as Seth Stodder describes, "the commanding heights of our tech economy."

The economic prospects of American Muslims compared to their European counterparts are dramatically different as well. Generally less educated and less well off, many European Muslims find themselves trapped in the high-unemployment economies of countries like France and Germany. In contrast, one 1999 study estimated that 52% of recent Muslim immigrants to the United States possessed graduate degrees. As James Fallows describes in "Declaring Victory" in the September 2006 issue of The Atlantic (subscription required):

The median income of Muslims in France, Germany, and Britain is lower than that of people in those countries as a whole. The median income of Arab Americans (many of whom are Christians originally from Lebanon) is actually higher than the overall American one. So are their business-ownership rate and their possession of college and graduate degrees. The same is true of most other groups who have been here for several generations, a fact that in turn underscores the normality of the Arab and Muslim experience [in the United States].

Demographic trends in Europe may only make matters worse. Muslim immigration continues unabated into Europe, with 150,000 new arrivals each year into Germany alone. And as countries such as Italy struggle to reach even replacement birth rates, the rapid growth of young, disaffected and disenfranchised Muslim populations across Europe offers the prospect of greater discord and further unrest. Last year's riots in France following the deaths of two Arab youths and a ban on headscarves in French schools almost certainly won't be the last.

The impact of colonialism's legacy, economic disadvantage and steep barriers to social mobility are clear in the attitudes of Muslims in Europe. "If you ask a second-generation American Muslim," says Robert Leiken, author of Bearers of Global Jihad: Immigration and National Security After 9/11, "he will say, 'I'm an American and a Muslim.' A second-generation Turk in Germany is a Turk, and a French Moroccan doesn't know what he is." 27 year-old Moussa Abdel Aziz, one of 10 children of Moroccan factory workers outside of Antwerp, described his plight, "We are born with this stamp on our forehead that says 'foreigner' that will never go away. People are less surprised seeing E.T. than seeing us. We are the true aliens."

Recent opinion surveys support the anecdotal data. A July 2006 study of Muslims in Britain, France, Germany and Spain by the Pew Research Center found that unemployment topped their list of concerns, followed by Islamic extremism. (Not surprisingly, the British voiced the greatest worry over radical Islam in a follow survey in August.) A more starting statistic involved national and religious self-identification, as 81% of Muslims in the UK, 69% in Spain, 66% in Germany and 46% in France viewed themselves as Muslims first and citizens of their respective countries second. (The Pew findings were consistent with those of a recent Channel 4 documentary in the UK.) These levels of Muslim primary self-identifications exceeded levels in Jordan, Egypt, Turkey and Indonesia.

In this context, the apparently growing appeal of extremist Islamic ideology and terror on Europe's "Muslim street" seems less mysterious. The combination of colonialism, racism, economic disadvantage and political powerlessness is an explosive one. Unless European societies find the will and the means to act soon, the meltdown of Lebanon's consociational model in the 1970's and the ensuing civil war offers one particularly dark vision of the future.

None of this is to suggest that the United States will be immune from homegrown terrorism within its Muslim communities. Nor it is to suggest that the innate justice or fairness of American society will save the United States from plots hatched by American residents on its own soil. But the circumstances and history of Muslims in America are clearly different than in Europe. Notwithstanding terror-related arrests in Florida, New York, Michigan and California, former National Security Council official Daniel Benjamin believes American Muslims "have been our first line of defense" and "pretty much immune to the jihadist virus." According to Mark Sageman, "The patriotism of the American Muslim community has been grossly underreported."

Sadly, that patriotism may be under assault. An August 2006 Gallop poll finds that almost four in 10 Americans believe Muslims should carry a special ID card. 22% said they would not want to have a Muslim as a neighbor. 34% of respondents felt that American Muslims were sympathetic to Al Qaeda while only 49% thought they were loyal to the United States.

Perhaps Americans aren't so different from Europeans after all.