"Since 9/11, there have been over 2,300 arrests connected to Islamist terrorism in Europe in contrast to about 60 in the United States." Thus writes Marc Sageman in his influential new book, Leaderless Jihad: Terror Networks in the Twenty-First Century (University of Pennsylvania Press).

This one statistical comparison inspires Sageman, in a chapter he calls "The Atlantic Divide," to draw sweeping conclusions about the superior circumstances of American Muslims. "The rate of arrests on terrorism charges per capita among Muslims is six times higher in Europe than in the United States." The reason for this discrepancy, he argues, "lies in the differences in the extent to which these respective Muslim communities are radicalized." He praises "American cultural exceptionalism," admonishes European governments "to avoid committing mistakes that risk the loss of good will in the Muslim community," and urges Europeans to learn from Americans.

Sageman's argument rehashes what Spencer Ackerman wrote in a New Republic cover story of late 2005, when he found that "Europe's growing Muslim culture of alienation, marginalization, and jihad isn't taking root" in the United States.

Marc Sageman, Leaderless Jihad: Terror Networks in the Twenty-First Century (University of Pennsylvania Press).

In fact, Sageman's numbers are scandalously inaccurate.

European arrests: His European number is inflated. The European Police Office (Europol) issued statistics showing that in 2007, 201 Islamists were detained in the European Union (other than the United Kingdom) on terror-related charges, compared to 257 in 2006. Earlier Europol statistics are less clear, but a close review of the evidence conducted for me by Jonathan Gelbart of Stanford University shows 234 arrests made in 2005, 124 in 2004. and 137 in 2003. In all, the total West European terrorism-related arrests appear to number less than 1,400.

U.S. arrests: According to the U.S. Department of Justice, Sageman's American figure is too low by a factor of almost ten. Department spokesman Sean Boyd indicated, according to a Fox News report, that "527 defendants have been charged in terrorism or terrorism-related cases arising from investigations primarily conducted after Sept. 11. Those cases have resulted in 319 convictions, with an additional 176 cases pending in court." Plus, as I documented at "Denying [Islamist] Terrorism" and its follow-up blog), politicians, law enforcement personnel, and the media are loathe to acknowledge terrorist incidents, so the real number of terrorism-related arrests is substantially higher.

Given that the Muslim population in the United States is about 1/7^th size of its West European counterpart (3 million vs. 21 million), using the figures of 527 arrests for the United States and 1,400 for Europe suggests that the Muslim per-capita arrest rate on terrorism-related charges in the United States is 2.5 times higher than in Europe, not, as Sageman asserts, 6 times lower. In fact, Sageman (who was offered a chance to reply to this article but declined) is off by a factor of about 15.

His error has major implications. If the United States, despite the much better socio-economic standing of its Muslims, suffers from 2.5 times more terrorism per capita than does Europe, socio-economic improvements are unlikely to solve Europe's problems.

This conclusion fits into a larger argument that Islamism has little to do with economic or other stresses. Put differently, ideas matter more than personal circumstances. As I put it in 2002, "The factors that cause militant Islam to decline or flourish appear to have more to do with issues of identity than with economics." Whoever accepts the Islamist (or communist or fascist) worldview, whether rich or poor, young or old, male or female, also accepts the ideological infrastructure that potentially leads to violence, including terrorism.

In policy terms, Americans have no reason to be smug. Yes, Europeans should indeed learn from the United States how better to integrate their Muslim population, but they should not expect that doing so will also diminish their terrorism problem. It could, indeed, even worsen.