As some very good comments indicated, the causes of terrorism are far from well understood, and neither Posner nor I claimed we had anywhere near the full answers. Still, some things are reasonably clear.
To be sure, there have been non-Muslim terrorists in Japan, Germany, Italy, and even the United States, but they have been less coordinated across countries, of shorter duration, and much less lethal. For reasons given in some comments, Islamic terrorism cannot be linked simply to the invasions of Afghanistan or Iraq since a considerable degree of destructive terrorism preceded these events.
I do not believe that the main explanation for the more extensive Muslim terrorism in Europe is that the less ambitious Muslim immigrants go to Europe. That may be true on average, but I believe their ambition is more likely to be sapped by high unemployment rates, high welfare payments, and greater discrimination. As I noted, Britain is partly a counterexample to my thesis on the connection between terrorism and restricive employment and other regulations since unemployment rates are low there too as well as in the US, and payments to those on the dole are not generous in Britain. But it has long been recognized that Britain has accepted foreigners much less readily than the United States, and that foreigners there feel less integrated into mainstream society.
Some of you stressed, correctly I believe, that terrorism may be greater in Europe partly because Muslims are a larger fraction of the populations of France, Britain, Spain, and some other European nations than of the US. That could have encouraged greater discrimination against Muslims in Europe, and hence greater resentment. Larger relative numbers also encourages bigger enclaves of Muslim populations where radical clerics and others can more easily espouse their brand of hatred of the West.
I did not claim that America is stronger on all family values than Europe. I was objecting to the claim that the French and other European labor and other legislation that reduced working time improved family values. Surely, fertility should be one of the most sensitive indicators of family values, and also sensitive to the amount of leisure, yet fertility rates are generally much lower in all European nations than in the US. And the growth in property crime, and to a lesser extent in violent crime, in Europe while American crime rates were falling is partly related to the much higher unemployment rates in Europe, especially among their Muslim and other minorities.
The data still indicate that America is generally the first choice of immigrants, Muslim as well as others, although perhaps by a smaller margin than in the past. The change in where immigrants go is partly due to the fact that it has become more difficult to get into the US, so more Muslim and others go to Germany, France, Spain, and other European nations.
Someone made a relevant point about the distinction between hours spent not working and leisure time. It has long been known that many Swedes, for example, spend much time painting their houses, etc because of the high rates of taxes on additional earnings. But it is also true that vacations are much longer in most European nations than in the US, and Europe has more work holidays.
Switzerland is indeed another interesting example. It is difficult to become a citizen there and foreigners are isolated. However, unemployment rates of immigrants are much lower there than in other European countries, and that could help explain the low levels of Swiss terrorism. Still, comparisons of terrorist activities in Switzerland, Britain, Spain, France, and other European nations is an excellent research topic.
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