The terrorist (I'm not using names here) who rammed his car into pedestrians on Westminster Bridge and ran it through the fence around the Houses of Parliament turns out to be a son of immigrants and was born in England: a second-generation terrorist. He's not the only one: second-generation terrorists include, according to an interesting analysis by Stephen Dinan in the Washington Times, the June 2016 Orlando Pulse nightclub murderer, the son of immigrants from Pakistan, one of the December 2015 San Bernardino shooters, the son of immigrants from Pakistan; one of the attackers in a May 2015 Garland, Texas, Muhammad cartoon drawing contest, the son of immigrants from Pakistan; the November 2009 Fort Hood assailant (classified as a perpetrator of "workplace violence" by the Obama administration), the son of Palestinian immigrants; the two terrorist bombers at the April 2013 Boston Marathon bombing, the sons of asylum seekers from the Chechnya province of Russia.
Opponents of restrictions on numbers of immigrants, refugees and asylum seekers from predominantly Muslim countries frequently make the argument that such restrictions wouldn't have kept these second-generation terrorists out of the nations where they committed their terrorist acts. That's true. But it's also true that if such restrictions had been applied to their parents when they sought to immigrate or sought refugee or asylum status, the second-generation terrorists wouldn't be here either. This doesn't settle the debate over whether we should block entry to people from countries where Islamist terrorism is common or to people from such countries who cannot be vetted. You can make serious arguments on both sides. But the phenomenon of frequent second-generation terrorism is something to weigh in the balance.
And it does seem to be a phenomenon, not just a series of unrelated anecdotes. Dinan provides a thoughtful overview from former NSA and CIA Director Michael Hayden: "Historically, the 'high stress' generation for American immigrants has been second generation. Mom and Pop can rely on the culture of where they came from. Their grandchildren will be (more or less) thoroughly American. The generation in between, though, is anchored neither in the old or in the new. They often are searching for self or identity beyond self."
Those leaders who have declined to identify Islamist jihadism as a motivation for terrorism seem to be operating on two assumptions, (a) that the American (or British) people will retaliate with mass violence against perceived Muslims and (b) that seeming to blame Islam will antagonize Muslims here and abroad and motivate them to be terrorists. My guess is that the number of people moved to support terrorism in line with concern (b) is very small, but I'm not sure and will put that issue to the side. But I'm absolutely sure that concern (a) represents a view of the American people so factually wrong as to amount to group libel. It sees the American people as a dim beast easily provoked to hateful rage. I think it's quite obvious to the vast bulk of the American people that most recent acts of terrorism here and abroad are committed by Muslims and that most, the vast majority, of Muslims in this country and in the world do not commit or support the commission of such acts. It's actually not too hard to keep these two ideas in your head at the same time: it just requires modest powers of observation.
And I think the same common sense that allows most people to understand these two things also allows them to understand that while the first generation of Muslim immigrants, refugees and asylum seekers may not commit many acts of terrorism, their sons and daughters—the second generation—may do so more frequently. It's not clear what policy you might advocate in response: perhaps just stronger programs of assimilation than many in our current university and media elites find congenial. But it's something you may want to keep in mind.