Donald Trump speaks in Manchester, N.H., on Monday on the aftermath of the Orlando shooting. (Jim Cole/Associated Press)

Each week, In Theory takes on a big idea in the news and explores it from a range of perspectives. This week we’re talking about radicalization and “homegrown terrorism.” Need a primer? Catch up here.

Martha Crenshaw is a senior fellow at the Center for International Security and Cooperation at Stanford University.

What do we mean when we talk about “homegrown extremism” or “radicalization” in the United States? Donald Trump claims that the threat of “radical Islam” is imported by immigrants from abroad, from regions where there is a history of terrorism against us and our allies. He refers to “thousands upon thousands of people” entering the United States, “many of whom have the same thought process” as the Orlando shooter. He asserts that they are forming “large pockets” of people who want to “slaughter us.”

Actually, we don’t know the motivations of the Orlando shooter, and we probably never will. We certainly do not know what thought processes immigrants might bring with them as they travel from the “many more Muslim countries” Trump mentioned earlier this week (the short list — in addition to Afghanistan, Iraq and Syria — would include Indonesia, Pakistan, Saudi Arabia, Lebanon, Jordan, Algeria, Tunisia, Morocco, Egypt, Libya, Somalia, Mali, Niger and Nigeria). Many of these potential immigrants might be fleeing jihadist violence in their home countries.

What we do know in terms of facts is that only a small number of people who could conceivably be acting out of sympathy with the jihadist cause, presumably the thought process in question, have attempted to use terrorist violence here. Few of them succeeded. My research — based on public records — counts around 220 perpetrators or would-be perpetrators of what we could consider jihadist-oriented acts of terrorism against the U.S. homeland since 1993. My co-author Gary LaFree and I include this analysis in our book “Countering Terrorism: No Simple Solutions,” forthcoming from Brookings Institution Press.

This is the group of people who expressed agreement with the goals espoused by al-Qaeda, the Islamic State or their associates and intended to or did use violence against targets in the United States. The list is narrow: It does not include individuals who expressed support for al-Qaeda or the Islamic State but took no action, or who gave money to suspect causes, or who wanted to go to fight for the Islamic State in Syria or al-Shabab in Somalia. We shouldn’t lump together all variations of “homegrown extremism,” because there isn’t necessarily any overlap between these different types of support.

On the other hand, within the category of violent jihadists in the United States, the standards of inclusiveness were generous. Sometimes a perpetrator only alluded vaguely to a jihadist cause as explanation for his or her plans or actions. This overall figure of around 220 includes the 19 hijackers on 9/11, who were al-Qaeda agents sent from abroad, not immigrants. But apart from those, there are only around 200 people whose “radicalization” we should be concerned about, not thousands by any means. And in fact, the large majority of those who wanted to or did attack here over the past 23 years — including the Orlando shooter — are U.S. citizens, not immigrants. Trump’s idea of banning immigration from suspect countries or by all Muslims would not have stopped them at all.

Over these two decades plus, from the first bombing of the World Trade Center in 1993 to the present, there were around 110 jihadist-oriented plots to use terrorism. Among that number, 16 attacks succeeded in causing casualties. Most plots involved only one person. The pathways to individual “radicalization” they followed, if any are apparent at all, do not reveal a consistent pattern. Ideas and beliefs can motivate violence, certainly, but in most cases motivations are impossible to establish. In almost every case, the possibilities are endless, from personal grievance to mental illness to social frustration. We cannot say whether an expression of allegiance to the principles of al-Qaeda or the Islamic State was a genuine reason or an excuse. The Orlando shooter was apparently incapable even of distinguishing between the Islamic State and its enemy Hezbollah.

The majority of the plots were foiled by government authorities, most often with the help of informants. The plans were typically intercepted at very early stages of the plot, as the perpetrators first indicated interest in engaging in violent jihad. Like the Orlando shooting, most of the 16 plots that resulted in physical harm involved the use of firearms, although in general explosives are the weapon of choice of terrorists. It is much easier in the United States to acquire a gun than it is to build a bomb that will explode.

It is clear is that the attackers and would-be attackers were not part of a social movement. There is no tide of “violent extremism” associated with Islam and carried by immigrants that is sweeping the country. The perpetrators of violence are not representative of any community.

Other perspectives:

Daniel Byman: War drives terrorism

Abigail R. Esman: Radical Islam tells a story. We must tell a better one.

David Sterman: It’s foolish to try to simplify the motives of terrorists

Hamza Yusuf: The Orlando shooter Googled my name. I wish he had reached out to me.