FBI officers walk past a memorial outside the Tree of Life synagogue in Pittsburgh.Brendan Smialowski / AFP / Getty

Little makes sense when it comes to the massacre of 11 worshippers in a Pittsburgh synagogue on Saturday. But after media identified the suspect, Robert D. Bowers, and duly excavated his apparent social-media postings, there was at least a hint of what was in his mind: By his own account, he was acting in the grip of an anti-Semitic conspiracy theory.

In this, Bowers resembles the man who allegedly mailed pipe bombs to more than a dozen public figures last week, and whose purported social-media accounts were similarly awash in delusions of shadowy forces scheming at oppression. Their experiences were in keeping with those of extremists the world over.

Where terrorists strike, whether it’s a lone extremist or a band of attackers, the perpetrators frequently embrace some kind of conspiratorial thinking. And anti-Semitism is a malevolent recurring theme. For jihadists, it’s the belief that Islam is under attack by a Judeo-Christian West. For neo-Nazis, it’s the belief that Jews are secretly controlling world governments and oppressing the “white” race. There is no single path to radicalization, and in most cases the destination is not violence. But there are patterns that do show up among individual violent extremists and terrorist groups, and the presence of conspiracy theories is a significant one.

The Pittsburgh suspect’s reported claim that he was acting to prevent the “slaughter” of his people at the hands of immigrants he thought were being brought to the United States by a Jewish refugee agency, for example, echoed the reasoning of another attacker who targeted a house of worship. Dylann Roof, who killed nine in a Charleston church in 2015, expressed similar anxieties about a white race overwhelmed by immigration in an manifesto he posted online. So did Anders Breivik, who killed 77 people, including many children, at a summer camp in Norway in 2011.

What ties these cases together is the chilling and warped logic that, in gunning down innocents, the attackers were somehow acting in self-defense.

It’s a logic that recurs across the ideological spectrum of radicals, from the far left and far right to jihadists and anarchists, according to a 2010 analysis of more than 50 extremist groups, mainly in the U.S. and Europe. That study found that each of those groups, without exception, embraced a form of conspiracy theory. Of course, some conspiracies actually have occurred (see: the Watergate break-in and cover-up). But the authors of the analysis, Jamie Bartlett and Carl Miller, observed that “what distinguishes conspiracy theories from genuine efforts to uncover actual conspiracies is that a conspiracy theory is not the most plausible account of events based on the available evidence.”

More recently, “if you spend any time in jihadist forums or spend any time in far-right forums, it is almost immediately apparent that these guys are swimming in conspiracy theories of all sorts—about the Jewish community, about the media,” the extremism researcher Amarnath Amarasingam told me in an interview. (A newly updated database of radicalized individuals of different ideologies, assembled by researchers at the University of Maryland, also shows conspiracy theories across the spectrum, but finds far-right extremists promoting them more frequently than jihadists or leftists.) The content of the specific theory varies depending on the ideology at issue, but it serves a dual function: to make sense of one’s own grievances, and to focus blame for them on an outside group, often a network of elites.

“The only thing that really does change,” Miller told me, “is who that cabal of shadow elites are.”

In the case of the Japanese doomsday cult Aum Shinrikyo, for example, the belief was that the government in Tokyo was conspiring with the United States and the Jews for world domination. Afrikaner nationalists have propagated the belief that the Illuminati controls the South African government.

Some would be funny if the potential consequences weren’t so serious—wackier conspiracy theories propagated in extremist forums include the belief that Hillary Clinton is a lizard person from outer space.

In 2016, the bizarre tipped over into the dangerous when the so-called Pizzagate conspiracy spread online. It claimed that Clinton was implicated in a pedophilia ring being run out of a pizza place in Washington, D.C., and it found eager propagators on social-media sites like Twitter and bulletin boards like 4chan. Ultimately, it inspired a man to enter the pizza place with an assault rifle in the apparent belief that he would bust up the activities. He was arrested without hurting anyone.

This is emphatically not to say that conspiratorial thinking causes violence. As with any trait common among extremists—childhood trauma, say, or any of a spectrum of political or economic grievances, or deep religious commitment—not all extremists have it, and the overwhelming majority of people who do have it never become extremists. Some conspiracy theories themselves are benign; they don’t necessarily serve as vehicles for identity-based grievances and the demonization of another group. It suggests no violent remedy to believe, for instance, that the moon landing was faked, or that Paul McCartney died in the mid-1960s and the Beatles replaced him with an imposter.

Bartlett and Miller describe the function of conspiracy theories instead as a kind of “radicalizing multiplier.” Amarasingam describes the process this way: “The conspiracy theory comes along and says, ‘Yeah, you’re absolutely right, this mysterious out-group is out to get you, is controlling everything, is orchestrating the downfall of the community that you hold so dear.’”

“You start to see the attacking of the out-group as not only justified, but morally required. That leap is quite a dangerous one, and almost always leads to violence of some kind,” he said. “You start to see attacking them as fundamentally necessary for your own survival and the survival of your people.”

Bartlett and Miller published their paper eight years ago and were surprised to find how far conspiracy theories were spreading “under the radar.” Since then, social media has aided their proliferation, both by facilitating access to conspiracy theories and by connecting like-minded individuals who might otherwise have remained isolated.

Meanwhile, trust in traditional purveyors of information, such as the media and the government, has declined in the U.S. and elsewhere, encouraging people to doubt official accounts and seek other sources of information, not all of them equally reliable. Finally, there is President Donald Trump himself, who came to political prominence promoting the “birther” conspiracy that his predecessor, Barack Obama, a natural-born U.S. citizen, was born in Kenya, and who has embraced conspiratorial language about immigrants, the media, and others consistently ever since.

Conspiracy theories thrive online, but they don’t necessarily stay there, Miller said. “Pizzagate is an obsession in 4chan one moment,” he noted, “and the next, someone’s bursting into a pizzeria with a rifle seeking to bust open a pedophile ring.” As the events of the past week show, the outcome can be significantly worse than that.

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