We've spent some time looking at the conspiracy theories that arose after the Boston Marathon bombing, but it's worth looking at the conspiracy theorist that allegedly started this all: Tamerlan Tsarnaev. The elder and more radical of the two brothers suspected of perpetrating the attack "believed in basically every conspiracy theory," as Dave Weigel put it, linking to an AP report showing that Tsarnaev was interested in everything from Alex Jones to "The Protocols of the Elders of Zion," a seminal anti-Semitic conspiracy tome.

It's not particularly surprising that Tsarnaev would be drawn to a wide range of conspiracy theories, as research shows that people prone to believing one conspiracy theory will likely believe many -- even if they're completely contradictory. And he fits a profile of a type of person likely to be drawn to conspiratorial thinking, considering he was allegedly alienated from and disgruntled with society.

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On top of Jones and anti-Semitic conspiracy theories, we have to add the one that seems to be the most important of all: The kind of anti-American conspiracy theories pushed by Islamists. For instance, the Washington Post reports that the brothers were apparently motivated by the wars in Iraq and Afghanistan and atrocities committed by U.S. soldiers there. As with many conspiracy theories, there is a grain of truth here -- American soldiers really have done some horrible things in those countries. But Tsarnaev went beyond the evidence by telling people that “in Afghanistan, most casualties are innocent bystanders killed by American soldiers." In fact, according to the U.N., the Taliban is responsible for the vast majority of civilian deaths -- 81 percent in 2012.

Anti-American and anti-Semitic conspiracy theories are foundational to al-Qaida and other radical groups' ideologies, according to Matthew Gray, a professor at Australian National University who wrote in his book "Conspiracy Theories in the Arab World," that "the speeches of Osama Bin Laden are peppered with conspiracist language and the assumptions that underline conspiracism."

Indeed, conspiracy theories are hardly unique to the United States and often run rampant in the Muslim world, as Eric Trager of the Washington Institute for Near East Policy wrote in the New Republic, and seem to be especially strong among Islamists. A 2011 Pew poll of residents of Lebanon, Jordan, Egypt, Turkey, Pakistan, the Palestinian territories and Indonesia found that the vast majority refused to believe that Arabs executed the terrorist attack on 9/11. "There is no Muslim public in which even 30 percent accept that Arabs conducted the attack," the study found.

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The Tsarnaev brothers didn't live in any of those countries, of course, but were influenced by rhetoric coming out of the Arab world, including al-Qaida's Inspire magazine, where they reportedly found the plan to build their bombs, as well as YouTube videos. And add to that the experience in Chechnya, where many anti-government conspiracy theories turn out to be fact. As Ben Smith wrote of the Tsarnaev parents' belief that the U.S. government set up their children:

The Tsarnaevs may sound like the craziest figures of the American fringe. But they come by their paranoia honestly: Russia's cynical and brutal governments have, for centuries, murdered their citizens in general, and their Chechen citizens and subjects in particular, under any number of pretexts ... This may sound paranoid. But paranoids can have real enemies. And you don't have to be crazy to believe Chechen allegations of baroque and brutal government conspiracies -- at least, not when they're directed at the Russian government.

The brothers' aunt, Maret Tsarnaeva, explained, "I am used to being set up. Before I left former Soviet Union countries, that's how I lived."