Meanwhile, conspiracy theories circulated on social media that liberals or Democrats had sent the packages as a “false flag” operation to paint conservatives as radicals before the Nov. 6 midterm elections. A classic false-flag attack is one in which a perpetrator dresses up as the enemy to create the impression that the enemy committed an atrocity.

Even with a suspect’s arrest, however, no one yet knows what motivated the sending of the explosive packages. And no matter what evidence emerges, conspiracy theories will almost certainly continue to circulate.

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Here are a few things the academic literature can tell us about such conspiracy theories:

1. We’ve seen many false-flag conspiracy theories in the U.S.

Every recent mass shooting has spawned false-flag theories. For instance, after the Parkland, Fla., school shooting, many conspiracy theories swirled on social media, alleging that the shooting was a hoax designed to undermine gun rights. David Hogg, a student activist from Parkland, was accused of being a “crisis actor” — a paid shill for a shadowy group that supposedly orchestrated the event. Similar accusations circulated after the 2016 Pulse nightclub mass shooting that killed 49 people; the on-camera killing of television journalist Alison Parker and photojournalist Adam Ward in Roanoke; and the Sandy Hook, Conn., elementary school shooting in 2012.

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These are not simply creations of the Internet. Not long after the 1941 Japanese attack on Pearl Harbor, conspiracy theorists accused President Franklin D. Roosevelt of allowing the attack to happen to help persuade reluctant Americans to support getting involved in World War II.

2. False-flag conspiracy theories are popular

Such theories are fairly popular. A nationally representative Farleigh Dickenson poll conducted in 2016 showed that 22 percent of Americans believed that it was possibly or definitely true that the 2012 Sandy Hook shooting was “faked, in order to increase support for gun control.” Representative polling of 2,068 Floridians done by Casey Klofstad and me in August showed that 15 percent agreed that mass shootings, like the ones in Parkland and Orlando, were hoaxes designed to justify taking away gun-owners’ weapons.

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Even when the evidence for such attacks is clear, some people are convinced that the truth is being hidden — and prefer alternative narratives.

3. False-flag beliefs coincide with people’s political views

So who believes conspiracy theories? People who, first, have an underlying predisposition toward seeing events and circumstances as the product of conspiracies — or I what call conspiracy thinking — and second, who have political views that coincide with the particular conspiracy theory.

For instance, Republicans with “conspiracy thinking” tend to believe that Democrats orchestrate conspiracies, while Democrats with “conspiracy thinking” believe that Republicans are the conspirators. Each side points fingers at the other, and in nearly equal numbers.

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When people believe conspiracy theories, it tends to be ones that give an advantage to their own side. In this case, because the attacks appear to have been carried out by a Republican — both from the choice of targets and the evidence currently emerging — Republicans who think conspiratorially may shift responsibility from their side by imagining a conspiracy, much as conspiracy-minded gun rights supporters suggest school shootings are actually false-flag operations orchestrated by those who want to chip away at the Second Amendment.

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4. False-flag conspiracy theories can have unintended consequences

Rather than helping to shift blame, conspiracy theories can backfire on the conspiracy theorists. That’s what happened after the Parkland school shooting. Not only did conspiracy theories fail to turn public opinion, but evidence shows that those outraged by the conspiracy theories shared them more often than people who actually believed them. As a result, Parkland student activists looked even more sympathetic: After facing a tragedy, they were targeted by unscrupulous Internet conspiracy theorists.

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5. There is a psychology to false-flag conspiracy theories

To one degree or another, each of us has a “cheater detector,” a psychological mechanism that helps us navigate a world in which some people play by the rules and others do not. In a series of experiments, psychologist Preston Bost found that when someone benefited from a tragedy, others were more likely to suspect that person had actually orchestrated the tragedy. This cheater detector, while useful, can result in suspicion of people without any basis for the suspicion. Grandchildren inherit money from grandmothers all the time, but it doesn’t mean they all pushed their grandmothers down the stairs.

Facts won’t end these theories. Even with a possible culprit in hand, at least some partisans will probably continue to believe theories that blame the other side, no matter what evidence points the other way.