Last Wednesday, the No. 1 trending video on YouTube was based on a lie. The video alleged that David Hogg, a 17-year-old survivor of the Parkland, Florida, school shooting, was an “actor.” After the shooting, Hogg had become a compelling and sympathetic proponent of gun control, making the rounds on cable news.

This YouTube video contained a clip of Hogg speaking on camera for a Los Angeles local news segment last summer. The video was captioned “DAVID HOGG THE ACTOR....” — the implication being that Hogg suspiciously shows up whenever news cameras are rolling.

YouTube took down the video and others like it — including one Infowars video headlined “David Hogg Can’t Remember His Lines in TV Interview.” But the video-sharing platform was hardly the only place similar conspiracy theories flourished, as Vox’s Matt Yglesias outlined. Right-wing celebrities suggest that George Soros was funding the anti-gun outcry. Elsewhere, outlets were pointing out that Hogg is the son of a retired FBI agent (the implication being that the FBI is out to get President Donald Trump).

We’ve seen this before. After a tragedy, conspiracy theories emerge, and they are powerful, mean, and damaging.

Related The science behind why fake news is so hard to wipe out

Consider the unending hurt of the parents of children slain during the Sandy Hook Elementary School massacre. For years, they have been charged with making up the whole tragedy (including the lives of their children).

Or the agony of the family of Seth Rich, the Democratic National Committee staffer who was murdered in an apparent robbery attempt in 2016. Despite zero evidence, conspiracy theorists and conservative pundits fanned suspicions that Rich was murdered by the Clinton campaign. “Seth’s death has been turned into a political football,” Rich’s parents wrote at the Washington Post.

Conspiracy theories are an infuriating plague on our media ecosystem. But they’re not inexplicable. We can understand why they spread so far and wide. Here are the two key reasons.

1) Conspiracy theorizing is a type of motivated reasoning

When it comes to political debates, we don’t use our smarts to uncover the truth. We use our smarts to protect the groups and worldview to which we adhere. This is called politically motivated reasoning, and it distorts how we see the world.

Asheley Landrum, a Texas Tech University psychologist who has studied conspiracy theorizing, suggests this might be what’s happening among those theorizing about the Parkland students.

“To counteract the kids’ powerful speech, a conspiracy narrative arises that allows individuals to dismiss or ignore [the kids’ perspectives],” she writes in an email.

Why would people want to counteract the kids? Well, because of where their pleas for greater gun control might lead: to greater gun control. And in the minds of right-wing conspiracy theorists, it’s easier to dismiss the kids outright than to actually engage with their arguments.

Psychologists have found that people aren’t necessarily afraid of facts — they’re afraid of where those facts lead. This is called “solution aversion,” and it helps explain why many conservatives are wary of the science of climate change — because many solutions to climate change involve increasing government oversight and regulations. Similarly, right-wingers may be discrediting the kids because they don’t want to see changes to gun laws.

One crucial thing to know about motivated reasoning is that you often don’t realize you’re doing it. We automatically have an easier time remembering information that fits our worldviews. We’re simply quicker to recognize information that confirms what we already know, which makes us blind to facts that discount it.

2) Conspiracy theories are a tool to cope with a painful, uncertain world

Psychological research also finds that some people are just more prone to believing in conspiracy theories than others. And it’s not because these people are necessarily unintelligent (though being a more analytic person is correlated with being less swayed by conspiracies.)

Instead, believing in conspiracy theories, psychological research informs us, is a coping mechanism to deal with uncertainty in the world.

“It’s a self-protective mechanism people have,” Jan-Willem van Prooijen, a psychologist who studies conspiracy theories, told me last year. The theories are a tool by which people can feel more in control and find explanations in a scary and turbulent world.

The thought of 17 kids being gunned down at school is horrible. Why wouldn’t we seek refuge in a theory that insists it wasn’t so bad after all?

Van Prooijen expanded on what makes people more susceptible than others to believing in the theories:

One of them is education level. Conspiracy theories are more likely among people who are [less] educated. But this doesn’t mean that people who are highly educated are immune. There’s also effects of political ideology. In our research, we find the more radical a person’s politics [both left- and right-wing], the more likely they are to be conspiracy theorists. Another is collective narcissism [a personality trait where people demand the group they belong to be admired], and people who believe their own nation and group is superior to others.

People who feel powerless and who are more pessimistic are also more likely to believe in conspiracy theories, van Prooijen finds. And this is where education and outreach can help. Achieving higher levels of education correlates to feeling more secure about the world, and this, in turn, seems to protect against a conspiratorial mindset.

Conspiracy theories are especially dangerous in the age of viral media

Conspiracy theories aren’t new. But today they, fueled by computer algorithms that award grabbiness and not truth and deliberate misinformation campaigns, spread like wildfire. And the big tech giants are often hesitant or sluggish in taking them down. (After the shooting in Las Vegas in October, Google featured a link to an anonymous message board notorious for fueling conspiracy theories in its “top stories” module.) Plus, there are whole media outlets — like Infowars — that have built huge followings peddling junk theories.

The theories are also dangerous because of a psychological phenomenon called the illusory truth effect. Its impact is subtle, but basically, it means that simply repeating a lie makes it more likely to be misidentified as truth.

Each time a reader encounters a conspiracy theory on Facebook, Google, or really anywhere, it makes a subtle impression. Each time, the story grows more familiar, and that familiarity casts the illusion of truth. The more we hear a piece of information repeated, the more we’re likely to believe it. This is true for “even things that people have reason not to believe,” Gord Pennycook, a psychologist who studies misinformation at Yale University, told me last year.

So consider this: When YouTube took down the conspiracy video last Wednesday, it had already been viewed 200,000 times.