In the book When Prophecy Fails, Leon Festinger chronicles an American cult from the middle part of the 20th century. Led by a housewife named Marian Keech, the cult members were big-time doomsayers, predicting that the world was about to be destroyed by a giant flood. Caused by aliens. Of course. But rather than digging out their ark-building tools, the cult members weren’t expected to be rescued from the flood by an alien visitor who would pick them up in a spaceship and fly them to safety. Of course.

As most reasonable people might expect, the flood didn’t happen. The UFO didn’t arrive. But this didn’t deter the cult members. Rather than admitting that they might have gotten the whole end-of-the-world thing wrong, they instead assumed that their hard work prevented the flood from happening. They had “spread so much light” that the world had been saved.

Bolstered by this position, the cult went public. Formerly they had avoided any public discourse. But now they began calling newspapers, sitting for interviews, and otherwise preaching their message loud and proud.

Their doubts and uncertainty didn’t make them wiser or more thoughtful. Doubt made them louder.

Why does this happen? A new study from two marketing professors at Northwestern University (David Gal and Derek Rucker, writing in the journal Psychological Science) indicates that people are more likely to proselytize when they doubt something. Shaken confidence, they write, actually increases advocacy.

In other words, if you have doubts about your religious (or political, or consumeristic) beliefs, you’re more likely to evangelize on behalf of those beliefs than someone who is actually more certain about what they believe.

“People proselytize to have their beliefs affirmed or validated, ” Rucker told Paul Kix at AOL News.

The point of it all, Rucker says, is that our beliefs are who we are, even (especially) our consumer beliefs. When those beliefs are challenged, we are challenged, and we spend more time trying to convince others of the product’s merit, because really we’re talking about our own.

The studies themselves are pretty interesting. Here’s how Kix describes them:

The first study tested 88 MBA students’ beliefs about animal testing on consumer products. The students read a paragraph about how animal testing is performed, said whether they still agreed with the practice, and then were asked to write about it, in an attempt to persuade others. Those who doubted what they were saying ended up writing more.

The second experiment primed the participants, some of whom were asked to describe two instances in which they felt uncertain. After that, these people were told to persuade someone whose diet was unlike their own (vegetarian, if the participant was a meat-eater, etc.) of the advantages of the participant’s diet. Same thing here: The people who had been primed to first discuss a situation in which they were uncertain wrote longer essays about the merits of their diet.

The third experiment yielded similar results. Undergrad students who used Macs were asked to persuade PC users. They did so, with a great deal of verbosity, if they thought the PC user would be open to hearing from the other side.

Honestly, these findings surprise me a little, because the thing that bothers me most about certainty is its arrogance. It doesn’t matter what kind of certainty — whether you’re sure Rob Bell is a heretic, whether you’re an evolutionary biologist arguing that religion is stupid, whether you’re convinced all firearms should be outlawed, whether you believe any form of gun control is always evil, or whether you’re an Apple evangelist convinced every person needs an iPad.

Because the world is a complicated place. It’s filled with complexity, and there are good arguments on behalf of most opinions. That’s why, if there’s no room for a glimmer of doubt in your religious, political or consumerist viewpoint, I have trouble taking you seriously — because it comes across as arrogance to me. It seems like you’re so convinced of your own rightness that you’re unwilling to look at all sides of an issue.

That kind of thinking annoys me, by default. (And I’m sure it annoys a lot of people who know me, too.) Much of this is probably tied to my introspective personality, but I’ve always thought a less certain approach was a humble and honest way to approach the world. It’s acknowledging the possibility of being wrong. It’s willing to be convinced otherwise. It’s flexible, thoughtful, reasonable, and pursues the truth rather than an agenda. And it keeps me from putting much effort into making sure everyone thinks the same way I do.

But this study says I might be wrong about that. Maybe doubters like me make the best advocates. Or, at least, the most persistent ones.

What do you think? Does this study ring true? Have you ever found yourself advocating passionately for an issue that you’re no longer quite sure about? Are our attempts at evangelism actually attempt to convince ourselves?