Humans are exquisitely social animals, and yet we’re vulnerable to some pretty stunning flaws in social cognition. Unfortunately, most of these flaws are on full display during a presidential campaign. Consider the false rumor, which can influence our beliefs even when it has been debunked. The most powerful example of this phenomenon, of course, is the swift-boating of John Kerry. It didn’t matter that every reputable news source found most of the charges to be misleading. The sheer fact that Kerry was being accused of lying was enough to impugn his honesty. Even when we’re found innocent, a little guilt remains.

And now it’s another campaign and the libelous lies are once again clotting up the airwaves. Consider the recent rumor that there’s a videotape of Michelle Obama saying “whitey”. Dave Weigel has thoroughly debunked the rumor – the “facts” of the videotape have mutated from day to day – but it’s still worth considering the impact of the rumor, even though it’s not true. I think one of the most troubling aspects of the way rumors work is that mere repetition is often confused with truth. When the brain hears a claim repeated endlessly – and that’s all cable news ever does – it can’t help but take that claim into account, even if the evidence is, at best, ambiguous. Basically, we’re the worst Bayesians ever.

But wait: it gets worse. Not only are we persuaded by false rumors that get repeated, but we’re persuaded even when the false rumors get repeated by one person. As Psy Blog notes, a recent study by Kimberlee Weaver and colleagues, found that “if one person in a group repeats the same opinion three times, it has 90% of the effect of three different people in that group expressing the same opinion.” Here’s the abstract:

Despite the importance of doing so, people do not always correctly estimate the distribution of opinions within their group. One important mechanism underlying such misjudgments is people’s tendency to infer that a familiar opinion is a prevalent one, even when its familiarity derives solely from the repeated expression of 1 group member. Six experiments demonstrate this effect and show that it holds even when perceivers are consciously aware that the opinions come from 1 speaker. The results also indicate that the effect is due to opinion accessibility rather than a conscious inference about the meaning of opinion repetition in a group. Implications for social consensus estimation and social influence are discussed.

That’s why one popular and persistent blogger, or one partisan hack on CNN or Fox News, can do so much damage. A single loud voice repeating bullshit is, as far as the brain is concerned, roughly equivalent to lots of voices repeating bullshit. And if lots of voices are repeating bullshit, then the bullshit must be true.

Of course, the paradox of this post is that, even though I set out to discredit the Michelle Obama rumor, I actually made things worse.