27 January 2006
It's a common truism, familiar to most people by now thanks to advertising and politics, that repeating things makes them more believable -- regardless of whether they're true or not. In fact, even if they know at the time that the information is false, people will still be more likely to believe something the more they hear it. This phenomenon, sometimes called the reiteration effect, is well-studied and well-documented. Nevertheless, from a statistical learning point of view, it is extremely counter-intuitive: shouldn't halfway decent learners learn to discount information they know is false, not "learn" from it?
One of the explanations for the repetition effect is related to source confusion -- the fact that, after a long enough delay, people are generally much better at remembering what they learned rather than where they learned it. Since a great deal of knowing that something is false means knowing that its source is unreliable, forgetting the source often means forgetting that it's not true.
Repetition increases the impact of source confusion for two reasons. First, the more often you hear something, the more sources there are to remember, and the more likely you are to forget at least some of them. I've had this experience myself - trying to judge the truth of some tidbit of information, actually remembering that I first read it somewhere that I didn't trust, knowing that I've read it somewhere else (but not remembering the details) and concluding that since there was some chance that this somewhere else was trustworthy, it might be true.
The second reason is that the more sources there are the more unlikely it seems that all of them believe it if it's false. This strategy makes some evolutionary and statistical sense. Hearing (or experiencing) something from two independent sources (or two independent events) makes it more likely that you can generalize on them than if you only experienced it once. This idea is the basis of getting large sample sizes: as long as the samples are independent, more samples means more evidence. Unfortunately, in the mass media today few sources of information are independent. Most media outlets get things from AP wire services and most people get their information from the same media outlets, so even if you hear item X in 20 completely different contexts, chances are that all 20 of them stem from the same one or two original reports. If you've ever been the source of national press yourself, you will have experienced this firsthand.
I tried to think of a way to end this entry on a positive note, but I'm having a hard time here. It's a largely unconscious byproduct of how our implicit statistical learning mechanisms operate, so even being aware of this effect is only somewhat useful: we know consciously not to trust things simply because we've heard them often, but so much of this is unconscious it's hard to fight. Education about it is therefore worthwhile, but better still would be solutions encouraging a more heterogeneous media with more truly independent sources.