When moderating comments, the goal is not to vote good posts up and bad posts down, but to make the vote total most accurately reflect the signals of all the people who voted on it. Since voters don't gain or lose anything by voting accurately, besides the satisfaction of knowing that their votes help the scores more accurately reflect post quality, they should always vote according to their private signal, and ignore the signals that others have given.
On the other hand, when signaling is tied together with some other choice, then information cascades can happen. The example that was given in my networks class was a case of two restaurants next to each other, where each potential patron can see how busy each restaurant is. In that case, people don't care about their signal, but just want to visit the better restaurant, and an information cascade is likely to occur. A similar occurrence happens with book purchases: if a book appears on a best-seller list, then that signals to everyone that it's good, but it may only be there because people bought it based on that signal. There are documented examples of clever publishers have buying copies of their own books to kick-start this effect.
An information cascade is a problem in group rationality. Wikipedia has excellent introductions and links about the phenomenon, but here is a meta-ish example using likelihood ratios.
Suppose in some future version of this site, there are several well-known facts:
Let's talk about how the very first reader would vote. If they judged the post high quality, then they would multiply the prior likelihood ratio (6:4) times the bayes factor for a high private signal (4:1), get (6*4:4*1) = (6:1) and vote the post up. If they judged the post low quality then they would instead multiply by the bayes factor for a low private signal (1:4), get (6*1:4*4) = (3:8) and vote the post down.
There were two scenarios for the first reader (private information high or low). If we speculate that the first reader did in fact vote up, then there are two scenarios for the second scenario: There are two scenarios for the second reader:
Note that now there are two explanations for ending up two votes up. It could be that the second reader actually agreed, or it could be that the second reader was following the first reader and the prior against their personal judgement. That means that the third reader gets zero information from the second reader's personal judgement! The two scenarios for the third reader, and every future reader, are exactly analogous to the two scenarios for the second reader.
This has been a nightmare scenario of groupthink afflicting even diligent bayesians. Possible conclusions:
Note: Olle found an error that necessitated a rewrite. I apologize.