The shopping madness exhibited on Black Friday is a great example of the power of social proof. Social proof is the psychological phenomenon where people assume the actions of others reflect correct behavior for a given situation.
Retailers have obviously exploited this tendency to great effect. Not only in the volume of shoppers who turn out (since everybody else is doing it, I should also) but in their often irrational herd-like behavior (let’s trample others to get those discounted towels).
Negotiators should be aware of this tendency and use it when appropriate. For example, market value (what others are paying for something right now) and precedent (what happened in previous similar situations) derive some of their power from social proof. This is true for independent standards (information from objective third-parties which supports your position) in general.