David Lewis's Convention: Imitation

Lewis discusses a situation where everyone wears a raincoat because they see others wearing raincoats. Say, the first person up and about puts one on as a lark, the second person sees her and says, "Oh, my, rain!" and puts his on, and so on. He makes the following important observation about the rationality of the situation:

It may be that everyone was completely reasonable in inferring and acting as he did -- although no one will think so when he learns what happened. The manifest irrationality of the group may not be due to any irrationality of its members. It is no mistake to expect rain when one sees people in raincoats, despite the bad results of doing so this time. -- p. 120

I think this idea has very important implications for the study my colleague and I have launched into on a general theory of the social cycle: group irrationality may arise in the presence of uniformly rational behavior on the part of individuals. (And it is nice to see it demonstrated in a situation with no ideological overtones: Lewis is clearly not striving to make a case for or against government regulation with this example!)