In Evolutionary Biology or Psychology, a nice-sounding but untested theory is referred to as a "just-so story", after the stories written by Rudyard Kipling. But, if there is a way to test the theory, people tend to consider it more likely to be correct. This is not a rational tendency.
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This post is part of the Rerunning the Sequences series, where we'll be going through Eliezer Yudkowsky's old posts in order so that people who are interested can (re-)read and discuss them. The previous post was Stop Voting For Nincompoops, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.
Sequence reruns are a community-driven effort. You can participate by re-reading the sequence post, discussing it here, posting the next day's sequence reruns post, or summarizing forthcoming articles on the wiki. Go here for more details, or to have meta discussions about the Rerunning the Sequences series.