Each part of the human brain is optimized for behaving correctly, assuming that the rest of the brain is operating exactly as expected. Change one part, and the rest of your brain may not work as well. Increasing a human's intelligence is not a trivial problem.
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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 The Uses of Fun (Theory), and you can use the sequence_reruns tag or rss feed to follow the rest of the series.
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