Gary Marcus, author of Kluge(amzn/b&n/abe/pwll) (a book I recommend), has an opinion piece in the Wall Street Journal that makes an important point: evolution does not produce rational, perfect, finely-tuned beings. It makes organisms that are good enough. Keep this in mind when looking at anything biological (like, say, an appendix), and it’s also true in economics, as Marcus points out — assuming that human beings will tend to make rational choices will lead to being fooled much of the time.