He does display an only superficial understanding of the tax code, however. A case in point is that he recently tried to say that state taxes didn't count in determining the effective tax rate because they were deducted, apparently failing to appreciate the difference between a deduction (which reduces only one's taxable income) and a credit (which is a dollar for dollar reduction in one's tax bill).

There are certainly more examples, but that's one that immediately came to mind. While I think that this may help to show that Joel IS crazy, however, it really doesn't explain WHY he is crazy. Possible explanations (not listed in the original post) might include severe head trauma, lead poisoning, or genetic predisposition.