The science of economics is founded on the notion that economic agents look out for their own financial interests in a rational way. We know of course this is wrong. The latest piece of evidence: Cindy McCain's balance of at least $225,000 on two American Express cards. The median interest rate on AmEx cards is about 14 percent, says Consumer Reports. Given that Ms. McCain is heiress of a beer distributorship, she may get something a little better. But not much. So she's paying more than $20,000 per year in non-deductible interest on balances that presumably she could easily pay off or refinance at a much better rate. But she doesn't. From the NYT, which also covers Barack and Michelle Obama's disclosures:

Another charge card, held by what was described as a “dependent child,” had also accumulated debts of $15,000 to $50,000. In addition, a credit card held jointly by the couple was carrying $10,000 to $15,000 in debt, the filing indicated, at a stiff 25.99 percent interest rate.

Mrs. McCain’s filing, however, indicated that she had substantial holdings in property and stocks — including shares in Anheuser-Busch, which this week became the target of a takeover bid that is expected to send its value climbing. Her land holdings included parcels in Arizona and California, one of which was sold last year for a profit of more than $1 million.

In other filings, the McCains have reported total household assets of $24.6 million to $39.5 million. In recently releasing a summary version of her 2006 tax return, Mrs. McCain reported income that year of more than $6 million, some $300,000 of which was derived from her salary as the chairwoman of Hensley, which was founded by her father.