A Republican friend and I had an enjoyable argument a few days ago about a thesis that I recently formulated. It goes like this: Our tax code is designed not just to raise revenue but also to carry out social policy. For example, Congress allows taxpayers to deduct home mortgage interest and charitable contributions from gross income to encourage us to borrow to buy houses and to give money to charity. The deduction for charitable contributions and the lower effective tax rate for married couples may also reflect the moral philosophy of Congress: it's more righteous to be married and give generously than to be single and stingy.
Similarly, income from certain sources (such as qualifying investments in offshore dependencies) is taxed at a lower rate, or not at all, to encourage taxpayers to invest in income-producing ventures in those locations.
It follows, then (I told my friend) that legislators who want to tax capital gains at lower rates than ordinary income (i.e., income from working) want to encourage people to realize capital gains -- and, conversely, discourage them from working. Alternatively, the difference in tax rates may embody the collective moral philosophy of our elected representatives, even if they don't know they're showing it: it's more praiseworthy to trade stocks for profit than it is to earn money from producing something in a factory, and Americans should thus be discouraged from working for a living.