The 2-income family gives a shocking amount of the extra money they scramble to earn to the government. I'm no tax expert, but my suspicion is that this happens because liberals like more taxes and conservatives like subsidizing the traditional family with a stay-at-home parent. Put those 2 forces together and we get the (perverse?) burdening of the 2-earner family.
Why don't more couples do the math and figure out that they should not do all that extra work for the government? Life is so much simpler with the 1-earner family, and the spouse who doesn't bring in the dollars can provide great economic benefits by directly performing work that would otherwise have to be paid for, most notably child care. Since this economic benefit isn't taxed, it's a double benefit. Instead of buying inferior childcare (or other services) with after-tax dollars, you perform the work that is worth that much money, and you're not paid, so you don't pay taxes on the value it represents.
-Ann Althouse in a recent post, to which Glenn Reynolds links with approval.