Tim Carney's got an idea:
[H]ere’s my proposal: All brackets, not just the 0% bracket, should get wider as household size grows. If a single gets $9,000 at the 10% rate, and a couple gets $18,000 at the 10% rate, a couple with a kid should get an additional $4,500 at that 10% rate (I’m counting kids as half an adult) for a total of $22,500 at the 10% rate.
Here’s why. I see the rates in a progressive tax code this way: When you’re dirt poor, we don’t tax you at all. When you actually have a bit of money, but are still in poverty, we tax you at 10%. When you are out of poverty, but still not really enjoying luxuries, we take 15% of your money. Once you have a bit of a cushion, we take 25% of your money. As you get wealthy, we take a higher portion.
Well, the distance in income between those categories — poverty, basic needs met, a bit of a cushion — grows as you have more dependents. So the distance between the income-tax-rate thresholds should grow, too.