The tax rate could easily be based on the average of the salaries of the (adult) individuals of the household. If you have one person making 100k supporting one

The tax rate could easily be based on the average of the salaries of the (adult) individuals of the household. If you have one person making 100k supporting one person, why not pay the same rate as two people making 200k to support two people. Instead, those two people currently pay the same rate as one person making 200k to support one person (more, technically, since it also moves them above the limits where they lose many writeoffs).