Robert VerBruggen argues that it’s unfair to tax dual-earner couples the same as single-earner couples with the same total income. The dual-earners should pay less because they tend to have higher costs. Even if they have no kids, they have higher commuting costs.
You’re never going to have a tax system that achieves fairness defined by taking account of every possible cost difference. What if the single-earner couple is taking care of an elderly relative? Should we create special tax rules to take account of this situation? What about couples who live further away from work and can’t easily change their location? Should they have to pay the same as people with lower commuting costs?
And the alleged unfairness in question — “we’re paying the same as a one-earner family with the same income but lower costs, even though one-earner families in general don’t make as much as two-earner families” — seems to me a bizarre one to spend time on.