United States tax complexity isn't because of the shape of the tax(income) function. That function is the most important part of tax to most Americans, but the

United States tax complexity isn't because of the shape of the tax(income) function. That function is the most important part of tax to most Americans, but the complexity comes from two other places: 1) Some people will game the system for economic advantage. Of the 321M Americans, a certain portion are both extremely clever and amoral. A lot of the code is a response to "They did what‽" 2) Americans tend to have an inbuilt belief that everyone should make it on their own which manifests into the government not "giving" things to people, even if it is to induce a person to accomplish the government's goal. Since it is politically difficult for the government to use payments to change people's behavior it is easier to "not take as much" instead and accomplish the same end with a tax credit. This pushes all sorts of policy and governance into the tax code.¹ ␄ ¹ As an example, I have in the past claimed "historic building renovation" tax credits. I have never renovated a historic building. My state wants to spend $X to preserve historic buildings, but they politically can't give $X to people preserving buildings. So instead, they give transferable tax credits worth $X to the renovators, who probably aren't paying much tax (because they are in the business renovating historic buildings, like that pays) and don't have a use for the credit. If you know the right finance person, you will find there is a quiet market that connects sellers and buyers, so you can buy credits and split the savings with the seller 80/20 if I recall. It's a stupidly complex system that only puts 80% of the state's cost into what it wanted done, but at least they didn't give anyone money to preserve buildings.