Apportionment
Apportionment is the constitutional requirement, in Art. I §2 cl. 3 and §9 cl. 4, that direct taxes be allocated among the states in proportion to the census population. A direct tax of $1 billion would be divided so that a state with 10% of the U.S. population would owe $100 million, regardless of how much wealth or income that state’s residents actually held.
The mechanism is awkward by design. It made it impractical to enact direct taxes that fell unevenly across the population, because the per-capita burden in poor states would be higher than in rich states under the same tax. The result is that the federal government has historically relied on indirect taxes — tariffs, excises, and (post-1913) income taxes — that don’t trigger the apportionment requirement.
Pollock v. Farmers’ Loan & Trust Co. (1895) struck down the 1894 income tax on the ground that, as applied to income from property, it was a direct tax requiring apportionment. The decision provoked a constitutional response: the Sixteenth Amendment, ratified in 1913, declared that taxes on incomes “from whatever source derived” need not be apportioned.
The amendment did not redefine “direct tax.” It carved out a specific exception for income taxation. This is why the apportionment requirement still applies — in principle — to other forms of direct taxation, including any prospective federal wealth tax. The amendment dissolved the income-tax problem; it left the broader doctrinal structure in place.