Apportionment determines how many representatives each subdivision of a country sends to a legislative body. It is distinct from districting (drawing the boundary lines within a unit) and from malapportionment (when seat-to-population ratios diverge significantly across units).
In the United States, apportionment of the 435 seats in the House of Representatives occurs every ten years following the decennial census, as required by Article I, Section 2 of the Constitution and the Fourteenth Amendment. Since 1941, Congress has used the method of equal proportions (the Huntington–Hill method) to divide seats among the fifty states, with each state guaranteed at least one seat. The 2020 census, for example, shifted seats away from states such as California, New York, Pennsylvania, Ohio, Illinois, Michigan, and West Virginia, and toward Texas, Florida, North Carolina, Colorado, Montana, and Oregon.
Different democracies use different mathematical formulas, each with known biases:
- D'Hondt method — favors larger parties or units; used in many European parliamentary systems.
- Sainte-Laguë method — closer to proportional; used in Germany, New Zealand, Norway, Sweden.
- Hamilton (largest remainders) method — used historically in the U.S. and currently in several Latin American systems.
- Huntington–Hill — minimizes relative differences in district population; current U.S. House method.
Apportionment can also refer to allocating seats within a legislature to parties under proportional representation, or distributing committee seats, treaty obligations, or budget contributions among members of an international body.
Landmark U.S. Supreme Court cases shaped the legal doctrine: Baker v. Carr (1962) made apportionment a justiciable issue, Wesberry v. Sanders (1964) applied "one person, one vote" to congressional districts, and Reynolds v. Sims (1964) extended it to state legislatures. Department of Commerce v. New York (2019) addressed the census questions that feed into apportionment data.
For Model UN delegates, the term also surfaces in debates over UN Security Council reform, IMF quota reallocation, and European Parliament seat distribution under the principle of degressive proportionality.
Example
After the 2020 U.S. Census, Texas gained two House seats while California lost one for the first time in its history, taking effect with the 2022 midterm elections.
Frequently asked questions
Apportionment decides how many seats each state or unit gets; redistricting draws the district boundaries within that unit. Apportionment happens first, then redistricting follows.
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