In the House of Representatives, states are represented according to their populations, with each state guaranteed at least one representative. The total number of representatives is capped at 435 as of 2014, so states gain or lose representatives according to population shifts.
The process for determining state population has changed several times since the Constitutional Convention in 1787. At the convention, delegates chose to hold a census each decade to assess the populations of the states, but controversy arose about whether slaves should be counted. They settled on the Three-fifths Compromise, which counted each slave as three-fifths the value of a free person for census purposes. After the Civil War, the 14th Amendment changed the value of every individual to a full count. However, only men 21 years of age and older were counted. Women were not given the right to vote until 1920. In 1971, the 26th Amendment lowered the voting age from 21 to 18.
The Constitutional Convention's decision on how to apportion seats in the House of Representatives and the Senate became known as the Great Compromise. Delegates from larger states insisted that their states should have more representation because their states contributed more finances and resources, while delegates from the smaller states voiced concerns about being continuously overwhelmed by the larger states. As a result, each state has two senators regardless of its population, although state representatives in the House are determined by population.