system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did the united states end up with a two party system?
gpt
Let's say that there are two political parties, A and B. Under the current US electoral system, whoever gets the most votes is elected.

So let's say third party C comes along, with beliefs that are slightly similar to party B. If C takes enough votes away from B, party A would often be elected, sometimes even without a majority of the votes (meaning, the number of B+C voters is greater than the number of A voters).

Some states and local areas have run-off elections, where if one candidate does not get a majority of votes, the top two vote-getters will have another election. In this example, if A and B get the most votes, the C voters would likely end up voting for the B candidate in a run-off election.

US elections don't have run-offs, so the C voter pool often becomes fearful of voting for C, because B seems to have more support, and A will likely win if C takes enough votes from B. So C voters vote for B out of fear that A will be elected, party B grows stronger, and party C never picks up steam. If run-off elections were allowed, party C voters wouldn't be as fearful and might eventually have a chance of gaining a foothold.

So why do state and local elections that hold run-offs usually have two parties? The influence of the national elections ends up trickling down to the local levels. However, you do tend to see more "independent" candidates at the lower levels of government.