The most common voting system used is called First Past the Post voting, FPP, or FPTP. FPTP is a fairly simple system, which is why it is used so often: Whoever has the most votes ranking them as first place wins. However, this has many problems!
Here are the points about the FPTP system:
Majority Criterion: A majority implies a plurality.
Condorcet Criterion: Clearly preferred candidates actually lose, due to the Spoiler Effect.
Pareto Criterion: Also by the Spoiler Effect, a pairwise loser still can win.
Irrelevance of Independent Alternatives (IIA): Yet another “Spoiler Effect” related problem, the dropping out of candidates that know they will lose will affect the election through the inverse of the Spoiler Effect.
Monotonicity Criterion: Once a candidate wins, any more votes for that candidate would increase the plurality.
Spoiler Effect Prevention: As many FPFP problems are related to this, this is important. Starting with a bipartisan system, there are 2 parties: Alpha and Omega. The outcome would have been 40% to Alpha and 60% to Omega. However, a new political party joins: Xi. Many Omega voters would rather vote for the Xi candidate, and the Xi party rises to almost equal status with the main 2 political parties. However, with the exact same preferences, the vote is divided this way: 40% to Alpha, 35% to Omega, and 25% to Xi. However, Alpha wins the election, instead of Xi. This is the main problem with FPTP, strategic voting. This is voting against a candidate, instead of for a candidate. This is the case with the election above: Xi voters agree most with Omega, and would vote for Omega to prevent Alpha from winning. This is where the spoiler effect comes in: Both Omega and Xi voters lost, as Alpha won the election. However, without Xi joining, Alpha would have lost! It also may be possible that the Alpha party supported Xi, as every vote for Xi is a vote away from Alpha’s main enemy, Omega.
Minority Rule Prevention: In a 30-party system, 5% may be enough to win a plurality!
Preference Proportionality: Slim pluralities can result in candidates spread out, and winning all localized elections.
Gerrymandering Prevention: This is highly susceptible to gerrymandering, as visible by some countries with FPTP voting systems, like the United States.
Political Diversity: Through this system, a study shows that FPTP elections tend towards bipartisan systems.
Definitive Outcome: This system always returns a winner.
As this particular voting system shows, a simple, seemingly logical system isn't always the best!