Your question about the relationship between voting systems and political parties implies that one causes the other. That's not the case. It's true that the number of parties represented in government varies with the type of voting system present. It's also true that there are a lot of cases where...
Your question about the relationship between voting systems and political parties implies that one causes the other. That's not the case. It's true that the number of parties represented in government varies with the type of voting system present. It's also true that there are a lot of cases where you have more or fewer parties represented that you might expect. If you want to know the details of the types of voting systems discussed here, look them up or click the links below.
The first-past-the-post voting system, used in the United States, the United Kingdom, Canada, and dozens of smaller countries, is commonly associated with two dominant political parties. That's certainly the case in the United States, though the number of Independent Party elected officials is increasing. Canada has six parties represented in its parliament, and the UK has seven.
All other voting systems are either hybrids of first-past-the-post and something else or versions of proportional representation. These other systems are generally thought to permit a greater number of parties to have representation in government. In general, these alternative systems give about the same number of parties seats in their national parliaments as first-past-the-post systems. Australia has thirteen parties represented in its federal parliament, Belgium has fourteen, Austria has six, France has seven, Germany has five, Japan has six, and India has seven. Italy's parliament is dominated by three coalitions, which have their own leadership and agenda and function as super-parties.
In all these cases, regardless of the voting system present, two parties dominate national politics. In some cases, a third and fourth party hold the balance of power between the two major ones, when those are about equal in representation, and they can decide which party gets to form a government.
So, even though there may be many smaller parties represented in a parliament—most with a single seat and many with fewer than ten seats—those smaller parties usually ally themselves with a dominant party, as in France, Germany, and with Independents in the United States, or they club together into coalitions that vote as blocs, as they do in Italy, Australia, and Belgium. For all intents and purposes, just about every country that's not a single-party state or a dictatorship has two dominant political parties even though they may have different voting systems.