French sociologist Maurice Duverger was one of the first to study the impact of electoral systems on government types (ie, does one party control the legislature, or are coalitions formed?) He posited that what is termed "majoritarian" electoral systems (individual with the most votes in a district gets one seat in the legislature - which is what the US has) will, in the long run, lead to a political system dominated by two parties.

His hypothesis has been corroborated by subsequent studies. I highly recommend Arend Lijphart's "Patterns of Democracy" for a overview of electoral systems and its consequences on government types.