Political polarization and the two-party system

America has been a two-party system throughout our history, and most Americans regard this arrangement as natural. But among the world's democracies it is fairly unusual: most democracies have four or more competitive political parties. Beyond being unusual, it is also harmful to our democracy, argues political scientist Lee Drutman. What is the connection between today's extreme political polarization and the two-party system? Is there anything we can do to change it?