One argument I see often made most recently here: http://www.abc.net.au/news/2016-05-14/why-do-we-have-a-two-party-preferred-system/7413578 http://www.abc.net.

One argument I see often made most recently here: http://www.abc.net.au/news/2016-05-14/why-do-we-have-a-two-party-preferred-system/7413578 http://www.abc.net.au/news/2016-05-14/why-do-we-have-a-two-p... is compulsory voting in Australia entrenches the dominance of Labor and the Coalition (the two established parties). However given the non-compulsory US political system is also dominated by two parties I'm not sure how true the argument is.