A recent publication in AEI discusses some of the reasons behind American success and it cites it's two-party system as one of those reasons. Two parties keep taxes low while many parties encourage higher taxes.
Under a PR system, several parties will compete, while in majoritarian systems, only two parties usually contest elections. If there are several parties, middle-class voters will support programs that tax the rich and benefit them, knowing that they can change their voting habits if a government wishes to tax them more. But if there are only two major parties, middle-class voters will worry that voting for leftist parties will mean more taxes for them, and so they will be inclined to support right-wing parties.
I don't see how this logic holds. In multi-party countries, these parties create coalitions and effectively transform into a handful of contending parties. A majority still have to approve the new government. In practice, three changes in the system come to mind: the minority view is more likely to be considered, platforms are more flexible because parties can mix and match their allies and, my favorite, politicians have to spend a great deal of time forming coalitions, time they can't spend governing.
The author, James Q. Wilson, is confusing voting for parties with voting for policies. If a vote to raise taxes for the rich were held to the people, they would probably cast in favor for it. But if a party wants to do that, there is still the danger that they could raise taxes on everyone. I'll buy that competition tends to create things people want but politicians tend be expections. They are package deals of package deals. Almost anything goes and everyone knows it.