The internal structure of these organizations is the subject of May’s law of curvilinear disparity. Richard Katz and Peter Mair argued that some of these organizations function as cartels. Applying the same formula that is used in other fields to compute the Simpson index and the Herfindahl–Hirschman index to these organizations, Laakso (“LOCK-soh”) and Taagepera (“TAH-gay-pay-rah”) quantified their effective number. The expressive, instrumental, and representative functions of these organizations were distinguished by Lipset and Rokkan, who argued that cleavage structures are responsible for their formation. According to Duverger’s (“doo-vair-ZHAY’s”) law, systems such as first-past-the-post tend to have exactly two of these organizations with a significant amount of power. For 10 points, the United States is an example of a system dominated by exactly two of what political organizations? ■END■
ANSWER: political parties [or political party]
<Social Science>
= Average correct buzz position