Analyzing the Evidence Exercises

Most political scientists agree that the ideology of individual justices affects the decisions handed down by the Supreme Court. For this reason, a great deal of attention is focused on the composition of the Court. When a vacancy arises, the attitudes of the individual nominated by the president have the potential to dramatically alter the Court’s overall ideological persuasion. The reason for this is the median voter theorem, which holds that under certain conditions, the outcome of a group’s decision making will reflect the ideology of the group’s median member.i

Recall from the introductory Analyzing the Evidence section of your textbook that the median is a statistical term referring to the observation in the fiftieth percentile (half the cases have a larger value while half are lower). The figure below shows agreement scores for the 2009 term of the Supreme Court, which indicate the proportion of cases in which pairs of justices voted in the same way. The vertical line on the right translates the pattern of voting to an ideological scale, with the most liberal justice at the top and the most conservative at the bottom.

Agreement Scores for the 2009 Term

1. Using the scale at the right, which justice is the Court’s “median voter”?
2. How does this justice fare when you consider the agreement scores? (That is, which justice is most often in agreement with the others?)
3. How do you explain this pattern?
As you’ve seen, the median voter is an influential person on the Court. Open this figure from the New York Times, which plots the ideology of individual justices of the Court from the late 1950s through the late 2000s.ii The bold line identifies the median justice of the Court throughout its history.
Who was the median justice in 2000?
How did this change with the retirement of Justice Sandra Day O’Connor in 2006?
Did O’Connor’s departure (along with Chief Justice Rehnquist’s) shift the court in a more conservative or more liberal direction? Why?

i Black, Duncan. 1948. “On the Rationale of Group Decision-making.” Journal of Political Economy 56: 23–34. 

ii New York Times, “Measuring the Conservatism of the Roberts Court,” July 24, 2010, (accessed 2/22/12). 

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