University of Chicago law professor Aziz Huq said it has been 80 or 90 years since the chief justice was also the court's median justice. "It creates some interesting questions, because the chief justice is supposed to be institutionally minded," he said. "We evaluate chief justices in terms of the performance of the court."

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The chief justice, however, also has shown an affinity in many circumstances for narrow, incremental rulings that pick up more votes, and legal observers say his strong sense of stewardship means he won't want the court to be seen as a partisan body that decides all of the nation's big legal issues on 5-4 votes.

"Roberts is a pretty cabined and cautious opinion writer, and that might be due to him being chief," said Mr. Huq.

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