You can read the study here, a quick summary from Reuters:

There is no evidence shareholder proposals increase a company's value in the short or long term, according to a study commissioned by the U.S. Chamber of Commerce released on Monday. "While proposals may be successful in making qualitative changes in companies' actions, there is little to no evidence that those changes have an impact on the bottom line of target firms," the report said.
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The study conducted by Navigant Consulting selects shareholder proposals from the labor federation AFL-CIO's 2002 to 2008 survey on key shareholder proposals. The study, which examined 166 shareholder proposals at 107 companies, said it found no evidence of an improvement in companies' stock market performance in either the short-run or the long-run.
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"(The study) adds to the growing body of evidence that union proxy campaigns provide no value to shareholders," said Thomas Donohue, the president of the Chamber of Commerce, the biggest U.S. business lobby.  "If union pension funds really want to help shareholders, they should stop squandering their members' retirement on political crusades through the proxy process," he said in a statement.