The Impact of Clean Air Act Amendments of 1990
on Electric Utilities and Coal Mines:
Evidence from the Stock Market
Shulamit Kahn, Boston University Department of Finance and Economics
Christopher R. Knittel, Department of Economics, University of California, Davis
Stock prices should reflect sudden changes in companies’ expected
profits due to new information about future environmental regulations.
We conduct an event study of President George H. Bush’s Clean Air Act
Amendment proposal of June 1989, which had surprising aspects. We find
that shares of 35 companies owning affected power plants did not noticeably
fall in value after the Bush announcement, nor after three preceding events
leading to this announcement. Instead, shares increased in value after
the announcement. In contrast, stock prices of practically all 12 coal
mining companies studied fell after the Bush announcement and after two
of the other three events (although significance levels make these results
not entirely conclusive). We argue that expected profits of electricity
companies did not fall because electricity price regulation and/or inelastic
electricity demand allowed cost increases to be passed through to customers.
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