Why does stock volatility increase when output declines? The theory of investment under uncertainty implies that political uncertainty may simultaneously increase volatility and reduce output. Though cause and effect are typically hard to separate, the transition from Imperial to Weimar Germany offers a natural experiment because major political events left clear traces on stock prices. Current and past increases in volatility are associated with output declines, consistent with U.S. experience. However, political events are more clearly the source of volatility, and the results support the view that the relationship between volatility and output reflects the joint effects of political factors.

Periodic antitrust attacks on corporations may have influenced stock prices. For the period 1904 to 1944, each antitrust case filed is associated with a 0.5 to 1.9 percent drop of the Dow, and each unexpected case with even larger drops. Other aspects of antitrust besides actual filings may help account for other movements, in particular the 1929 Crash. Historical evidence bears on the question of whether antitrust is exogenous and also links antitrust and the “corporation problem.” These results illustrate the sorts of real factors aside from changes in concurrent output that may account for stock price volatility.

Periodic antitrust attacks on corporations may have influenced stock prices. For the period 1904 to 1944, each antitrust case filed is associated with a 0.5 to 1.9 percent drop of the Dow, and each unexpected case with even larger drops. Other aspects of antitrust besides actual filings may help account for other movements, in particular the 1929 Crash. Historical evidence bears on the question of whether antitrust is exogenous and also links antitrust and the “corporation problem.” These results illustrate the sorts of real factors aside from changes in concurrent output that may account for stock price volatility.