This paper analyzes the empirical behavior of stock-return volatilities prior to and subsequent to the ex-dates of stock splits. The evidence demonstrates rather unambiguously that there is, on the average, an approximately 30% "arbitrary" increase in the return standard deviations following the ex-date. The increase holds for both daily and weekly data, and it is not temporary. No explanatory confounding variables, such as institutional frictions affecting price observations, have been identified. We view the findings as being essentially inconsistent with the notion of "rational pricing."
Ohlson, James, and Stephen Penman. "Volatility Increases Subsequent to Stock Splits: An Empirical Aberration." Journal of Financial Economics 14, no. 2 (June 1985): 251-267.
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