We propose a simple indicator function based on the aggregate tug-of-war between overnight and intraday traders, and use it to identify two types of trading days: quiet and noisy days. We analyze these days and document that the security market line is upward sloping on quiet days and downward sloping on noisy days. This result is robust to a number of additional tests. Moreover, the result holds on both (i) important macroeconomic and earnings news days and (ii) other days, challenging some proposed explanations in the literature. We present and test a mechanism based on the over-correction hypothesis to rationalize the finding.
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