Comovement revisited

We find, unlike earlier studies, that there is no rise in the market betas of stocks that enter the S&P 500 index when the estimated factor model is that of Fama and French (1993). We also find that SMB and HML factor betas decline after the stocks are added to the index. This decline is explained by strong increases in earnings and in the market value of the event stocks in the period around—and, in particular, prior to—their inclusion in the index. We suggest that inclusions to the S&P 500 index are informative events that trigger a reassessment of the risk of newly added firms by drawing the broad market’s attention to their extraordinary growth in size and profitability.