The authors found that the rise in popularity of index trading—assets
invested in index funds reached more than $1 trillion at the end of
2010—contributes to higher systematic equity market risk. More equity
index trading corresponds to increased cross-sectional trading commonality,
which precipitates higher return correlations among stocks. Consistent with the
accelerating growth of passive trading, the authors found that equity betas have
not only risen but also converged in recent years.