This paper finds that the market betas of value and small stocks have decreased by about 75% in the second half of the twentieth century. The decline in beta can be related to a long-term improvement in economic conditions that made these companies less risky.

In the first three decades of CRSP data, value stocks have higher betas than growth stocks. Later on, the ranking is reversed and the gap in beta widens. What makes growth strategies nowadays bear more market risk than value strategies? What are the causes of the reversal in the ranking of...