Jason T. Greene

Market Diversity and the Performance of ActivelyManaged Portfolios

Agapova, Ferguson, and Greene examine a theoretically motivated measure of the “size effect” known as market diversity and link it to the relative returns of institutional actively managed portfolios. Market diversity reflects how disperse or concentrated capital is across firms in the market, with changes in market diversity reflecting movement of capital between relatively large firms to relatively small firms. Changes in market diversity explain a statistically and economically significant amount of variation in the relative returns of actively managed institutional large-cap strategies. The authors estimate that an increase (decrease) in market diversity of 1% leads to an average increase (decrease) in relative returns of approximately 30 basis points, with higher tracking error strategies showing relatively more sensitivity. They find that another measure of the size effect, the Fama–French small-minus-big factor, explains less of the variation in actively managed large-cap strategies’ relative returns and is rejected in favor of changes in market diversity as the underlying explanatory variable for actively managed strategies’ relative returns. The authors suggest that market diversity provides academics and practitioners an important measure of market conditions when evaluating the performance of actively managed portfolios.

The Sources of Portfolio Returns: Underlying Stock Returns and the Excess Growth Rate

This paper decomposes portfolio returns into the underlying sources arising from the constituent stocks’ growth rates, as well as their variances and covariances. We employ this method to show that the difference between large and small stock portfolio returns is driven by a portfolio “excess growth rate” that is induced by the higher volatility of small stocks’ returns and not by the average growth rate of small stocks. Therefore, the “size effect” is not a small firm effect, but a small firm portfolio effect driven by the excess growth rate of the portfolios. In contrast, portfolios of high book-to-market stocks outperform due to higher average levels of growth by the constituent stocks and not due to their variance-covariance structure. Our results demonstrate the importance of considering the sources of portfolio performance as possibly distinct from the performance of the portfolio’s underlying stocks when designing and interpreting studies of portfolio performance, corporate events, or the cross-section of stock returns.