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

Abstract

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.

Keywords: Portfolio Returns, Portfolio Growth Rates, Size Effect, Long-Term Stock Returns

JEL Classification: G11, G12, G14