In 1981, Rolf Banz, a graduate student at the University of Chicago, investigated the returns on stocks using the database provided by the Center for Research in Security Prices (CRSP). He found that
small stocks systematically outperformed large stocks, even after adjusting for risk as defined within the framework of the capital asset pricing model.
Some analysts maintain that the superior historical returns on small stocks are
compensation for the higher transaction costs of acquiring or disposing of these securities. This means that there may be an
extra return for illiquidity. Yet, for long-term investors who do not trade small stocks, transactions costs should not be of great importance. The reasons for the excessive returns to small stocks are difficult to explain from an efficient markets standpoint.
Although the historical return on small stocks has outpaced that of large stocks since 1926,
the magnitude of the small stock premium has waxed and waned unpredictably over time.