This study clears up misunderstandings regarding the diversification of unsystematic risk. Contrary to conventional wisdom, there is no evidence investors can, or have ever been able to, easily form portfolios containing negligible exposure to unsystematic returns. Because well-diversified...

Firm-specific risk climbed steadily between 1962 and 1999, but fell sharply between 2000 and 2003. We hypothesize that changes in the composition of the market, rather than fundamental changes in the economy or return-generating process, drive these changes in aggregate firm-specific risk over...

Contrary to conventional wisdom, there is no evidence investors can, or have ever been able to, easily form portfolios containing negligible exposure to unsystematic returns. Because well-diversified portfolios are the bedrock upon which so much financial theory is built, investors' inability to...