What Drove the Increase in Idiosyncratic Volatility during the Internet Boom?

Aggregate idiosyncratic volatility spiked nearly fivefold during the Internet boom of the late 1990s dwarfing in magnitude a moderately increasing trend While some researchers argue that this rise in idiosyncratic risk was the result of changes in the characteristics of public firms others argue that it was driven by the changing sentiment of irrational traders We present evidence that the marketwide decline in maturity of the typical public firm can explain most of the increase in firm specific risk during the Internet boom Controlling for firm maturity we find no evidence that investor sentiment drives idiosyncratic risk throughout the Internet boom