IPO underpricing during the boom: a block-booking explanation

A bank can efficiently underwrite individually difficult to value IPOs by offering them as a package deal to a stable coalition of investors (block-booking). Block-booking banks set offer prices to equalize downside risk across their offerings, not expected returns. Examining US IPOs over the 1986 to 2003 period, I find that this is so. Given the return distribution on non-tech IPOs during non-boom years, equalizing downside risk implies that the average initial return on tech/boom IPOs equals 48% (actual value: 46%). The block-booking theory accounts for both the direction and magnitude of differences in average initial returns across IPO types.