We document how mechanical buying by CRSP-index-tracking funds 5 days post-IPO affects stock returns and IPO deal structure. Using a difference-in-differences design, we show that expected indexer demand causes Fast-Track IPOs to outperform their non-Fast-Track counterparts by over five percentage points, peaking at the index inclusion date and reverting significantly within 3 weeks. Anticipated CRSP index inclusion also affects IPO terms, with Fast-Track IPOs raising 6% more capital than their non-Fast-Track counterparts. Our findings support a proposed index rule change to eliminate a $5.8 billion “shadow tax” paid to intermediaries by index fund investors and firms raising capital through IPOs. (JEL G12, G14)

We find that firms are the primary sellers who clear the market for index fund buying, providing shares at a nearly one-for-one rate. Most demand-side institutions trade in the same direction as index funds rather than accommodating passive demand. We use two instruments for index fund demand and show that firms causally respond to exogenous passive demand, with prices serving as the coordinating mechanism. Firms satisfy passive demand mostly through nonprimary market issuance, for example, through employee stock-based compensation. Our results suggest that passive investing has systematically supplied capital to firms by enabling equity issuance over the last two decades.