We examine the impact of trading on markets partially exempt from National Market System requirements (“dark venues”) on equity-market quality. We find evidence consistent with the notion that dark venues rely on their special features to segregate order flow based on asymmetric information risk, which results in their transactions being less informed and contributing less to price discovery on the consolidated market. Except for the execution of large transactions and trading in small stocks, the effects of dark-venue order segmentation are damaging to overall market quality. Our results have important implications for the regulation of international equity markets.

We investigate competition between traditional stock exchanges and new dark trading venues using an important difference in regulatory treatment. Securities and Exchange Commission required minimum pricing increments constrain some stock spreads, causing large limit order queues. Dark pools allow some traders to bypass existing limit order queues with minimal price improvement. Using a regression discontinuity design, we find that spread constraints significantly weaken exchanges׳ competitiveness. As more orders migrate to dark pools, the probability of subsequent order execution there increases, raising liquidity. The ability to circumvent time priority of displayed limit orders is one cause of the rapid rise in US equity market fragmentation.