Stock markets fragmentation, volatility and final investors

The 2000s in equity markets are marked by two major regulatory shocks: RegNMS in the United States, and MiFID in the European Union. Simultaneously, there is a massive increase in the proportion of high-frequency trading, and market orders volume. However, trading volumes do not significantly increase. We propose a theoretical model describing the effects of stock markets fragmentation on two types of investors optimization problems: “intermediary” high-frequency and “final” investors. Volatility has a permanent and a transitory component, whose weights depend on market fragmentation via the share of non-marketable orders of intermediary investors. The trading volume of final investors depends on market fragmentation both directly via transaction costs, and indirectly via total volatility. Finally a shock in fragmentation may lead to a decrease in trading volume, enhanced in the case of an equity markets crisis by a rise in the components of volatility.