Working Paper No. 80 (March 2015)
JEL Classifications: G12, G14, G19, D4

Valeria Caivano (
CONSOB, Research Department


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The huge increase of HFT activity in recent years has posed the crucial question of whether it is beneficial for financial markets to both researchers and regulators. Recent academic research has studied the impact of HFT on different measures of market quality, such as liquidity, transaction costs, market integrity and efficiency, though the results are sometimes non conclusive. This study focuses on the impact of HFT on stock price volatility over the period 2011-2013 for a sample of 35 blue chips traded on Borsa Italiana. High frequency traders (HFTrs) are identified according to two methods. The first one, based on public information on the trading strategies of market participants, led us to identify 14 traders (so called "pure" HFT firms). The second one includes the main investment banks active in the European markets, since they carry out some proprietary trading which could take the form of HFT (as stemming from the evidence reported in ESMA, 2014). These approaches allow the identification of a lower and upper bound for the actual share of HFT on total trading volume. Potential endogeneity of HFT is controlled through an instrumental variable approach, using as an instrument the introduction of a new trading platform that eased the HFT activity by decreasing the latency. Results show that an exogenous increase of HFT activity causes a statistically and economically significant increase in volatility. In details, an increase by one standard deviation of HFT activity carried out by "pure" HFT firms raises volatility by an amount between 0.5 and 0.8 standard deviations. This means that, if HFT activity increases by 10 percentage points the annualized intraday volatility increases by an amount between 4 and 6 percentage points depending on the specification used. If we also take into account the activity carried out by investment banks the impact of an increase by 10 percentage points of HFT activity leads to an increase of annualized volatility by an amount between 3 and 5 percentage points. This paper adds to the existing literature by providing new empirical evidence from the Italian market. Furthermore, it contributes to the policy debate, which had recently led the European regulators to introduce new rules aimed at mitigating possible negative effects of HFT.

The author thanks Francesco Scalese for very helpful contribution on the construction of the database used in this study. Of course, the author is the only responsible for errors and imprecisions. The opinions expressed here are those of the author and do not necessarily reflect those of Consob.

ISSN 2281-3519