High-frequency trading - split seconds Constraining the relentless advance of technology is rarely easy. But that is no excuse for not trying when its potential effects may be damaging. High-frequency trading, via ultra-fast computerised programmes, does not pose huge societal risks. But it does bring very limited general benefits. Advocates claim that it provides liquidity and improves price discovery, but these gains can prove ephemeral if the supply of liquidity is itself highly volatile. And while the evils of HFT are often overstated, its proliferation has on occasion hampered robust, transparent "quality" markets to the detriment of other investors. High-frequency traders may not have initiated the 2010 "flash crash" in the US - when the total US stock market capitalisation fell by $1tn in 30 minutes. But their presence made the tailspin harder and faster. European regulators are right to tackle HFT as part of their current overhaul of market rules, the so-called Mifid II legislation. The question is how. Numerous ideas have been mooted, some of which look better than others. Circuit breakers are a useful tool, and the proposal that market venues should be able to halt trading in a harmonised fashion is sensible. Imposing unexecuted order to trade ratios also has merit - although some differentiation to reflect underlying product liquidity is also desirable. Insisting on a minimum resting time - so orders have to stay in the market for at least half a second, say - is more problematic: traders could simply widen spreads to compensate. And requiring algorithmic traders to quote continuously needs a carve-out for truly exceptional circumstances. European parliamentarians decided their stance on Wednesday. But this is a first step only. Reaching agreement with the European Union's member states may prove as protracted as HFT is swift.