Now thanks to a new report by ESMA titled "High-frequency trading activity in EU equity markets" we know: in the lifetime total of all orders, HFT accounts for 76% of all orders by, 49% of all trades and 43% of total value traded.
http://www.zerohedge.com/news/2014-12-19/hft-accounts-76-all-orders-europethe day the EU market slips, there will be no mercy. robots just dont do mercy.
I think the bigger problem is now that every time the robots figure out that it is time the market crashed then the humans panic and turn the robots off, reset and claim 'fat finger' foul or otherwise ... and this gives rise to those crazy spikes and flash crashes, etc.
Then there are the robots doing the work for the Fed and Treasury (ESF) that are more like closed-loop control algorithms working large multi-variate arrays of pricing to 'stabilise' the whole system using cash as a control input, priming humans expectations of pricing and thus financial incentive structures ... which all seems to work fine on 'normal' days but start demanding massive cash inputs on panic days (like 'hey the system just disappeared $700 billion, how did that happen? start bailing and pumping or this thing is gonna lock up')