I think your argument is essentially an argument against speculation. All of the fears you cite apply equally to any form of speculation. The problem is, many

I think your argument is essentially an argument against speculation. All of the fears you cite apply equally to any form of speculation. The problem is, many things count as speculation -- a farmer who produces a commodity buying insurance against a poor crop is ultimately a futures contract. The way I think of HFT is that it's like increasing the fidelity of a recording from 11 kilohertz to 44.1 kilohertz. There are a lot more intermediate data points, but the underlying rationality (or irrationality) of pricing trends will be the same. Before HFT, it was more likely that a bid/ask spread would be higher ... reflecting the risk aversion of the market making firm... With HFT, that spread decreases b/c the liquidity is offered as part of a small timeframe strategy which may or may not pay off. If you weren't concerned with that short timeframe, then it makes no difference to you whether your order was filled by one party (a traditional market maker) or 100 parties (each a different HFT algorithm). I think to assuage your fear I'd probably just say that it's very difficult to move large numbers of shares at a price that's all that different from the previous price... So any HFT algorithm will necessarily be dealing either with largely uncontroversial prices or very small share volumes. Considering that, I think the overall risk of rogue algorithms leading to a harmful feedback loop is not significantly increased by HFT, and HFT was mostly a scapegoat in the recent crash.