This rise of the robots certainly seems to have helped ordinary investors. Bid-ask spreads—the difference between what buyers want to pay and sellers want to be paid—have fallen dramatically the past 20 years. Part of this is because, since 2001, stock prices have gone from trading in fractions to pennies—which has allowed them to be increasingly precise. Another part is that electronic trading, though not super-fast, has made markets more liquid. And the last part is that HFT has added even more liquidity, eliminating bid-ask spreads that would have been too small to do so before. Indeed, researchers found that Canadian bid-ask spreads increased by 9 percent in 2012 after the government introduced fees that effectively limited HFT.

That doesn't mean, though, that HFT is unambiguously good. It's not. In fact, it might not even be ambiguously good. As Noah Smith points out, we just don't know enough to do any kind of cost-benefit analysis. Now, we do know that smaller bid-ask spreads, which cut the cost of trading, are one benefit. But how much of one is it? Bid-ask spreads are down to around 3 basis points today—from 90 basis points 20 years ago—so even if curbing HFT increases them, say, 9 percent like it did in Canada, we're not talking about a big effect. There might be diminishing returns to liquidity that we've already hit, and then some.

Then there are the costs. Michael Lewis' new book, Flash Boys, describes some of them. In it, there's Lewis' requisite group of plucky outsiders—is there another kind?—taking on a rotten status quo. Except this time, they're not really outsiders; they're big bank traders. And they've figured out that the market doesn't work like it should for big investors, like pension and mutual funds, because of the algobots. But it's a little bit more complicated than that. Here are the three biggest, though hard to quantify, costs of HFT.

1. Market-taking, not market-making. Lewis' protagonist, a trader named Brad Katsuyama, had a problem. Every time he tried to buy stock for a client, he could only get a little bit of what was supposed to be there at the price he saw. Now, oddly enough, he could get all the stock he saw at one particular exchange, but he had to pay more at all the others. What was going on?

Well, he was being front-run. HFT firms pay public and private exchanges to see their incoming orders. That's why Katsuyama was getting all of his order filled at the exchange closest to him—that is, as the fiber optic cable lies—but nowhere else. The HFTers were seeing his order at the first exchange and then racing to buy all the rest of the stock he wanted everywhere else, so they could sell it to him for more. This happens all the time: Nicholas Hirschey of the London Business School found that HFT funds only tend to buy aggressively right before everybody else does.