Stock markets are supposed to be efficient. Rational actors continually process data in a methodical way and spit out the appropriate price. But today, the market is increasingly peopled by machines–algorithms, programs, trading bots, and high-frequency traders swapping large and small bits of stock at an insanely rapid pace. High-frequency trading accounts for about half the volume on U.S. stock exchanges. And in recent years, we’ve learned that dispassionate machines and software can act in ways that seem highly irrational.