(I’ve selected one interesting bit, but there’s more; I recommend reading the whole thing)
When a market anomaly shows up, the worst possible question to ask is “what’s the fastest way for me to exploit this?” Instead, the first thing to do is to steelman it as aggressively as possible, and try to find any way you can to rationalize that such an anomaly would exist. Do stocks rise on Mondays? Well, maybe that means savvy investors have learned through long experience that it’s a good idea to take off risk before the weekend, and even if this approach loses money on average, maybe the one or two Mondays a decade where the market plummets at the open make it a winning strategy because the savvy hedgers are better-positioned to make the right trades within that set.[1] Sometimes, a perceived inefficiency is just measurement error: heavily-shorted stocks reliably underperform the market—until you account for borrow costs (and especially if you account for the fact that if you’re shorting them, there’s a good chance that your shorts will all rally on the same day your longs are underperforming). There’s even meta-efficiency at work in otherwise ridiculous things like gambling on 0DTE options or flipping meme stocks: converting money into fun is a legitimate economic activity, though there are prudent guardrails on it just in case someone finds that getting a steady amount of fun requires burning an excessive number of dollars.
These all flex the notion of efficiency a bit, but it’s important to enumerate them because they illustrate something annoying about the question of market efficiency: the more precisely you specify the definition, and the more carefully you enumerate all of the rational explanations for seemingly irrational activities, the more you’re describing a model of reality so complicated that it’s impossible to say whether it’s 50% or 90% or 1-ε efficient.
Some interesting thoughts on (in)efficient markets from Byrne Hobart, worth considering in the context of Inadequate Equilibria.
(I’ve selected one interesting bit, but there’s more; I recommend reading the whole thing)