Are markets irrationally exuberant or irrationally depressed today? It’s hard to tell.
This difficulty is no surprise. It's the central prediction of free-market economics, as crystallized by Hayek, that no academic, bureaucrat or regulator will ever be able to fully explain market price movements. Nobody knows what "fundamental" value is. If anyone could tell what the price of tomatoes should be, let alone the price of Microsoft stock, communism and central planning would have worked.
More deeply, the economist’s job is not to "explain" market fluctuations after the fact, to give a pleasant story on the evening news about why markets went up or down. Markets up? "A wave of positive sentiment." Markets went down? "Irrational pessimism." ( "The risk premium must have increased" is just as empty.) Our ancestors could do that. Really, is that an improvement on "Zeus had a fight with Apollo?" Good serious behavioral economists know this, and they are circumspect in their explanatory claims.
But this argument takes us away from the main point. The case for free markets never was that markets are perfect. The case for free markets is that government control of markets, especially asset markets, has always been much worse.
In fact, the behavioral view gives us a new and stronger argument against regulation and control. Regulators are just as human and irrational as market participants. If bankers are, in Krugman's words, "idiots," then so must be the typical treasury secretary, fed chairman, and regulatory staff. They act alone or in committees, where behavioral biases are much better documented than in market settings. They are still easily captured by industries, and face politically distorted incentives.
If you believe the Keynesian argument for stimulus, you should think Bernie Madoff is a hero. He took money from people who were saving it, and gave it to people who most assuredly were going to spend it. Each dollar so transferred, in Krugman's world, generates an additional dollar and a half of national income. The analogy is even closer. Madoff didn't just take money from his savers, he essentially borrowed it from them, giving them phony accounts with promises of great profits to come. This looks a lot like government debt.
If you believe the Keynesian argument for stimulus, you don't care how the money is spent. All this puffery about "infrastructure," monitoring, wise investment, jobs "created" and so on is pointless. Keynes thought the government should pay people to dig ditches and fill them up.