Brad DeLong can't decide whether or not Greenspan made a mistake when he kept interest rates low after the collapse of the dot.com bubble:
Sympathy for Greenspan, by J. Bradford DeLong, Commentary, Project Syndicate: In the circles in which I travel, there is near-universal consensus that America’s monetary authorities made three serious mistakes that contributed to and exacerbated the financial crisis. ... US policymakers erred when:
-the decision was made to eschew principles-based regulation and allow the shadow banking sector to grow with respect to its leverage and its compensation schemes, in the belief that the government’s guarantee of the commercial banking system was enough to keep us out of trouble;
-the Fed and the Treasury decided, once we were in trouble, to nationalise AIG and pay its bills rather than to support its counterparties, which allowed financiers to pretend that their strategies were fundamentally sound;
-the Fed and the Treasury decided to let Lehman Brothers go into uncontrolled bankruptcy in order to try to teach financiers that having an ill-capitalised counterparty was not without risk, and that people should not expect the government to come to their rescue automatically.
There is, however, a lively debate about whether there was a fourth big mistake: Alan Greenspan’s decision in 2001-2004 to push and keep nominal interest rates on US Treasury securities very low in order to try to keep the economy near full employment. In other words, should Greenspan have kept interest rates higher and triggered a recession in order to avert the growth of a housing bubble? ...
Full employment is better than high unemployment if it can be accomplished without inflation, Greenspan thought. If a bubble develops, and if the bubble ... collapses, threatening to cause a depression, the Fed would have the policy tools to short-circuit that chain. In hindsight, Greenspan was wrong. But the question is: was the bet that Greenspan made a favourable one? ...
I am genuinely unsure as to which side I come down on in this debate. ... What I do know is that the way the issue is usually posed is wrong. People claim that Greenspan’s Fed “aggressively pushed interest rates below a natural level.” But what is the natural level? In the 1920’s, Swedish economist Knut Wicksell defined it as the interest rate at which, economy-wide, desired investment equals desired savings, implying no upward pressure on consumer prices, resource prices, or wages as aggregate demand outruns supply, and no downward pressure on these prices as supply exceeds demand.
On Wicksell’s definition — the best, and, in fact, the only definition I know of — the market interest rate was, if anything, above the natural interest rate in the early 2000’s: the threat was deflation, not accelerating inflation. The natural interest rate was low because, as the Fed’s current chairman Ben Bernanke explained at the time, the world had a global savings glut (or, rather, a global investment deficiency). ...
Greenspan’s mistake — if it was a mistake — was his failure to overrule the market and aggressively push the interest rate up above its natural rate, which would have deepened and prolonged the recession that started in 2001.
But today is one of those days when I don’t think that Greenspan’s failure to raise interest rates above the natural rate to generate high unemployment and avert the growth of a mortgage-finance bubble was a mistake. There were plenty of other mistakes that generated the catastrophe that faces us today.
I have argued the Fed's decision to keep interest rates low contributed to the bubble, but was not itself the sole cause of it. As to whether the Fed made a mistake, I'll just note that the tradeoff wasn't quite as stark as Brad implies, i.e. there were other policy instruments that Fed could have used to limit the housing bubble. Regulation is certainly one means the Fed had to that end, but Fed communication could have helped too. If Greenspan had, for example, told people to stay away from mortgages because they were toxic rather than implicitly encouraging them to invest in housing, things might have been different.
Would limiting the bubble through regulation, communication, or other means have limited the employment response, the primary worry? I don't think so, at least not enough to matter. The money would have been invested somewhere, housing had an opportunity cost after all, so the next best alternatives would have been pursued to the extent that they were profitable (and many would have been, just not as profitable - apparently anyway - as investing in housing and mortgages). So people still would have been employed somewhere as the money was invested, just not in housing, and that would have helped to insulate us from the housing crash. (And a lot of them might still have those jobs, unlike the people who depended upon the housing markets for employment.)
So narrowly, keeping interest rates low and employment high was the right thing to do. The mistake was letting all of the action brought about by those low rates, or most of it anyway, occur in a single sector, housing, rather than using regulation and other means to limit the flow of resources into the housing market in pursuit of profits based upon the misperception of risk. Those resources could have been redirected into other sectors and put to productive use rather than wasted building houses nobody wants, and achieving this result did not require the Fed to aggressively raise the target rate, it only needed to use the other tools it already had available.
Unfortunately, however, those tools were not used, and the ideology Greenspan brought to the Fed played a large role in this outcome.