Anyone trying to understand the crisis of 2007-2008 should be paying close attention. The central mystery of the crisis (and, indeed, many other crises) is how a relatively minor initial loss—$500 billion to $2 trillion, of the same order of magnitude as the change in the value of equities during a weak month—somehow led to a much larger collapse in the economy as a whole. As Brad DeLong puzzled in late 2008:

And our models for why the risk discount has taken such a huge upward leap in the past year and a half are little better than simple handwaving and just-so stories. Our current financial crisis remains largely a mystery: a $2 trillion impulse in lost value of securitized mortgages has set in motion a financial accelerator that we do not understand at any deep level but that has led to ten times the total losses in financial wealth of the impulse.

Now imagine that we have a shock to demand for liquidity in general rather than cash. For whatever reason, investors want to park more of their portfolio in T-bills and repos, and less in illiquid bonds and equities. At the same time, imagine that the supply of liquidity is actually decreasing—the “AAA” tranches of mortgage-backed securities are no longer acceptable collateral, and in general it’s much harder to transform real, risky assets into liquid, riskless ones. What happens? Remember that with a shock to cash demand, nothing happened as long as the Fed targeted the federal funds rate: it would supply whatever additional cash was necessary to keep the interest rate at its target level. But with shocks to liquidity supply and demand, the world isn’t so rosy. At least traditionally, the Fed doesn’t target the liquidity premium. (Though QE/QE2 may be viewed as attempts to do just that.) Classic “open-market operations” are just trades of base money for T-bills—one liquid asset for another—leaving supply unchanged. As a result, if short-term supply and demand for liquid assets are inelastic, even a small shock can produce a large spike in the liquidity premium, and a rise in interest rates in general. In exactly the same way that a few gas station owners demanding cash could raise the price of credit across the economy, a small decline in supply or increase in demand for liquidity can force up the interest rates paid by all businesses and consumers, leading to a severe contraction. Now, if the Fed realizes what’s happening, it can offset the effect of an increase in the liquidity premium by decreasing the federal funds rate. But this standard reaction may be delayed, and eventually it becomes impotent: the federal funds rate hits zero, and exchanging money for T-bills can do no more.

At this point, the fate of all assets in the economy is determined by the idiosyncrasies of the market for liquidity—which, like the market for cash, may be very small relative to the economy as a whole. A few pension funds deciding to move $100 billion (<1% of GDP) from real estate into liquid assets can—by increasing demand in an inelastic market—cause a noticeable runup in all interest rates, interest rates that govern almost $50 trillion in household financial assets. On the supply side, we’ll see the same effect from a change in market expectations that renders useless $1 trillion in repo collateral.... And once the economy has reached the zero lower bound—and money is just another liquid asset—minor shocks to the supply and demand of liquid assets can have catastrophic consequences. We saw this in 2008. Even as the Fed eventually brought the federal funds rate down to zero, the rates that actually mattered in the economy shot upward.

What does this all mean? For one, it suggests that the quasi-monetarists were right. As David Beckworth argues, there was a great liquidity shock in 2007-2009 that made monetary policy effectively very tight. This doesn’t mean that monetary policy can fix every problem—not all zero lower bound episodes are caused by a liquidity shock, and in those cases zero is a very real barrier. But when a shortage of liquid assets is the main issue, the Fed has a simple remedy: trade money for illiquid assets.... We already know how a quantitatively trivial shock can devastate an economy: the formidable Volcker tightening, which led to the highest unemployment rate since the Great Depression, was just a small blip in the monetary base. We understand all this with the market for money. Yet somehow we haven’t managed to make the connection to the market for liquidity.

Now we understand why demand for money--what I call liquidity--is so interest-inelastic. You need money to buy stuff. If you don't have money, you can't buy stuff--and so when you are short of money you cut back your spending because you must and so build your money balances back up.

But why is the demand for what Matt calls "liquidity" and I call "safety" so interest-inelastic in a financial crisis? It's not that you have to cut back on your spending on currently-produced goods and services--you have plenty of cash money. But you are unwilling to part with some of your cash money because it is now--and here our terminological problem begins--part of your holdings of liquid cash money are now in the the delong-safe or the rognlie-liquid tranche of your portfolio that you feel you must retain at all costs.

But why must you retain it? Why not buy risky assets when there is blood in the streets? Why not become a stabilizing speculator and become a supplier of rather than a demander of delong-safe or rognlie-liquid assets?

We know why people don't turn around and become suppliers of liquid cash money when the money stock contracts: they can't, for nobody else's liabilities are good as payment for transactions in currently-produced goods and services. But surely Berkshire Hathaway or Microsoft or Northrup-Grumman could have sold lots of bonds at attractive values. Why didn't they?

That's the question I don't think I have a very good answer to. And that's why I wrote:

our models for why the risk discount has taken such a huge upward leap in the past year and a half are little better than simple handwaving and just-so stories. Our current financial crisis remains largely a mystery: a $2 trillion impulse in lost value of securitized mortgages has set in motion a financial accelerator that we do not understand at any deep level but that has led to ten times the total losses in financial wealth of the impulse.