July 5 (Bloomberg) -- There is only one real law of economics: the law of supply and demand. If the quantity supplied goes up, the price goes down.

Back in the third quarter of 2008, the public held about $5.3 trillion of U.S. Treasury bills, notes and bonds. As the recession hit, tax revenue plummeted, and government spending rose, that total reached $9.4 trillion by mid-2011.

We’re on target to have $10.7 trillion outstanding by mid-2012 -- doubling the Treasury debt held by the public in just four years. Supply and demand tells us that a steep rise in Treasury borrowings should produce a commensurate fall in Treasury bond prices and thus higher interest rates -- and that increase should crowd out other forms of interest-sensitive spending, slowing productivity growth.

Yet the market has swallowed all these issues without so much as a burp. By all accounts, it’s smacking its lips in anticipation of the next tranches.

In the years of the Clinton budget surpluses -- remember those? -- the U.S. government was repaying $60 billion of debt each quarter. The Bush administration worked hard to make that surplus evaporate. It succeeded.

From 2002 to 2007, the Treasury issued, on average, $70 billion of debt per quarter. Like many watching this shift, I concluded that this expanded supply would exert substantial pressure on interest rates to rise.

Treasury Demand

The demand for Treasuries was inordinately high, in part because the supply of alternatives was low. Lacking confidence, corporate executives held back investment, reducing private issues of bonds. In addition, China and other emerging economies, eager to keep their currency values low, directed dollars earned from exports into U.S. Treasury debt. Reinforcing this demand, wealthy individuals around the world purchased Treasuries as a hedge.

Thus by late 2007, the 10-year U.S. Treasury rate was exactly where it had been when the Clinton surpluses ended at the close of 2001. “How long could this go on?” we wondered. Eventually the market’s appetite for Treasury bonds at high prices and low interest rates had to reach its limit, right? Supply and demand isn’t just a good idea -- it’s the law.

Discovering the Limit

At the end of 2008, as the economy collapsed and the pace of net Treasury debt increases quintupled, it seemed we were about to discover that limit. I presumed we had a little time for expansionary fiscal policy to boost the economy -- a year, maybe 18 months -- before the bond-market vigilantes would arrive. They would demand higher interest rates on Treasury bonds, which would begin seriously crowding out the benefits of fiscal stimulus. The U.S. government would have to react, pivoting from fighting joblessness, via deficit spending, to reassuring the bond market via long-run tax increases and spending cuts to Medicare and Medicaid.

But it didn’t happen in 2009. It didn’t happen in 2010. And it isn’t happening in 2011. There are no signs from asset prices that the market is betting heavily that it will happen in 2012. Looking at the yield curve, it appears the market intends to swallow every single bond that the Treasury will issue in the foreseeable future -- and at high prices. The prices of inflation-protected bonds suggest that the market expects the new Treasury issues to be devoured without any acceleration in inflation.

Keynes Disciple

Although I worked for three years in the Clinton Treasury Department, and am a card-carrying member of the economist guild, I predicted none of this. Like most of my peers, I was wrong. Yet the most interesting thing is that I could have -- should have -- been right. I had read economist John Hicks; I just didn’t quite believe him.

Hicks, one of the clever young Brits dotting i’s and crossing t’s in the writings of John Maynard Keynes in the 1930s, was responsible for the workhorse formulation of Keynesian economics -- the IS-LM model -- that has been the bane of many an intermediate macroeconomics student. It was his version of the IS-LM model that formalized and elevated a key insight: that interest rates paid by creditworthy governments would remain low after a financial crisis. This formulation holds even in the face of enormous budget deficits that greatly expand the supply of government bonds.

A financial crisis initiates a sudden flight to safety among bondholders -- widening interest-rate spreads, diminishing the private sector’s desire to sell bonds to raise capital and encouraging individuals to save more and consume less as they, too, hunker down. Thus bond prices rise, and interest rates drop. As rates fall, firms see that they can get capital on attractive terms and so issue more bonds; households see the low interest rate earned on their savings and lose some of their desire to save. The market heads toward equilibrium.

Safeguarding Wealth

But something else happens on the path to equilibrium. The decline in interest rates and the rise in savings are accompanied by an increased desire among businesses and households to safeguard more of their wealth in cash. As a result, the speed with which cash turns over in the economy, the velocity of money, falls. And as the velocity of money falls, total spending falls, workers are fired, and their savings evaporate with their incomes.

Thus the equilibrium turns negative, with high unemployment and low capacity utilization.

In responding to a small financial disruption, the Federal Reserve can inject more money into the economy by buying bonds for cash, increasing the amount of cash so that even at the lower velocity of money we retain the same volume of spending. This eases the decline in interest rates, spending, employment and production into a decline in interest rates alone.

Little Difference

But when rates become so low that there’s little difference between cash and short-term government bonds, open-market operations cease having an effect; they simply swap one zero-yielding government asset for another, with their hunger to hold more safe, liquid assets unsatisfied.

This is the liquidity trap.

In this situation we need deficit spending. The government spends and borrows, creating more of the safe, cashlike assets that private investors want. As these bonds hit the market, people who otherwise would have socked their money away in cash -- diminishing monetary velocity and slowing spending -- buy bonds instead. A large, timely government deficit thus short-circuits the adjustment mechanism, avoiding the collapse in monetary velocity.

Hicks’s conclusion: As long as output remains depressed and there is slack in the economy, printing more bonds will have negligible effect in increasing interest rates.

Special Case

I had read Hicks. I even knew Hicks. But I thought that his era, the Great Depression, had passed. Sitting in my first graduate economics class in 1980, I listened to Marty Feldstein and Olivier Blanchard -- two of the smartest humans I am ever likely to see -- assure me that Hicks’s liquidity trap was a very special case, into which the economy was unlikely to wedge itself again. Yet it did.

On my shelf is a slim, turn-of-the-millennium volume by Paul Krugman titled “The Return of Depression Economics.” In it he argued that we mainstream economists had been too quick to ditch the insights of Hicks -- and of economists Walter Bagehot and Hyman Minsky. Krugman warned that their analysis was still relevant, and that if we dismissed it we would be sorry.

I am sorry.

(Brad DeLong, a former deputy assistant secretary of the U.S. Treasury, is a professor of economics at the University of California at Berkeley, where he is chair of the political economy major. The opinions expressed are his own.)

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To contact the writer on this story: Brad DeLong at delong@econ.berkeley.edu.

To contact the editor responsible for this story: Francis Wilkinson at fwilkinson1@bloomberg.net.