In five easy steps:
Step one: Suppose Robinson Crusoe spends 30% of his time building or expanding his home, and 70% of the time catching fish and collecting fruit. There’s no money in his island economy, but you could say that in terms of hours worked, GDP is 70% consumption and 30% investment. Then Crusoe injures his arm, and his productivity falls by 10%. Call this a “real business cycle”. Economic theory predicts that Crusoe will engage in “consumption smoothing”. Most of the decline in output will show up in less investment, with very little change in consumption. He’ll keep eating, but postpone the expansion of his hut.
Step two: All business cycles predict that investment will be unusually procyclical, much more than consumption. But some models also suggest that investment shocks cause business cycles. Keynesians worry that a drop in animal spirits might depress investment, and this would reduce AD via the multiplier effect. Austrians worry that an excessive burst in investment might lead to malinvestment, and the resulting hangover could lead to a recession.
I believe those theories are misleading. Most modern recessions (except this one) are caused by unstable monetary policy (i.e. unstable NGDP.) Investment is highly procyclical, but that would certainly be true even if investment played no causal role in the recession, as in the deserted island example.
Step three: When people like Keynes and Hayek developed their business cycle theories, however, investment shocks really did cause business cycles. That’s because most developed countries used to be on some form of the gold standard. When the nominal price of gold is stabilized by monetary policy, i.e. when gold is the medium of account, then an increase in demand for gold is deflationary. It also reduces NGDP.
Barsky and Summers showed that falling nominal interest rates increase the demand for gold, which is why nominal interest rates were positively correlated with the price level under the gold standard. A negative investment shock would reduce real interest rates. Because expected inflation was near zero under the gold standard, this led to lower nominal interest rates. Lower nominal rates increased the demand for gold, which is deflationary under a gold standard.
Thus under a gold standard, negative investment shocks really could cause recessions, just as Keynes postulated. But not for the reason he assumed. Just as fish don’t notice that they are wet, Keynes never understood the way his worldview was shaped by the lack of fiat money policy regimes. In the rare cases where fiat money was used (say Germany in the early 1920s), Keynes temporarily became a monetarist.
Step four: Once we shifted to fiat money, investment shocks should no longer have led to business cycles. Monetary policy should be adjusted to offset any change in the equilibrium interest rate resulting from an investment shock, and thus maintain steady NGDP growth. But the Fed tends to be a bit behind the curve when there is a sudden change in the natural rate of interest, so investment shocks still do matter.
In the early 2000s, the Fed did an OK (but not perfect) job responding to the fall in the natural rate of interest after the bursting of a business investment boom. Unemployment peaked at 6.3% in the mild recession that followed the tech boom. That’s much better than the way they responded to the end of the investment boom of 1929!
In contrast, the Fed was far behind the curve in responding to the fall in the natural rate of interest after the bursting of a residential investment boom in 2007-08. This error was partly caused by the Fed freaking out over high headline inflation when oil prices soared in 2007-08.
Step five: So people keep thinking that investment plays a causal role in the business cycle, even though the actual problem is that the investment shocks cause the Fed to inadvertently tighten monetary policy. A successful monetary regime would cause people to no longer see investment shocks as the cause of business cycles. If you favor classical economics, you should root for a successful monetary policy, as this sort of policy regime causes people to see the economy in classical terms.
The best way of persuading people that investment shocks don’t cause business cycles is to adopt NGDP level targeting, and have NGDP futures “guardrails” guiding policy. The ideal monetary policy is one where the market always expects 4%/year NGDP growth over the next year (or two during a pandemic.) If the markets always expect 4% NGDP growth, then people won’t worry about investment shocks causing recessions.
Robinson Crusoe understood that the decline in his housing investment didn’t cause the fall in GDP. Rather his injured arm caused the “recession”, and he decided to smooth consumption during the downturn. A good monetary policy will make us all as smart as Robinson Crusoe. It will make the economy more “classical”. Less investment just means more consumption.