Bryan Caplan and Paul Krugman are right: the Austrian business cycle theory (ABCT) is flawed . Economists of the Austrian school need to retrace from first principles and construct a clear and logical theory of the business cycle. Such a theory has to provide a plausible explanation for why entrepreneurs are fooled by credit expansion and why the consequences for the economy are so severe.
I here try to provide a starting point for such a revised theory based on my own entrepreneurial experience of the market mechanism and key principles of Austrian economics.
The principle of subjective value
One of the key propositions of Austrian economics is that both utility and cost are subjective. We all value different things in life, and the true cost of any action is opportunity cost, i.e. the value of the highest-valued alternative forgone in taking the action . Hayek placed such importance on this concept of subjective value that he defined his subject of study not as "economics" (which Aristoteles defined as the art of household management) but as "catallactics", the order brought about by the mutual adjustment of many individual economies in a market .
But for some reason Hayek decided to leave these principles behind when he constructed his theory of the business cycle. Suddenly money, and especially the time value of money, took center stage. The result is in my opinion a rather contrived explanation based on aggregate concepts such as the "natural interest rate" and "relative price changes". This has been widely criticized  and I don't want to add insult to injury.
Instead, let's see how the argument goes if we for a moment forget about monetary aggregates and instead stay true to the principle of subjective value.
Credit expansion lowers opportunity cost
Remember that all costs are really opportunity costs. In a monetary catallaxy opportunity costs depend on two things: how accessible money is and what alternative use money has. This statement may perhaps surprise some of my readers because mainstream economics often keeps the former of these out of the equation (as a constant income) and treats the rest separately as a "utility maximization problem" . But if we listen closely to the definition, "the opportunity cost of any action is the value of the highest-valued alternative forgone in taking that action", it is quite obvious that the highest-valued alternative to buying a car may very well be the free time and rest you could have had instead of working extra hours to pay for the car.
Credit expansion lowers opportunity costs simply because it tends to make money more accessible to more people. For example, there is a world of difference between refinancing your mortgage to buy a car and working extra night shifts to buy the same car. In the first case the buyer may hardly see an opportunity cost at all, whereas in the latter case the buyer has obviously already foregone something valuable, her time and energy, in order to afford the car.
Austrians often make a distinction between loans to productive businesses and those made to consumers. In general they are in favor of the former but more ambivalent about the latter. From this viewpoint I think there's a simple explanation for that ambivalence: when a loan is made to a business that invests in productive capacity the borrowed money doesn't come easy to anybody. The entrepreneur knows she will lose her business unless she can put the money to productive use and pay back the loan, and the employee or supplier that gets payed from those borrowed funds has to work just as hard for their money. This means that the loan will not have a significant impact on anybody's perceived opportunity costs.
Compare this to the case where a loan is made to a home buyer. The buyer pays the seller. The seller has most likely lived in the home for some time over which the property has appreciated (and money has depreciated). The seller therefore makes a significant profit with very low perceived risk. Other home owners see those profits and regard them as easily available to them as well. It is true (as you are perhaps arguing to yourself) that the seller often has to buy a new home. Everybody needs somewhere to live and so on. But that just means the money is transferred to another seller, and another, until there is either somebody who can use the profit for consumption or a dead person (in which case the money is inherited and with some probability put towards consumption).
In my opinion this concept of credit that lowers perceived opportunity costs is distinct from the "unsound credit" that Mises so often lamented. If my understanding is correct Mises was referring to credit extended to borrowers that are or will be unable to pay the money back. To me that seems of less importance. If a loan is extended to a business that does its best to invest it wisely (in wages and capital goods) then that should have the same effect on the subjective perception of opportunity costs for those involved regardless of if the business fails or succeeds. It can perhaps even be argued that lending to a business that fails would tend to increase perceived opportunity costs, for the lender that is, who can no longer look forward to repayment.
How credit expansion can distort a market
If (some sorts of) credit expansion lowers opportunity costs for many people then it should come as no surprise that it causes a boom. But how can it cause the massive damage known as a recession?
It would be a serious blunder to neglect the fact that inflation also generates forces which tend toward capital consumption. One of its consequences is that it falsifies economic calculation and accounting. It produces the phenomenon of illusory or apparent profits.
- Ludwig von Mises
Entrepreneurs rely on the market response to guide them towards profitable new businesses. For new products and services (which have not yet been "commoditized") the market response can be very inelastic, i.e. it varies only slightly with price. In developed markets some goods can even have a negative price elasticity. It is therefore incorrect to only consider prices when studying distortions of the market mechanism. Important information is also conveyed through quantities demanded at each price level.
Credit expansion will sooner or later lead to price increases. But the effect on opportunity costs is much more immediate. This in turn has a sharp and immediate effect on the market response, not in terms of prices but in terms of quantities demanded at a certain price. This is what fools entrepreneurs. They see "strong demand" for their new product or service at profitable price levels, but their customers are only willing to pay those prices (or even any price) as long as money is easy to come by and has no important alternative use. The market response to a new product or service is incredibly difficult to predict, but nearly impossible to second guess.
This can also be understood as a distortion of the profitability requirement itself. Profitability doesn't depend only on market prices. It also depends (perhaps even more so) on market quantities. For example, just because you can sell a single burger for $10 at some street corner doesn't mean you can open a profitable burger joint there (you have to sell hundreds every day). When credit expansion lowers opportunity costs for consumers they tend to be less careful with their money. Entrepreneurs interpret this as demand for their product or service. Investors and bankers carefully review the numbers, which look good, and chip in. The result is investment in an economic activity which is dependent on continued credit expansion for its profitability. When the boom is over demand vanishes and "phony profits evaporate", leaving a fooled entrepreneur with a failing business behind.
There may be a third way to understand this problem. In a sound catallaxy market participants trade value for value with money acting as the medium of exchange. If we isolate a single market participant, say a car buyer, and regard the interaction between this participant and the rest of the catallaxy then an interesting observation can be made. In an undistorted market the car buyer would have to offer the rest of the catallaxy something of value in exchange for the car. Let's for a moment ignore money, which is only the medium of exchange, and say that he offers his labor. Since free trade is voluntary he would only make that trade if he placed a higher expected value on the car than on his labor. We rely on this one-way valve principle when we assume that a catallaxy as a whole will trend towards value creation and capital accumulation. But when credit expansion lowers opportunity costs this property breaks down. The car buyer now expects to trade nothing but some small portion of his constantly increasing home equity, or rather whatever distant alternative use that equity has, in return for the car. I think this may to some extent explain what Mises referred to as "capital consumption", especially when considering the fundamental human bias of hyperbolic discounting of future rewards .
The anatomy of a boom-bust cycle
Now lets apply this understanding to a typical boom-bust cycle.
There is no means of avoiding a final collapse of a boom brought about by credit expansion. The alternative is only whether the crisis should come sooner as a result of a voluntary abandonment of further credit expansion or later as a final and total catastrophe of the currency system involved.
- Ludwig von Mises
First an expansion in (some forms of) credit lowers opportunity costs for many people. This distorts the market response, at this stage not so much in prices as in quantities demanded at a certain price. Entrepreneurs misinterpret this as genuine demand for their products and services and therefore "malinvest". From this perspective it should come as no surprise that the products and services produced would tend to be of a dispensable nature, such as luxury goods, travel and experiences. The longer the boom continues the more economic activity sustained only by credit expansion is built up.
If credit expansion continues for long enough prices will start to raise. When they finally do people are forced to make more and more difficult trade-offs between alternative uses of money, i.e. their opportunity costs go up. This increase in opportunity costs as perceived by many people will sharply lower demand for products and services that previously relied on credit expansion for their profitability. This will be seen as deteriorating economic conditions, causing banks to tighten their lending standards and credit expansion to slow. This in my mind explains the short burst of relatively strong price inflation often seen just before a crash.
Once credit stops expanding and starts to contract money will become even harder to come by than usual, causing the process above to work in reverse. A Keynesian may conclude that in uncertain times the demand for money itself becomes excessive thereby causing a fall in aggregate demand. An Austrian may instead conclude that in uncertain times many market participants may perceive future consumption as a highly valued alternative to immediate consumption options, because access to money in the future is uncertain and/or expected to be laborious. But perhaps it is sufficient to simply say that when credit is contracting money is harder to come by than when credit is expanding. In any case this has the effect of increasing perceived opportunity costs and distorting the market response in the opposite direction: quantities demanded at profitable price levels become too small to sustain even businesses that would under more normal circumstances be profitable.
Conclusions and future work
A clear and logical theory of the business cycle based on the key principles of subjective value, opportunity cost, heterogeneity of goods and voluntary trade in a free market is urgently needed. More work is necessary to understand the effects of various forms of credit on opportunity costs. It seems however that the tendency of credit expansion to lower opportunity costs is a plausible explanation for "malinvestment", and perhaps also for the "phony profits" and "capital consumption" concepts that are often thrown around by Austrians but rarely explained.
The framework laid out here also explains why entrepreneurs are fooled by credit expansion and why the consequences for the economy are so severe. Plausible explanations are further provided for why luxury goods are hardest hit in a recession and why a short burst of price inflation is often observed just before a crash.
Finally, the proposed understanding of the business cycle becomes remarkably similar to the Keynesian model during the bust, with the exception that not all of the reduction in economic activity is "spare capacity" or an "output gap"; some of the economic activity built up during the boom would not have been profitable in the first place had it not been for unsustainable credit expansion and the resulting lowering of opportunity costs.
I plan to follow up this post with one aimed at those more familiar with mainstream economics. That post will include a mathematical formulation of a "generalized utility maximization problem" (involving both income and consumption and taking the combinatorial nature of choices into account) most likely as a Mixed Integer Non-Linear Programming (MINLP) problem . I think this could fit nicely as a microfoundation  to an agent-based model  for further study. I know how Austrians feel about mathematics but I'm hoping they will be willing to compromise if it means proving the mainstream economists wrong on their home turf. ;)
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 Bryan Caplan, Why I Am Not an Austrian Economist; http://econfaculty.gmu.edu/bcaplan/whyaust.htm.
 Paul Krugman, The Hangover Theory; http://www.slate.com/articles/business/the_dismal_science/1998/12/the_hangover_theory.html.
 The Concise Encyclopedia of Economics: Austrian School of Economics; http://www.econlib.org/library/Enc/AustrianSchoolofEconomics.html.
 Wikipedia: Catallaxy; http://en.wikipedia.org/wiki/Catallaxy.
 Social Democracy for the 21st Century: Bloggers Debate the Austrian Business Cycle Theory; http://socialdemocracy21stcentury.blogspot.com/2012/02/bloggers-debate-austrian-business-cycle.html
 Wikipedia: Utility maximization problem; http://en.wikipedia.org/wiki/Utility_maximization_problem.
 Wikipedia: Hyperbolic discounting; http://en.wikipedia.org/wiki/Hyperbolic_discounting.
 Mixed Integer Nonlinear Programing, M. Bussieck and A. Pruessner (2003); http://www.gamsworld.eu/minlp/siagopt.pdf.
 Wikipedia: Microfoundations; http://en.wikipedia.org/wiki/Microfoundations.
 Wikipedia: Agent-based model; http://en.wikipedia.org/wiki/Agent-based_model.