Thresholds: does supply respond symmetrically in both directions?
Sometimes supply and/or demand curves exhibit threshold effects. There may be some minimum consumption required at (almost?) any price or income level; “basic needs” like housing and food may be modeled this way. Mayo provides an example, using the so-called "Stone-Geary" model. Another threshold may appear when you consume a lot; at some point consuming more chocolate cake or beer will make you sick (especially if you consume them together). There may be some market threshold where government freezes production and consumption, as when there is a limit on the number of building permits issued. O'Sullivan is one of many sources with such a model. Another case is when there’s an existing supply of housing that is hard to get rid of in a short run while you can easily build more than existing supply. All these thresholds can introduce "kinks to our curves:
|
Exhibit 8 |
If demand increases, increasing prices are a signal to suppliers to build, or upgrade space.
If demand falls, decreasing prices are a signal to suppliers to demolish, reduce maintenance, or convert space to some other use.
Question: does the supply response work equally well in both directions?
|
Exhibit 9 |
Here in Exhibit 9 we depart from two of our previous assumptions. We are looking at a single market and counting houses as before, but this time let’s assume that we are examining the stock of all houses, new and existing, instead of the flow of new construction. Let us further assume that over several years we can readily build some new houses, but we cannot so easily get rid of the old ones. This is not strictly true, of course; but experience teaches us that it does take time to reduce the stock of housing through depreciation and demolitions.
Suppose we started P
0Q
0. If demand shifts up and the price rises to P
1, then developers build (Q
1–Q
0) new units, taking us to P
1Q
1. But if demand shifts down, the price falls to P
3, as drawn we can’t get rid of the excess stock, and we are still stuck at Q
3=Q
0.
To the extent there is some elasticity in the downward direction, this effect will be mitigated. Rotate the bottom of the supply curve a little towards the origin while holding the pivot at P
0Q
0 fixed: the price decline is moderated, as the new Q
3 (call it Q
3' as you draw it in) falls a little bit below the old Q
0.
In reality, declining cities like Detroit and Baltimore and Milwaukee often find it difficult to reduce the existing stock as rapidly as the decline and demand would suggest. That’s why there were periods in the 2000’s where you could find old but livable houses in parts of Detroit for $10,000 or less.
Time; and the Definition of the Good
Early models of real estate were focused on land markets in the land market was viewed as perfectly inelastic in supply. David Ricardo and Henry George were two of the authors who popularized these views of markets and their work is still extremely influential. On the face of it the Ricardo – George view seems plausible as in the words of attributed to a number of authors including Will Rogers and Mark Twain, “Son, buy land; they ain’t making any more of it.”
But in the 1940s, Richard Ratcliff pointed out that what was more often relevant is not the supply of land in the aggregate, but the supply of land for particular use; and this latter supply is often at least somewhat elastic.
A moment’s introspection will suggest that the narrower we draw our definition of a market, the more elastic the supply. At one extreme, consider the total amount of land available on spaceship earth. It’s a pretty good approximation to treat that is fixed. When you examine a country, the same assumption is usually made; although the United States could always invade Canada or Mexico to increase our supply of land, that’s rare, and costly. (Recall that we have done so in the past although not the not in the last few hundred years; see Elliott. Cross your fingers that we are not about to do so again.)
Many projects add to the supply of land by draining swamps, infill projects and the like. Chicagoans know that Michigan Avenue used to be the
Lake Michigan shoreline before infill using rubble from the Great Fire of 1871. New York City residents are familiar with Battery Park City, for example, which was built on fill from the original World Trade Center excavation. Other famous examples of such land reclamation include projects hundreds of years ago in China and the Netherlands, to But again at the national level these are usually small enough to neglect, except in some of the smaller places like Singapore and Macau. At the city level these may be important projects, but with a few possible exceptions (Holland?) they generally do not dominate basic outcomes in large national land markets.
A city’s land supply may also grow by annexing land from neighboring jurisdictions, as analyzed by Rusk. At the city level, growth through annexation and infill begins to become more relevant. But as we narrow the type of land use were considering, supply becomes more elastic still, as Ratcliff emphasized. Suppose we consider not just the supply of land in Madison, but the supply of residential land. This can be increased not just by annexation or filling in part of Lake Monona, but also the much easier process of rezoning.
If we consider the supply of land for multifamily housing, the supply is more elastic still because it’s often easier to up-zone some single-family land to multifamily than it is to rezone agricultural and extend the infrastructure needed for multifamily to as yet undeveloped areas. In the most extreme case, the supply of land for your house or my house is more or less perfectly elastic. It’s unlikely that either of us is such a large part of market demand that we can affect the price. In other words, the supply curve we face as individuals is perfectly horizontal. Thus Ratcliff demonstrated that the supply of land for particular uses can be very elastic even as the total supply of land for all uses is less so.
Sometimes the classical land model of Ricardo and George is a good approximation, when we examine the supply of land in the very short run, or at a very large level (e.g. a country). Sometimes, it's not a good model, and we turn to Ratcliff and others. We note in passing that followers of Henry George, take a very strong view, at times fanatical, that the classical model is relevant in all places and times, in all situations
Tant pis pour ils. We'll discuss the pros and cons of "Henry George Theorems" at another point.
Thomas Carlyle Was Wrong About Slavery; and Also About Economics
This introduction hardly exhausts the topics of supply and demand, but you can see we've already exhausted the
verbal capacities of most parrots. The "Romantic"
Thomas Carlyle was an influential writer in his day, still read and much cited but on this, as on slavery, he was over-matched against Enlightenment figures including 19th century economists like
John Stuart Mill.
Concluding Thoughts
|
Exhibit 10 |
Whenever you draw, or view, supply and demand curves, ask:
- Which two variables – P and Q (most common), Q and Y (Engel curves), Q and demographics, etc. – are we drawing?
- Curves isolate 2 variables, ceteris paribus. What are the other “demand shifters” and “supply shifters” in our supply and demand multivariate functions?
- Are we looking at stocks, or flows, of Q?
- How is the good defined? How is price measured?
- What are the units of observation? Are we looking at individual firms/consumers, or aggregating up to a market? Are we looking at a submarket (e.g. rental demand only – is there a supply/demand for owner-occupants “off stage?”)
- Are we looking at the short run or the long run? Can you put a time stamp on it? (Overnight? Six months? A decade?)
- What do we know about the shape and the “steepness” of the curves?
- Are there related markets we should consider?
We've covered a lot of ground here, but there's much more to discuss in future posts, for example:
- How to measure price and quantity in
the housing market.
- Stocks and flows, in general equilibrium in the DiPasquale
and Wheaton four-quadrant model.
- How are expectations set in the housing
market?
- Key elasticities of supply and demand: what have we learned?
- Consumer
surplus, producer surplus, and their use in analyzing housing programs.
References Cited, and Further Reading
There are a number of references here, including some historical ones for those interested in tracing out the pedigree of the concepts. But he basic concepts here -- supply, demand, equilibrium, elasticity -- have passed into the toolbox of "common knowledge" and can be used without reference to the original sources. Some time ago, Pam Woodall, then editor of The Economist, forgot that when I sent her some basic material on supply and demand for a special center section on real estate. She kindly recognized my contribution, but by labeling one of my charts that could be interpreted as exaggerating my contribution to economics. Sharp-eyed friends were quick to mock what they claimed were my attempts to appropriate the work of giants, some of whom we've met in this post.
I come from a culture (Pennsylvania) where if you don't mock a friend at every opportunity, well, you're not much of a friend.
On the charge of claiming authorship of supply and demand, I plead innocence! 😉
|
Exhibit 11 |
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