The Industrial Organization Of Markets With Two-Sided Platforms - Part 1.

Published in Competition Policy International (print ISSN 1554-0189, online ISSN 1554-6853), Spring 2007, Vol. 3, No. 1. Competition Policy International is a free publication. To order or download additional copies, visit eSapience.org.

I. INTRODUCTION

Many diverse industries are populated by businesses that operate "two-sided platforms." These businesses serve distinct groups of customers who need each other in some way, and the core business of the two-sided platform is to provide a common (real or virtual) meeting place and to facilitate interactions between members of the two distinct customer groups. Two-sided platforms are common in old-economy industries such as those based on advertising-supported media and new-economy industries such as those based on software platforms and web portals. They play an important role throughout the economy by minimizing transactions costs between entities that can benefit from getting together.

In these businesses, pricing and other strategies are strongly affected by the indirect network effects between the two sides of the platform. As a matter of theory, for example, profit-maximizing prices may entail below-cost pricing to one set of customers over the long run and, as a matter of fact, many two-sided platforms charge one side prices that are below marginal cost and are in some cases negative. These and other aspects of two-sided platforms affect almost all aspects of antitrust analysis-from market definition, to the analysis of cartels, single-firm conduct, and efficiencies.1

This paper provides a brief introduction to the economics of two-sided platforms and the implications for antitrust analysis.

Two-sided platforms were first identified clearly in pioneering work by Jean-Charles Rochet and Jean Tirole, which began circulating in 2001.2 A significant theoretical and empirical literature quickly emerged, and the subject has become a very active area of research in economics.3 For the purposes of this paper, it is helpful to clarify some terminology that is used in the economics literature and which sometimes causes confusion. Rochet and Tirole used the term "two-sided markets" to refer to situations in which businesses cater to two interdependent groups of customers. The term "market" was meant loosely and does not refer to how that term is often used in antitrust. This paper refers to "two-sided platforms" but it is synonymous with "two-sided markets" as used in much of the economics literature. How to determine what market a two-sided platform competes in, from an antitrust perspective, is one of the questions considered here.4 Two-sided platforms often compete with ordinary (single-sided) firms and sometimes compete on one side with two-sided platforms that serve a different second side.

II. ECONOMIC BACKGROUND ON TWO-SIDED PLATFORMS

A heterosexual, singles-oriented club offers some intuition on the economics of two-sided platforms. A nightclub, such as Bungalow 8 in Manhattan, provides a platform where men and women can meet and search for interactions and potentially dates. The club needs to get two groups of customers on board its platform to have a service to offer either one: it needs to get both men and women to come. Moreover, the relative proportion of men and women matters. A singles club with few women will not attract men, and a club with few men will not attract women. Pricing is one way to get the balance right. The club might want to offer women a break if they are in short supply (through a lower price or free drinks). Or it might want to ration the spots to ensure the appropriate number of women; popular clubs typically have queues waiting outside, and women are picked out of line disproportionately.

The dating club example motivates the informal definition of a two-sided platform that we introduced in the beginning paragraph. There are two groups of customers-men and women. Members of each group value members interacting with members of the other group. And the platform provides a place for them to get together and interact. By doing so it enables members of these two groups to capture various benefits from having access to each other.

In their 2006 paper, Rochet and Tirole have proposed a formal definition:

"A market5 is two-sided if the platform can affect the volume of transactions by charging more to one side of the market and reducing the price paid by the other side by an equal amount; in other words, the price structure matters, and platforms must design it so as to bring both sides on board."6

To satisfy this definition, "the relationship between end-users must be fraught with residual externalities" that customers cannot sort out for themselves.7 That is clear in the case of the dating environment. In contrast, in the textbook wheat market there are no externalities connecting buyers and sellers, and the price structure doesn't matter: a tax on wheat levied on buyers has the same effect on quantity as the same tax levied on sellers.

In addition, it must not be possible for the two sides to arbitrage their way around the price structure chosen by the platform. Men and women, for example, want to be able to search for dates among a large number of opposites. It is hard to conceive of a practical mechanism for women to reward men who come to a singles club but who they reject. Likewise, for the other two-sided platform industries we consider it is difficult, if not impossible, for customers on one side to make side payments to customers on the other side. As a result the platform owner can institute a pricing structure to harness indirect network effects, and it is not feasible for customers to defeat this pricing structure through arbitrage. Generally, one can think of two-sided platforms as arising in situations in which there are externalities and in which transactions costs, broadly considered, prevent the two sides from solving this externality directly. The platform can be thought of as providing a technology for solving the externality in a way that minimizes transactions costs.

It is helpful to review four different types of two-sided platforms: exchanges, advertiser-supported media, transaction devices, and software platforms.8

A. Exchanges

Exchanges have two groups of customers, who can generally be considered "buyers" and "sellers." The exchange helps buyers and sellers search for feasible contracts- that is where the buyer and seller could enter into a mutually advantageous trade-and for the best prices-that is where the buyer is paying as little as possible and the seller receiving as much as possible. (In organized exchanges, such as the New York Stock Exchange, it is often more useful to think of the two sides as liquidity providers-specialists or market-makers who quote prices to both buyers and sellers and thus bring liquidity to the market-and liquidity consumers- ordinary customers who accept liquidity providers' offers.9) We use the term buyers and sellers here loosely. The term, "exchanges," covers various matchmaking activities such as dating services and employment agencies. It also covers traditional exchanges such as auction houses, internet sites for business-to-business, person-to-business, and person-to-person transactions, various kinds of brokers (insurance and real estate) and financial exchanges for securities and futures contracts. Finally, exchanges include a variety of businesses that provide brokerage services. These include publishers (readers and authors), literary agents (authors and publishers), travel services (travelers and travel-related businesses), and ticket services (people who go to events, and people who sponsor events).

Exchanges provide participants with the ability to search over participants on the other side and the opportunity to consummate matches. Having large numbers of participants on both sides increases the probability that participants will find a match. Depending on the type of exchange, however, a larger number of participants can lead to congestion. That is the case with physical platforms such as singles clubs or trading floors. Moreover, participants may derive some value from having the exchange prescreen participants to increase the likelihood and quality of matches.

Some exchanges charge only one side. For example, only sellers pay directly for the services provided by eBay. This is also true for real-estate sales in the United States. Other exchanges charge both sides, although the prices may bear little relation to side-specific marginal costs. Internet matchmaking services charge everyone the same, for instance, while, as we mentioned, physical dating environments sometimes charge men more than women. Auction houses charge commissions to buyers and sellers. Insurance brokers historically charged both insurance customers and insurance providers in some types of transactions (some have agreed not to charge both as a result of settlements of lawsuits brought by the New York State Attorney General).

B. Advertising-Supported Media

Advertising-supported media such as magazines, newspapers, free television, and web portals are based on a two-sided business model. The platform either creates content (newspapers) or buys content from others (free television). The content is used to attract viewers. The viewers are then used to attract advertisers. There is a clear indirect network effect between advertisers and viewers-advertisers value platforms that have more viewers; the extent to which viewers value advertisers is the subject of more debate but we suspect that viewers value advertisers more than they might admit.10

Most advertising-supported media earn much of their revenues-and probably all of their gross margin-from advertisers.11 Print media are often provided to readers at something close to or below the marginal cost of printing and distribution. 12 In some cases-such as yellow page directories and some newspapers- they are provided for free. Free television is just that. And most web portals- Google and Yahoo for example-receive revenue only from advertisers.

C. Transaction Systems

Any method for payment works only if buyers and sellers are willing to use it. Humans switched from barter when they were agreed on a standard medium for exchange-such as metallic coins or seashells. Governments facilitated this by ensuring the integrity of coins (to various degrees) and by using government-issued coinage for buying and selling. Cash, which has no intrinsic value in most modern economies, provides a payment platform because buyers and sellers expect that other buyers and sellers will use it. Of course the government facilitates this with various laws and through its own buying and selling activities.

For-profit transaction systems are based on the same principles although they have challenges that governments-which at least in principle can create a platform by fiat-do not necessarily have. Although bank checks and travelers' checks are also examples of for-profit transaction systems, we focus on payment cards, which have been the subject of significant competition policy scrutiny in many countries.

Diners Club started the first two-sided payment system in 1950. Before then stores issued payment cards to their customers for use only at their stores. Diners Club began by getting a set of restaurants to agree to take its card for payment; that is to agree to let Diners Club reimburse the restaurant for the meal tab and then in turn collect the money from the cardholder. It also persuaded individuals to take its card and use it for payment. Starting with a small base in Manhattan it grew quickly throughout the United States and other countries.

Diners Club initially charged restaurants seven percent of the meal tab; cardholders had to pay an annual fee, which was offset in part by the float they received as a result of having to pay their bills only once a month. As a result Diners Club earned most of its revenue-and most likely all of its gross margin-from merchants. Other entrants into the charge and debit card businesses have followed this same approach. Determining who pays in the case of credit cards is a bit more complicated since that product bundles a transaction feature (for which the cardholder pays little) and a borrowing feature (for which the cardholder incurs finance charges). However, it is safe to say that merchants are the main source of revenue for credit cards held by people who do not revolve balances.

American Express, Discover, and, until its recent absorption into MasterCard, Diners Club, set prices to merchants-the merchant discount, which gives rise to a positive variable transaction price-and to cardholders-annual fees and various rewards which may give rise to negative variable transaction prices. Card associations such as MasterCard and Visa have been examples of cooperative two-sided platforms. For a transaction to be consummated there has to be an agreement on the division of profits and the allocation of various risks between the entity that services the merchant and the entity that services the cardholder. Most card associations set this centrally as, in effect, a standard contract between the businesses that service the two sides. Typically, they agree that the entity that services the merchant pays a percentage of the transaction-the "interchange fee"-to the entity that services the cardholder. This fee ultimately determines the relative prices for cardholders (issuers obtain a revenue stream which they compete for) and merchants (acquirers pass the cost of the interchange fee onto merchants). This centrally set fee has been the subject of litigation and regulatory scrutiny, as we discuss below.13

D. Software Platforms

A software platform provides services for applications developers; among other things, these services help developers obtain access to the hardware for the computing device in question. Users can run these applications only if they have the same software platform as that relied on by the developers; developers can sell their applications only to users that have the same software platform they have relied on in writing their applications.

Software platforms are central to several important industries. These include personal computers (e.g., Apple, Microsoft); personal digital assistants (e.g., Palm, Treo); 2.5G+ mobile telephones (e.g., Vodafone, DoCoMo); video games (e.g., Sony PlayStation, Xbox); and digital music devices (e.g., Creative Zen Micro, Rio Carbon). With the exception of video games, the software platform owners make most of their revenue, and all of their gross margin, from the user side; developers generally get access to platform services for free, and they obtain various software products that facilitate writing applications at relatively low prices. Videogame console manufacturers, on the other hand, typically receive most of their gross margin from licensing access to the software and hardware platforms to game developers; they sell the videogame console at close to or below manufacturing cost.

Software platforms facilitate a market for applications by reducing duplicative costs. Application programs need to accomplish many similar tasks. Rather than each application developer writing the code for accomplishing each task the software platform producer incorporates code into the platform. The functions of that code are made available to application developers through an application program interface (API). The user benefits from this consolidation as well since it reduces the overall amount of code required on the computer, reduces incompatibilities between programs, and reduces learning costs.14 An important consequence of this reduction in cost is an increase in the supply of applications for the platform, an increase in the value of the software platform to end users, and positive feedback effects to application developers.

E. Methods For Minimizing Transactions Costs

The fundamental role of a two-sided platform in the economy is to enable parties to realize gains from trade or other interactions by reducing the transactions costs of finding each other and interacting. Two-sided platforms do this by matchmaking, building audiences, and minimizing costs. Different platforms engage in these activities to different degrees. Software platforms are mainly about minimizing duplication costs, advertising-supported media in mainly about building audiences, and exchanges are mainly about matchmaking. But they all seem to engage in each to some degree. All platforms help reduce costs by providing a virtual or physical meeting place for customers. We will see that these platforms all minimize transactions costs by through matchmaking, audience-making, and cost minimization through the elimination of duplication.15

MySpace provides an example of how a two-sided platform engages in all three functions. It is a popular internet site where young people can post their profiles and develop networks of friends. It provides matchmaking between the people who sign up as well as the advertisers who would like to meet them. It builds audiences for advertisers as well as members-particularly musicians-who want to make themselves known. And it reduces the costs to people of getting together by providing a common meeting place.

III. ECONOMIC PRINCIPLES

The theoretical economics literature on two-sided platforms is relatively new. Economists have derived many results based on stylized models that apply to some of the industries described above. The precise results are sensitive to assumptions about the economic relationships among the various industry participants. Even for these special cases it has turned out to be challenging to derive results without making further assumptions about the precise nature of the demand, cost, and indirect network effects relationships.16 Nevertheless, several principles have emerged that seem to be robust. They appear to depend only on the assumptions that the platform has two groups of customers, that there are indirect network externalities, and that the customers cannot solve these externalities themselves.

A. Pricing

To see the intuition behind pricing consider a platform that serves two customer groups A and B. It has already established prices to both groups and is considering changing them.17 If it raises the price to members of group A fewer As will join. If nothing else changed the relationship between price and the number of As would depend on the price elasticity of demand for As. Since members of group B value the platform more if there are more As fewer Bs will join the platform at the current price for Bs. That drop-off depends on the indirect network externality which is measured by the value that Bs place on As. But with fewer Bs on the platform, As also value the platform less leading to a further drop in their demand. There is a feedback loop between the two sides. Once this effect is taken into account, the effect of an increase in price on one side is a decrease in demand on the first side because of the direct effect of the price elasticity of demand and on both sides as a result of the indirect effects from the externalities.

A few equations will make this point more sharply for readers familiar with the concept of elasticity. The situation described just above can be summarized by two demand functions: QA = DA(PA, QB) and QB = DB(PB, QA). The first of these gives participation by members of group A as a function of the price charged to group A and participation by group B, and the second gives participation by members of B similarly. Let eI = -(∂DI/∂PI)(PI/QI), for I = A,B. These are the own-price elasticities for each group, holding constant participation by the other (i.e., ignoring the externalities linking the two groups). Let ӨIJ = (∂DI/∂QJ)(QJ/0I for I,J = A,B and I ≠ J. These elasticities measure the strengths of the externalities connecting the two groups. In the normal two-sided case, both would be expected to be positive. Finally, let EI = -(dQI/dPI)(PI/QI) for I = A,B. These are the ordinary own-price elasticities, computed assuming other prices remain constant but allowing participations (quantities) to vary. Differentiating both demand functions totally with respect to either price, and solving, yields:

EI = EI/(1 - ӨIJ ӨJI); I,J = A,B; I ≠ J

Even if the As are not particularly price-sensitive, and as long as the externalities between the groups are strong (in either direction!), participation by group A may be highly sensitive to the price its members are charged, and similarly for group B. Even a small response by group A to a price change will trigger a response by group B, which in turn will produce a response by A, and so on. (The equation above assumes that these response sequences converge.)

The platform of course would like to find the prices that maximize its profits by taking these same sorts of considerations into account. For a single-sided business that would occur by selecting the output at which marginal revenue equals marginal cost and then charging the corresponding price for this quantity from the demand curve. (This equilibrium is often described by the Lerner formula that says that the price marginal-cost margin equals the inverse of the own-price elasticity of demand.) For two-sided platforms three results appear to be robust:

The optimal prices depend in a complex way on the price sensitivity of demand on both sides, the nature and intensity of the indirect network effects between the two sides, and the marginal costs that result from changing output of each side.

The profit-maximizing, non-predatory price for either side may be below the marginal cost of supply for that side or even negative.

The relationship between price and cost is complex, and the simple formulas that have been derived for single-sided markets do not apply.

For many platforms it is possible to charge two different kinds of prices: an access fee for joining the platform and a usage fee for using the platform. Although these are interdependent, one can think of the access fee as mainly affecting how many customers join the platform and the usage fee as mainly affecting the volume of interactions between members of the platform. Most software platforms charge access fees to users-they have to license the software platform but then can use it as much as they want-and do not charge access or usage fees to developers. Videogame console vendors, though, charge a usage fee to game developers-a royalty based on the numbers of games that are sold; users pay this usage fee indirectly through their purchase of games for the console. Payment card systems generally charge merchants a usage fee but no access fee. Cardholders may pay an access fee (the annual card fee); they often pay either no usage fee or a negative one (to the extent they receive rewards based on transactions volume).

The profit-maximizing reliance on access versus usage fees depends on many factors including the difficulty of monitoring usage and the nature of the externality between the two sides. Cardholders care about card acceptance, for instance, while merchants care about usage. It thus seems sensible not to charge merchants for access and not to charge consumers for usage.

The empirical evidence suggests that prices that are at or below marginal cost are common for two-sided platforms. Table 1 summarizes some relevant evidence.

B. Design Decisions

Two-sided platforms are in the business of encouraging customers to join their platforms and stimulating them to interact with each other once they have joined. They design their platforms with this in mind. This can lead to decisions that in a narrow sense harm one side.

A simple example is a shopping mall. Shoppers would prefer to get to stores in the least amount of time. Merchants would like to maximize the amount of foot traffic outside their stores and therefore the number of potential shoppers. Shopping malls are sometimes designed to encourage shoppers to pass by many stores (e.g., by putting the up and down escalators at different ends of the mall).

Advertising-supported media are another obvious example. Viewers would like to gain access to the content-and perhaps even the advertisements of their choice-in the most convenient way. Some magazines are laid out to make it difficult even to find the table of contents or to find the continuation of an article without thumbing through many advertisements. Television watchers might benefit from having advertisements clustered at the beginning or the end of each program, but television providers (in the United States, at least) typically intersperse the advertisements and precede them perhaps with a cliffhanger to discourage viewers from taking a long break.

Two-sided platforms may also bundle features that directly benefit side A but harm side B (putting aside the indirect externalities from increasing the participation of side A).19 All software platforms include features for example that do not benefit most users. However, some developers value each of these features and in particular value knowing that any user of the software will have that feature and therefore be able to run its applications. All payment card systems require merchants that take their cards for payment to take any of their cards for payment, regardless of who presents it or which entity issued it. Some merchants would benefit from being selective-taking cards only from people who lack cash, for example. But this would reduce the confidence that cardholders have that their cards will be taken at stores that display the acceptance mark. (We will see later that special cases of these requirements, linking acceptances of credit and debit cards, have given rise to tying claims. This paragraph is not meant to suggest that tying could not be used in an anticompetitive way by two-sided platforms but rather to point out that there is an additional efficiency explanation for at least one aspect of this practice that does not arise in one-sided businesses.)

C. Rules And Regulations

Given that platforms promote interactions between customers and seek to harness indirect network externalities it should come as no surprise that two-sided platforms have an incentive to devise rules and regulations that promote these externalities and limit negative externalities between customers. The most sophisticated rules and regulations may be those employed by exchanges. All exchanges have rules against "front-running," for instance. This practice occurs when a broker receives a large purchase order from a customer, first buys on his own account, and then executes the customer order, which drives the price up slightly, and then sells on his own account and pockets the resulting profit. Banning this practice directly harms brokers, but it makes buyers and sellers more confident that they are getting the best price possible, and thereby boosts volume on the exchange.

Cooperative two-sided platforms have further need for rules and regulations because the behavior of their members can affect the value of the two-sided platform as a whole. Visa, for example, has rules that govern the appearance of cards issued by members, to provide some uniformity for the common brand, as well as to prevent members from using the brand inappropriately. The system also has rules that address disputed transactions. Acquirers would have an incentive to favor their customers (merchants) in a dispute while issuers would favor their customers (cardholders). The system's rules attempt to find a balance between these competing interests, to increase the attractiveness of the system as a whole.

IV. INDUSTRIAL ORGANIZATION OF MARKETS WITH TWO- SIDED PLATFORMS

Casual empiricism shows that industries with two-sided platforms are quite diverse. We explain some of the basic determinants of this heterogeneity from a theoretical perspective and then document aspects of it by surveying industries in which two-sided platforms are central.

A. Determinants Of Platform Size And Structure

Five fundamental factors determine the relative size of competing two-sided platforms. Table 2 summarizes the factors we discuss below and their effect on size (with a "+" indicating that there is a positive association between size and the factor).

1. Indirect Network Effects

Indirect network effects between the two sides promote larger and fewer competing two-sided platforms. Platforms with more customers of each group are more valuable to the other group. For example, more users make software platforms more valuable to developers and more developers make software platforms more valuable to users. These positive-feedback effects make platforms with more customers on both sides more valuable to both sets of customers. To take another example, a payment card system whose cards are taken at more merchants is more valuable to card users-that is why we see card systems touting their acceptance ("MasterCard: No card is more accepted.") in consumer advertisements.

If there were no countervailing factors, we would expect that indirect network effects would lead two-sided platforms to compete for the market. First movers would have an advantage, all else being equal. We would have the familiar story that the firm that obtains a lead tends to widen that lead as a result of positive-feedback effects and therefore wins the race for the market.20 Other firms could compete with this advantage only if they offered consumers on either side something that offset the first mover's size advantage.

Indirect network effects may decline with the size of the platform. For example, the probability of finding a match increases at a diminishing rate with the number of individuals on either side (buyers or sellers, men or women).21 At some point positive externalities from more participants may turn into negative externalities in the form of congestion as discussed below.

2. Economies And Diseconomies Of Scale 

For many two-sided platforms there would appear to be significant fixed costs of providing the platform. This should lead to scale economies over some range of output. For example, card payment systems have to maintain networks for authorizing and settling transactions for cardholders and merchants (and for their proxies-issuers and acquirers-in the case of association-based payment systems such as MasterCard). The costs of developing, establishing, and maintaining these networks are somewhat independent of volume. To take another example, there is a fixed cost of developing a software platform but a low marginal cost of providing that platform to developers and end users. In some cases the scale economies may mainly operate on one side. For example, there are scale economies in providing newspapers to readers (there is a high fixed cost of creating the newspaper and a relatively low marginal cost of reproducing and distributing it) but not in providing space to advertisers. Lastly, some physical platforms such as trading floors and singles clubs have scale economies at least in the short run, up to their capacity levels.

Diseconomies may set in at some point for various reasons on one or both sides. For example, to persuade existing end users to replace (i.e., upgrade) their existing software platforms software, platform vendors have to add features and functionality. Many of these improvements may be designed to encourage application developers to write new or improved applications for the platform that in turn benefit end users. However, as software platforms have gotten larger and more complex, it has become more expensive and time consuming to add features and functionality. The most recent version of the Apple OS took four months longer to develop than the previous version.22 Microsoft's Vista operating system has also been plagued with very long delays.

3. Congestion And Search Optimization 

Several design issues tend to limit the size of two-sided platforms. Physical platforms such as trading floors, singles clubs, auction houses, and shopping malls help customers search for and consummate mutually advantageous exchanges. At a given size expanding the number of customers on the platform can result in congestion that increases search and transaction costs.23 It may be possible to reduce congestion by increasing the size of the physical platform, but that in turn may increase search costs. Indeed, to optimize searching for partners, two-sided platforms may find that it is best to limit the size of the platform and pre-screen the customers on both sides to increase the probability of a match. One might argue that singles-type clubs do this explicitly (deciding who can get into an "exclusive" club) or implicitly (compare church-oriented singles groups and Club Med resorts). We will return to this subject below in discussing platform differentiation. Congestion may arise on one side alone. For example, increasing the volume of advertising in a newspaper may not only crowd out the content that attracts the readers but also result in a cacophony of messages that reduces the effectiveness of any particular advertisement.

4. Platform Differentiation And Multi-Homing

Platforms can differentiate themselves from each other by choosing particular levels of quality (what is known as "vertical differentiation") with consumers choosing the higher or lower quality of platform depending on the income and relative demand for quality. There are, for example, upscale and downscale malls. Platforms can also differentiate themselves from each other by choosing particular features and prices that appeal to particular groups of customers (what is known as "horizontal differentiation"). Thus there are numerous advertising-supported magazines that appeal to particular segments of readers and advertisers (e.g., Cape Cod Bride or Fly Fisherman).

Horizontal differentiation can result in customers choosing to join and use several platforms-a phenomenon that Rochet and Tirole have called "multi-homing". Customers find certain features of different competing platforms attractive and therefore rely on several. Payment cards are an example of multi-homing on both sides. Most merchants accept credit and debit cards from several systems, including ones that have relatively small shares of cardholders. Many cardholders carry multiple cards, although they may tend to use a favorite one most often.24 Advertising-supported media also has multi-homing on both sides-advertisers and viewers rely on many differentiated platforms. Other two-sided platforms have multi-homing only on one side. Most end-users rely on a single software platform for their personal computers, for instance, while many developers write for several platforms.

B. Empirical Evidence On Two-Sided Industry Structure

It is possible to see some regularities across industries in which two-sided platforms appear to be the dominant form of organization. Table 1 above and Table 3 reveal several features:

It is relatively uncommon for industries based on two-sided platforms to be monopolies or near monopolies. Some industries based on two-sided platforms have several large differentiated platforms, while others have many small platforms that are differentiated by location as well as along other dimensions.

Source: Adapted from David S. Evans, The Antitrust Economics of Multi-Sided Platform Markets, 20 YALE J. ON REG. 325 (2003). Industry share data from United States Census Bureau, 2002 Economic Census, available at http://www.census.gov/econ/census02/guide/INDSUMM.HTM; "Top 20 U.S. Daily Newspapers by Circulation," Newspaper Association of America (2001), at http://www.naa.org/info/facts01/18_top20circ/index.html (accessed Feb. 21, 2007); Stephen Labaton, U.S. Backs Off Rules for Big Media, NY TIMES, Jan. 28, 2005; A. Gillen & D. Kusnetzky, Worldwide Client and Server Operating Environments 2004-2008 Forecast, IDC MARKET ANALYSIS, No. 32452 (Dec. 2004); Schelley Olhava, Worldwide Videogame Hardware and Software 2004-2008 Forecast and Analysis, IDC MARKET ANALYSIS, No. 31260 (May 2004); THE NILSON REPORT, No. 828 (Feb. 2005); THE NILSON REPORT, No. 833 (May 2005).

Multi-homing on at least one side is common. Horizontal product differentiation tends to be the norm.

Asymmetric pricing is relatively common. Many two-sided platforms appear to obtain the preponderance of their operating profits (revenues minus direct costs) from one side. A nontrivial portion of two-sided platforms appear to charge prices that are below marginal cost or below zero.

Footnotes

1. See David S. Evans, The Antitrust Economics of Multi-Sided Platform Markets, 20 YALE J. ON REG. 325 (2003) and Julian Wright, One-Sided Logic in Two-Sided Markets, 3 REV. OF NETWORK ECON. 44 (2004).

2. Jean-Charles Rochet & Jean Tirole, Platform Competition in Two-Sided Markets, 1 J. EUR. ECON. ASS'N 990 (2003). Some of the key issues were identified in the context of payment cards in an important contribution Wlliam F. Baxter, Bank Exchange of Transactional Paper: Legal and Economic Perspectives, 26 J.L. & ECON. 541 (1983). There are also literatures for particular industries that also provide precursors.

3. See Conference on Competition Policy in Two-Sided Markets (Institute d'Economie Industrielle, U. Toulouse) (Jun. 29 - Jul. 1, 2006), available at http://idei.fr/doc/conf/tsm/programme.pdf.

4. Although, for the most part, we will use the term two-sided platform the reader should note that some platforms have more than two distinct groups of customers. Digital media platforms, for example, often have four: users, developers, hardware makers, and content providers.

5. Note that the word market below is being used in the loose manner that is the custom among economists and not in the antitrust sense. The Rochet-Tirole definition would be more precise if it said "A two-sided platform business exists if ...."

6. Jean-Charles Rochet & Jean Tirole, Two-Sided Markets: A Progress Report, RAND J. ECON. (Autumn 2006).

7. As a result a necessary condition for a market to be two-sided is that the Coase theorem does not apply to the transaction between the two sides. For more details, see Rochet & Tirole (2006), id.

8. For discussion, see DAVID S. EVANS, ANDREI HAGIU, & RICHARD SCHMALENSEE, INVISIBLE ENGINES: HOW SOFTWARE PLATFORMS DRIVE INNOVATION AND TRANSFORM INDUSTRIES, ch. 3 (MIT Press 2006).We refer there to software platforms more generally as shared input facilities. Armstrong uses the term "competitive bottlenecks" to refer to certain shared-input facilities. Although his discussion is analytically sound, his term is pejorative and has a meaning in competition law that differs from the one he assigns to it. See MARK ARMSTRONG, COMPETITION IN TWO-SIDED MARKETS (EconWPA, working paper, 2005).

9. Bernhard Friess & Sean Greenaway, Competition in EU Trading and Post-Trading Service Markets, 2 COMPETITION POL'Y INT'L (2006).

10. See, e.g., James M. Ferguson, Daily Newspaper Advertising Rates, Local Media Cross-Ownership, Newspaper Chains, and Media Competition, 26 J.L. & ECON. 637 (1983) ("Readership studies show that advertising, especially retail advertising, is considered as important as, or more important than, editorial content.") and R.D. Blair & R.E. Romano, Pricing Decisions of the Newspaper Monopolist, 59 SOUTHERN ECON. J. 731 (1993) ("circulation demand rises with increases in the quantity of advertising"). Other studies have shown that, unlike Americans, readers in certain European countries are averse to advertising. See, e.g., Nathalie Sonnac, Readers' Attitudes Toward Press Advertising: Are They Ad- Lovers or Ad-Averse?, 13 J. MEDIA ECON. 249 (2000). On the other hand, TiVo and other related products that permit ad avoidance and deletion are very popular currently, with one study citing that TiVo viewers skip about 60 percent of commercials. See A Farewell to Ads?, THE ECONOMIST, Apr. 15, 2004.

11. In a two-sided platform there is no rigorous way to define the profit "earned" by one side or the other. Not only are there typically costs that are common to both sides (the floor of the New York Stock Exchange, for instance), outlays that build business on one side of the market (via product enhancement, say) will also tend, via the externality, to build business on the other side. By "gross margin" we mean the difference between revenue and the variable costs, if any, that depend entirely on the volume on only one side of the market. The cleanest examples of such a cost would be the manufacturing costs of videogame consoles or the marginal printing costs of newspapers or yellow page directories.

12. Blair & Romano, supra note 10.

13. DAVID S. EVANS & RICHARD SCHMALENSEE, THE ECONOMICS OF INTERCHANGE FEES AND THEIR REGULATION: AN OVERVIEW (MIT Sloan,Working Paper, 2005), in Interchange Fees in Credit and Debit Card Industries 73-120 (Federal Reserve Bank of Kansas City, 2005).

14. See Evans, Hagiu, & Schmalensee, supra note 8.

15. See DAVID S. EVANS & RICHARD SCHMALENSEE, CATALYST CODE: THE STRATEGIES BEHIND THE WORLD'S MOST SUCCESSFUL COMPANIES (Harvard Business School Press 2007).

16. That is, the models are based on assuming particular functional forms-e.g. linear-for relationships.

17. To keep matters simple we consider the case where each side is charged a membership fee as in MARK ARMSTRONG, COMPETITION IN TWO-SIDED MARKETS (EconWPA,Working Paper, 2005). More generally, platforms are natural businesses for two-part tariffs involving an access fee and a usage fee.

18. This table shows pricing structures that are common in these industries. In many cases, fees will differ from these pricing structures. For example, some clubs offer free entry to women, some magazines offer free subscriptions, some videogame players pay fees for on-line play, and some payment cardholders do not pay fees for their cards and/or get usage based rewards. For dating clubs, usage fees for men and women refer to fees for drinks in the club. For real estate, the usage fee for sellers refers to the fee for selling a house; there is typically no fee for using the system to list or show a house. For shopping malls, the negative usage fee for shoppers refers to the free parking that is commonly available. For videogame consoles, players do not pay a fee for using the console, although they do pay for video games to the game developer (which in some cases is the same firm that makes the console and in other cases pays a royalty to the console manufacturer). For payment cards, cardholders are also subject to penalty fees, such as for exceeding credit limits or for late payments; we have not included these fees in the table.

19. See Rochet & Tirole (2006), supra note 6.

20. See, e.g., David S. Evans & Richard Schmalensee, A Guide to the Antitrust Economics of Networks, 10 ANTITRUST MAG. 36 (1996) and CARL SHAPIRO & HAL R. VARIAN, INFORMATION RULES: A STRATEGIC GUIDE TO THE NETWORK ECONOMY (Harvard Business School Press 1999).

21. See Evans, supra note 1.

22. For Apple OS release dates, see Jason Snell, Jaguar unleashed: Mac OS X 10.2 Arrives, MACWORLD, Sept. 1, 2002; Sarah Stokely, Apple Sets Panther Release Date, IDG DATA, Oct. 10, 2003.; and, Steven Musil, This Week in Tiger: Apple releases Mac OS X 10.4, CNET NEWS, Apr. 29, 2005.

23. For a general discussion on matching, search, and congestion see, for example, Robert Shimer & Lones Smith, Matching, Search, and Heterogeneity, 1 ADVANCES IN MACROECONOMICS (2001) and Mark Rysman, Competition Between Networks: A Study of the Market for Yellow Pages, 71 REV. ECON. STUDIES 483 (2004b).

24. MARK RYSMAN, AN EMPIRICAL ANALYSIS OF PAYMENT CARD USAGE (Boston University Department of Economics,Working Paper, 2004).

David S. Evans is Chairman of eSapience, Ltd. in Cambridge, MA, Managing Director of the Global Competition Policy Practice at LECG, Cambridge, MA and Executive Director of the Jevons Institute for Competition Law and Economics and Visiting Professor at University College London.

Richard Schmalensee is Professor of Economics and Management at the Massachusetts Institute of Technology (MIT) and the John C Head III Dean of the MIT Sloan School of Management, Cambridge, MA.

The content of this article is intended to provide a general guide to the subject matter. Specialist advice should be sought about your specific circumstances.

Mr David Evans 

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