Using the Hypothetical Monopolist Test to Define the Relevant Market for Ad Tech

Rohan Shah —  23 September 2021

Policymakers’ recent focus on how Big Tech should be treated under antitrust law has been accompanied by claims that companies like Facebook and Google hold dominant positions in various “markets.” Notwithstanding the tendency to conflate whether a firm is large with whether it hold a dominant position, we must first answer the question most of these claims tend to ignore: “dominant over what?”

For example, as set out in this earlier Truth on the Market post, a recent lawsuit filed by various states and the U.S. Justice Department outlined five areas related to online display advertising over which Google is alleged by the plaintiffs to hold a dominant position. But crucially, none appear to have been arrived at via the application of economic reasoning.

As that post explained, other forms of advertising (such as online search and offline advertising) might form part of a “relevant market” (i.e., the market in which a product actually competes) over which Google’s alleged dominance should be assessed. The post makes a strong case for the actual relevant market being much broader than that claimed in the lawsuit. Of course, some might disagree with that assessment, so it is useful to step back and examine the principles that underlie and motivate how a relevant market is defined.

In any antitrust case, defining the relevant market should be regarded as a means to an end, not an end in itself. While such definitions provide the basis to calculate market shares, the process of thinking about relevant markets also should provide a framework to consider and highlight important aspects of the case. The process enables one to think about how a particular firm and market operates, the constraints that it and rival firms face, and whether entry by other firms is feasible or likely.

Many naïve attempts to define the relevant market will limit their analysis to a particular industry. But an industry could include too few competitors, or it might even include too many—for example, if some firms in the industry generate products that do not constitute strong competitive constraints. If one were to define all cars as the “relevant” market, that would imply that a Dacia Sandero (a supermini model produced Renault’s Romanian subsidiary Dacia) constrains the price of Maserati’s Quattroporte luxury sports sedan as much as the Ferrari Portofino grand touring sports car does. This is very unlikely to hold in reality.[1]

The relevant market should be the smallest possible group of products and services that contains all such products and services that could provide a reasonable competitive constraint. But that, of course, merely raises the question of what is meant by a “reasonable competitive constraint.” Thankfully, by applying economic reasoning, we can answer that question.

More specifically, we have the “hypothetical monopolist test.” This test operates by considering whether a hypothetical monopolist (i.e., a single firm that controlled all the products considered part of the relevant market) could profitably undertake “a small but significant, non-transitory, increase in price” (typically shortened as the SSNIP test).[2]

If the hypothetical monopolist could profitably implement this increase in price, then the group of products under consideration is said to constitute a relevant market. On the other hand, if the hypothetical monopolist could not profitably increase the price of that group of products (due to demand-side or supply-side constraints on their ability to increase prices), then that group of products is not a relevant market, and more products need to be included in the candidate relevant market. The process of widening the group of products continues until the hypothetical monopolist could profitably increase prices over that group.

So how does this test work in practice? Let’s use an example to make things concrete. In particular, let’s focus on Google’s display advertising, as that has been a significant focus of attention. Starting from the narrowest possible market, Google’s own display advertising, the HM test would ask whether a hypothetical monopolist controlling these services (and just these services) could profitably increase prices of these services permanently by 5% to 10%.

At this initial stage, it is important to avoid the “cellophane fallacy,” in which a monopolist firm could not profitably increase its prices by 5% to 10% because it is already charging the monopoly price. This fallacy usually arises in situations where the product under consideration has very few (if any) substitutes. But as has been shown here, there are already plenty of alternatives to Google’s display-advertising services, so we can be reasonably confident that the fallacy does not apply here.

We would then consider what is likely to happen if Google were to increase the prices of its online display advertising services by 5% to 10%. Given the plethora of other options (such as Microsoft, Facebook, and Simpli.fi) customers have for obtaining online display ads, a sufficiently high number of Google’s customers are likely to switch away, such that the price increase would not be profitable. It is therefore necessary to expand the candidate relevant market to include those closest alternatives to which Google’s customers would switch.

We repeat the exercise, but now with the hypothetical monopolist also increasing the prices of those newly included products. It might be the case that alternatives such as online search ads (as opposed to display ads), print advertising, TV advertising and/or other forms of advertising would sufficiently constrain the hypothetical monopolist in this case that those other alternatives form part of the relevant market.

In determining whether an alternative sufficiently constrains our hypothetical monopolist, it is important to consider actual consumer/firm behavior, rather than relying on products having “similar” characteristics. Although constraints can come from either the demand side (i.e., customers switching to another provider) or the supply side (entry/switching by other providers to start producing the products offered by the HM), for market-definition purposes, it is almost always demand-side switching that matters most. Switching by consumers tends to happen much more quickly than does switching by providers, such that it can be a more effective constraint. (Note that supply-side switching is still important when assessing overall competitive constraints, but because such switching can take one or more years, it is usually considered in the overall competitive assessment, rather than at the market-definition stage.)

Identifying which alternatives consumers do and would switch to therefore highlights the rival products and services that constrain the candidate hypothetical monopolist. It is only once the hypothetical monopolist test has been completed and the relevant market has been found that market shares can be calculated.[3]

It is at that point than an assessment of a firm’s alleged market power (or of a proposed merger) can proceed. This is why claims that “Facebook is a monopolist” or that “Google has market power” often fail at the first hurdle (indeed, in the case of Facebook, they recently have.)

Indeed, I would go so far as to argue that any antitrust claim that does not first undertake a market-definition exercise with sound economic reasoning akin to that described above should be discounted and ignored.


[1] Some might argue that there is a “chain of substitution” from the Maserati to, for example, an Audi A4, to a Ford Focus, to a Mini, to a Dacia Sandero, such that the latter does, indeed, provide some constraint on the former. However, the size of that constraint is likely to be de minimis, given how many “links” there are in that chain.

[2] The “small but significant” price increase is usually taken to be between 5% and 10%.

[3] Even if a product or group of products ends up excluded from the definition of the relevant market, these products can still form a competitive constraint in the overall assessment and are still considered at that point.