The But-For World: The Conceptual Foundation

Antitrust damages analysis rests on a single organizational idea: the but-for world. An economist is tasked with reconstructing what market conditions would have looked like absent the challenged conduct. For example, what prices would have prevailed, what output would have been sold, the profits that would have been earned. That is then compared to what actually happened. Damages are the difference between the two: but-for and actuals.

This framing is deceptively simple. The actual world is observable; the but-for world is not. It must be constructed through economic reasoning and empirical evidence, and every assumption embedded in that construction is a potential point of attack. The discipline of a good damages analysis lies in building the counterfactual from data and economic logic rather than from advocacy, and in isolating the effect of the unlawful conduct from the many other factors that move prices and profits in the real world such as input cost changes, demand shocks, macroeconomic conditions, entry and exit, and lawful competitive behavior by the defendant(s).

Because the but-for world is constructed, courts and opposing experts often scrutinize it closely. The economist must be able to explain not only what the counterfactual looks like but why it is the right counterfactual, grounded in the specific theory of harm the plaintiff has advanced. A damages model disconnected from the liability theory is vulnerable regardless of how sophisticated its econometrics appear.

Two Families of Harm: Overcharges and Lost Profits

Antitrust damages generally fall into two broad categories: overcharges or lost profits.

Overcharges refer to the injury suffered by purchasers who paid inflated prices as a result of anticompetitive conduct. In a price-fixing conspiracy, for example, the overcharge is the difference between the price actually paid and the competitive price that would have prevailed in the but-for world, multiplied by the quantity purchased.

Lost profits are the injury suffered by a firm, often a competitor, that was excluded, foreclosed, or otherwise harmed in its ability to compete. When a competitor forecloses a rival through exclusive dealing, tying, or predatory conduct, the rival’s damages are typically measured by the profits it would have earned in the but-for world but did not earn because of the exclusionary conduct.

The two categories demand different analytical tools and methods. Overcharge estimation focuses on price: the economist models the but-for price and measures the gap. Lost-profits estimation focuses on the plaintiff’s own business performance: the economist models the revenues and costs the plaintiff would have realized and derives the profit it was denied. A single case can involve both. For example, when a firm is both a purchaser paying inflated input prices and a competitor foreclosed from a downstream market.

Principal Methodologies Employed by Economists

Economists rely on several established methodologies to construct the but-for world. Most damages models use one or a combination of the approaches outlined below.

Before-and-After

The before-and-after method compares market outcomes during the period of alleged anticompetitive conduct to outcomes in a benchmark period when the conduct was absent. This is typically before the conspiracy began, after it ended, or both. The core assumption is that, controlling for other observable factors, the “clean” period reveals what prices or profits would have looked like during the “dirty” period.

The method’s appeal is that it uses the same market, the same products, and often the same firms, which reduces the risk that unobserved differences distort the comparison. Its weakness is that market conditions change over time. If demand grew, input costs rose, or the competitive structure shifted between the benchmark and impact periods, a naive comparison will confound those changes with the effect of the conduct. In such comparisons, it is important to use a regression framework to hold constant the other determinants of price.

Yardstick (Benchmark) Analysis

The yardstick method compares the affected market to a similar but unaffected market. This might include a different geographic region, a comparable product, or an analogous industry that was not affected by the challenged conduct. The unaffected market serves as a proxy for the but-for world.

The validity of a yardstick analysis turns entirely on comparability. The benchmark market must be similar in the economically relevant respects. For example, cost structures, demand characteristics, and competitive dynamics should be similar so differences in outcomes can be attributed to the conduct rather than to preexisting differences between the markets.

Regression and Econometric Modeling for Antitrust Damages

Regression analysis is the workhorse of estimating antitrust damages. By modeling price (or profit) as a function of the conduct alongside a set of control variables such as input costs, demand shifters, seasonality, product characteristics, macroeconomic conditions, etc. the economist can isolate the incremental effect of the unlawful conduct while holding other influences constant.

A common specification uses the logarithm of price as the dependent variable, which allows coefficients to be interpreted in approximate percentage terms and often improves the statistical properties of the model. Fixed effects can absorb unobserved heterogeneity across products, regions, or time periods, and interaction terms can allow the estimated effect to vary across product types or customer segments. The economist must also attend to the mechanics that make a regression credible: testing the sensitivity of results to specification choices and ensuring that the standard errors properly reflect the structure of the data.

Regression’s strength is its ability to control for confounding factors transparently and to produce a quantifiable measure of statistical reliability. Its vulnerability is specification: opposing experts will probe whether the right variables were included, whether the functional form is appropriate, and whether the results are robust to reasonable alternative choices. A regression that produces a large overcharge estimate but proves fragile to specification changes invites problems.

Estimating Overcharges

In a typical overcharge analysis, the economist estimates the but-for price and computes damages as the overcharge per unit multiplied by the affected volume of commerce. When the direct purchaser is not the ultimate consumer (think a distributor who buys at inflated prices and resells downstream), the question of how much of the overcharge was passed on to indirect purchasers becomes central. Pass-through analysis affects both the allocation of damages among plaintiffs at different levels of the chain and, in some jurisdictions, whether particular plaintiffs have standing to recover at all. Expert economists estimate pass-through rates using the same regression toolkit, modeling how downstream prices respond to changes in upstream costs.

Volume of commerce is another consideration for an economist. The overcharge percentage is only half the calculation; it must be applied to the correct base of affected transactions. Defining that base (which products, which customers, which time period) is a crucial and necessary step.

Estimating Lost Profits

Lost-profits analysis reconstructs the plaintiff’s but-for financial performance. The economist estimates the revenues the plaintiff would have earned absent the exclusionary conduct and subtracts the costs it would have incurred to generate those revenues, yielding the incremental profit that was lost.

The revenue side typically requires modeling the market share or sales trajectory the plaintiff would have achieved in a competitive but-for world. This is often anchored to its performance before the conduct began, to the performance of comparable firms, or to the growth of the overall market. The cost side requires distinguishing incremental costs, which would have been incurred to serve the additional business, from fixed costs, which would not vary with the lost sales. Only incremental costs are properly deducted; treating fixed costs as if they scaled with output would understate the profit that was lost.

Lost-profits models face a particular tension between ambition and defensibility. A plaintiff naturally wants to claim it would have captured a large share of a growing market. But projections that assume aggressive, unproven growth invite the charge that the damages are speculative or unsupported. The economist’s role is to ground the but-for trajectory in evidence such as historical performance, comparable firms, documented business plans, market data, etc. rather than optimistic assumptions.

Disaggregation and Causation

A defendant’s conduct often involves several distinct strands. This can include some challenged as unlawful and others lawful or not at issue. Courts require that damages be attributable to the unlawful conduct specifically, not to lawful competition, the plaintiff’s own business missteps, or general market conditions.

This is the problem of disaggregation. When a plaintiff alleges several theories of harm, or when only some of a defendant’s actions are found unlawful, the economist must be able to apportion damages across conduct strands and isolate the portion caused by the actionable conduct. A damages model that lumps all harm together, without a principled way to separate lawful from unlawful causes, is exposed to the argument that it overstates recoverable damages. Building disaggregation into the model from the outset, rather than treating it as an afterthought, is a hallmark of a well-constructed analysis.

Causation also interacts with the legal standard for the certainty of damages. Courts have long recognized a distinction between proving the fact of damage, which is held to a demanding standard, and proving the amount of damage, where some latitude is permitted once injury is established. This asymmetry reflects a practical reality: the defendant’s own wrongful conduct is often what makes precise measurement difficult, and courts have been reluctant to let wrongdoers escape liability by pointing to uncertainty they created. Even so, the amount cannot be pure conjecture; it must rest on a reasonable basis in evidence.

The Class Certification Dimension

In class actions, damages methodology carries weight well before trial. To certify a class, plaintiffs must generally show that antitrust impact and damages can be established through evidence common to the class rather than through thousands of individualized inquiries. The expert economist’s model is often the centerpiece of this showing: it must demonstrate that impact was widespread and that damages can be calculated on a classwide basis using a common methodology.

Standards, Rigor, and the Expert Economist’s Obligations

Whatever methodology is chosen, the expert economist’s analysis will be tested against the standards governing expert testimony.

This places a premium on transparency and discipline. Assumptions should be stated explicitly and defended on economic grounds. Results should be tested for robustness across reasonable alternative specifications. The distinction between what the data show and what the economist infers should remain clear throughout.

Conclusión

Quantifying antitrust harm is where economic theory meets the exacting demands of litigation. The expert economist’s role is to build a defensible counterfactual and to measure the gap between it and reality using methods that are tested and withstand adversarial scrutiny. Whether the injury takes the form of overcharges paid by purchasers or profits lost by excluded competitors, the analytical discipline is the same: ground the counterfactual in evidence, control rigorously for confounding factors, isolate the harm caused by the unlawful conduct, and connect every step of the calculation to the underlying theory of liability.

Done well, a damages analysis does more than produce a number. It tells a coherent economic story about what the market would have looked like in the absence of the challenged conduct—a story credible enough to persuade a court, survive cross-examination, and support the weight the case places on it.