University of Chicago Law Review

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Under the rule of prosecution history estoppel, patent applicants who amend their claims during the course of patent prosecution assume a significant risk: the risk that a court will later construe the changes as concessions that should be read to limit patent scope. This risk is exacerbated by strong evidentiary presumptions under which courts are to assume, unless the patentee presents sufficient evidence to the contrary, that every change triggers estoppel, and that the resulting estoppel forfeits everything except that which the revised claim language literally describes. The justification for these presumptions is that, implemented in this fashion, prosecution history estoppel makes patent scope more predictable. In this Essay, I argue that the benefit comes at too high a price. Drawing on a large empirical study of patent prosecution, I show that, because of these evidentiary presumptions, estoppel is dangerously sensitive to differences among patent examiners and differences across technology categories. That is, estoppel treats similar applications in dissimilar ways, not because of differences on the merits, but instead because of the personal characteristics of the examiners involved and because of differences inherent to the types of technology at issue. A better rule, I argue, would minimize the significance of examiner and technology disparities by reversing the current evidentiary presumptions and thus recognizing estoppel only where there is clear evidence that the applicant and the examiner intended to forfeit a given scope of coverage.