Berger's research focuses on constitutional law. Overarching themes in Berger's writings includes judicial decision making in constitutional cases, with special attention to deference, fact finding, rhetorical strategies, and other under-theorized factors that help shape judicial opinions in constitutional cases. Berger has also written extensively about lethal injection litigation. Berger has testified in the Nebraska legislature about a variety of constitutional issues, including free speech, lethal injection, and the process for amending the U.S. Constitution. Berger is also the faculty advisor to the Law College's chapter of the American Constitution Society and to the Community Legal Education Project, which sends law students into Lincoln public schools to teach about the Constitution.