Langbein's research has focused on taxation, with a particular emphasis on U.S. tax treaty policy. In his early work, he co-authored the article "United States Tax Treaty Policy: An Overview," in which he discussed the primary objective of U.S. tax treaties—eliminating double taxation and promoting international trade.[4]
In the mid-1980s, Langbein published an article, entitled "The Unitary Method and the Myth of Arm's Length," which questioned conventional wisdom concerning the manner in which corporate profits are allocated for tax purposes among various countries in which multinational enterprises operate. The established method for doing this allocation was the "arm's length" method, which determined the allocation based on the hypothetical transfer prices the different national components of the enterprise would charge each other at "arm's length." This article questioned the extent to which this method historically constituted the antithesis of alternative methods of allocation based on productive factors located in the various jurisdictions involved. The article identified ways in which the arm's length method is theoretically defective and facilitates tax evasion and avoidance by multinational enterprises.[5]
When it enacted the major 1986 tax reform, Congress mandated a comprehensive study of transfer pricing by the Treasury Department. The department completed this study in 1988, and its chapter on the theory of transfer pricing relied on Langbein's article, particularly its emphasis on the "continuum price problem" generated by the arm's length standard. Langbein responded to the publication of this White Paper in several articles, including a proposal for reform presented at the University of Michigan's annual tax policy conference in 1991.[6]
The United States amended its regulations governing transfer pricing in 1993 and 1994 and worked with the Organization for Economic Co-operation and Development (OECD) in developing international guidelines reflecting the approach embodied in the revised United States regulations. Bill Clinton, during his 1992 election campaign, had advocated reforms along the lines suggested by Langbein. After taking office, the then-President supported watered-down proposals, which embodied substantial concessions to the OECD.[7]
From 2010 to 2012, reports exposed flaws in 1990s transfer pricing rules, showing many corporations paid minimal taxes. This led to congressional hearings and the adoption of the G20 and OECD's base erosion and profit shifting (BEPS) initiative to reform transfer pricing and the international tax system. In response to the completion of the first phase of this BEPS project, Langbein in 2018, along with German scholar Max Fuss, published a study approving of the reform effort but critical of its limitations at that stage.[8]
The BEPS initiative entered a second phase, with the adoption in 2021 of the two-pillar approach to international tax reform.[9]
In 2001, Langbein authored a book Federal Income Taxation of Banks and Financial Institutions, where he addressed tax challenges for financial institutions, including the Alternative Minimum Tax, currency transactions, interest expenses, and compliance issues.[10] In a collaborative 2018 study with Max R. Fuss, he highlighted how the 1990s tax regime allowed "stateless" income, limiting corporate profit taxation, and how the BEPS initiative aimed to address this, though its rules remained complex and needed refinement.[11] Extending this focus, he demonstrated that the Tax Cut and Jobs Act shifted the U.S. from a global tax system to a modified territorial system, with a focus on how income was allocated across jurisdictions, particularly regarding international intangible income.[12]
Langbein also explored whether congressional actions during the thrift crisis, particularly supervisory goodwill, constituted a "taking" under the Takings Clause, concluding that the Supreme Court's Winstar decision suggested significant government actions affecting private rights in regulated industries may require compensation.[13]