Research

Publication

• Kei Ikegami, Kyohei Okumura, and Takumi Yoshikawa, A Simple, Fast, and Safe Mediator for Congestion Management, AAAI '20 (Oral and Poster presentation)
• Kei Ikegami, Ken Onishi, and Naoki Wakamori, Competition-Driven Physician-Induced Demand, Journal of Health Economics, Vol. 79, September 2021

Working Paper

• Kei Ikegami, Exposure Design for Two-Sided Platforms
  Abstract

  I study exposure design on two-sided platforms where users search sequentially. I show that an exposure rule that maximizes flow match surplus can cannibalize the platform’s long-run user value by thinning future options of weak participants. I formalize this wedge and pose the platform’s problem of maximizing user value—the sum of equilibrium continuation values. To solve it, I develop a tractable algorithm, Nested Fixed-Point Sinkhorn Annealing (NFSA), which nests fixed point problem inside a Sinkhorn algorithm and anneals to the zero-temperature limit. Taking a doctor–spot-job platform as an application filed, I estimate preferences of participants and exposure rules implemented in the platform, then compute the user-value-optimal exposure rule by applying a variant of NFSA. The optimal rule raises continuation values on both sides, makes post-side exposure less even, places less weight on geographic proximity, and shifts older doctors toward higher-hour posts while steering younger doctors toward higher-salary posts.
  Presentations

  ERATO weekly meeting
• Kei Ikegami, Bargaining over Leasing Contracts (new draft coming soon)
  Abstract

  This paper investigates how kinked revenue-sharing contracts are negotiated and how they embed bargaining power. Using proprietary panel data on tenant contracts and sales in shopping malls, I estimate a structural model in which a risk-averse tenant and mall renegotiate contracts based on realized sales. The results show that the mall-to-tenant power ratio shapes contract terms through two channels: (i) risk posture—powerful malls can insist on low-variance, high-fixed-rent contracts; and (ii) sales expectations—pessimistic forecasts cap fixed rent and shift weight to commissions. Because these forces offset, contract form alone seldom signals power imbalance. Counterfactual simulations imposing fairer bargaining weights indicate that, after risk adjustment, total rent can triple even when the mall is relatively weak.
  Presentations

  ESWC 2025, IIOC 2025, APIOC 2024, UTMD rising star in market design, NYU seminar

• Kei Ikegami, Atsushi Iwasaki, Akira Matsushita, and Kyohei Okumura, Evaluating the Efficiency of Regulation in Matching Markets with Distributional Disparities (New draft: July 7, 2025) Accepted for EC'25 (Exemplary paper award for the Empirics track)

  Abstract

  Cap-based regulations are widely used to address distributional disparities in matching markets, but their efficiency relative to alternative instruments such as subsidies remains poorly understood. This paper develops a framework for evaluating policy interventions by incorporating regional constraints into a transferable utility matching model. We show that a policymaker with aggregate-level match data can implement a taxation policy that maximizes social welfare and outperforms any cap-based policy. Using newly collected data from the Japan Residency Matching Program, we estimate participant preferences and simulate counterfactual match outcomes under both cap-based and subsidy-based policies. The results reveal that the status quo cap-based regulation generates substantial efficiency losses, whereas small, targeted subsidies can achieve similar distributional goals with significantly higher social welfare.
  Presentations

  SITE 2025, EC 2025, IIOC 2025, APIOC 2024, NYU seminar, JEA annual meeting 2024

• Kei Ikegami, Ken Onishi, and Naoki Wakamori, Joint Venture Formation in Procurement Auctions (New draft: Oct 23, 2024)

  Abstract

  We propose a model for joint venture formation in the context of procurement auctions. This model enables us to identify the formation mechanism, which is necessary for simulating counterfactual auction settings. We estimate the model using newly collected Japanese procurement auction data. Our estimates reveal the presence of cost synergies: joint ventures are more likely to be cost-effective compared to standard bidders. Despite this pro-competitive effect, our simulation indicates that excessive encouragement of joint ventures hinders cost-effective procurement by reducing competition. This anti-competitive effect arises from the diminished incentive to enter the auction due to the possibility to compete with joint ventures.
  Presentations

  AEA 2024, Otaru University of Commerce, Yokohama National University, NYU seminar, APIOC 2022

• Kei Ikegami and Hikaru Kawarazaki, Social Learning in Budget Formulation: A Case of Adaptation to Natural Disasters

  Abstarct

  This paper studies social learning among policymakers when formulating a budget. Focusing on the disaster prevention policy, we model local governments’ expenditures as a response to the expected future disaster risk. Through this model, we associate the correlation of the expenditures between local governments with the connection in the social learning about the risk. We use Japanese administrative data over 20 years to estimate a sparse social learning network. Moreover, as the network formation determinants, we find significant effects of (1) the inflow of internal migration between local governments, (2) the risk of future earthquakes, and (3) the risk preference of the local citizens.
  Presentations

  KCL, UCL, IFS, GRIP, AASLE Annual Conference at the University of Tokyo, and the Applied Econometric Conference at Waseda University

Work in Progress

• Kei Ikegami and Kan Kuno, Wage Stagnation in Daycare Industry: A Two-Sided Market Perspective