National Bureau of Economic Research

The US stock market has become more concentrated in recent years. Between 2015 and 2024, the share of the 10 largest stocks in total market capitalization rose from 13 to 31 percent, with the “Magnificent 7” companies alone accounting for roughly one-third of the S&P 500 by the end of 2024. This concentration poses a practical challenge for the thousands of investment funds that must comply with long-standing diversification rules designed for a far less top-heavy market.

Every time a consumer swipes a credit card, the merchant pays an interchange fee—typically around 1.9 percent of the transaction value—most of which funds the rewards that cardholders receive. Because merchants generally charge the same prices regardless of how customers pay, consumers who use cash or debit cards effectively help finance the rewards enjoyed by credit card users. In Who Pays for Payments? (NBER Working Paper 35067), Mark L.

Declining fertility and the resulting aging of a nation’s population can affect a number of macroeconomic outcomes. By slowing labor force growth, it can reduce GDP, increase wages, reduce firm entry, and lower innovative activity. By increasing the demand for assets, it can reduce interest rates and raise capital intensity and labor productivity.

We would like to thank Andrew Atkeson, Markus Brunnermeier, Dan Cao, Darrell Duffie, Joachim Hubmer, Mark Huggett, Magnus Irie, Tim Landvoigt, Cedomir Malgieri, Emile Marin, Peter Maxted, Sebastian Merkel, Konstantin Milbradt, Simon Oh, Stavros Panageas, Dimitris Papanikolaou, Sergio Rebelo, Tano Santos, Lawrence Schmidt, Dejanir Silva, Sanjay Singh, Fabrice Tourre, Stijn van Nieuwerburgh, Toshihiko Mukoyama, Gianluca Violante, and conference and seminar participants at Northwestern Kellogg,...

For helpful feedback and suggestions, we thank Omer Ali, Alex Bartik, Pat Bayer, Rebecca Diamond, Jonathan Dingel, Ingrid Gould Ellen, Robert French, Ed Glaeser, Jesse Gregory, Lu Han, Gordon Hanson, Daniel Hartley, Nathaniel Hendren, Larry Katz, David Phillips, Raven Molloy, Chris Palmer, Elizabeth Roberto, Esteban Rossi-Hansberg, Matthew Staiger, Chris Timmins, and numerous seminar participants. For excellent research assistance, we thank Akash Banerjee.

I am grateful to Humoyun Abdumavlon, Omer Ali, Sascha O. Becker, Stephan Heblich, Sharun Mukand, Max Steinhardt, Patrick A. Testa, Gabby Toborg, and Sebastian Garcia Torres for helpful discussion and comments. Replication files for all worked examples in this guide are available at: https://doi.org/10.3886/E249897V2 The views expressed herein are those of the author and do not necessarily reflect the views of the National Bureau of Economic Research.

We thank Max Croce, Luigi Bocola, John Cochrane, Tim De Silva, Arvind Krishnamurthy, James Parron, and the participants of the Stanford macro-finance lunch, the IMF-Bocconi University-Bank of Italy Conference, and the Italian Ministry of Economy and Finance Annual Report Conference on Italian Bonds for helpful comments and discussions.

We thank Marcella Alsan, Colleen Carey, Emma Dean, Coleman Drake, Kelsey Moran, Alberto Ortega, and Julian Reif, as well as seminar participants at Agency for Healthcare Research and Quality (AHRQ), American University, the 2026 AEA/ASSA Annual Meeting, the 2026 Annual Health Economics Conference (AHEC), and the 2026 NBER Geography and Health meeting for helpful comments on earlier versions of this manuscript. Some of the results in this paper are based on restricted-use and/or proprietary data.

We are grateful to Sebastian Di Tella, Chris Tonetti, and to seminar participants at Brown, Pacific Macro 2026, Stanford, and Yale for helpful discussions. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research. Copy Citation Philip Trammell and Charles I. Jones, "When GDP Misleads: Inferring Living Standards from the Value of a Statistical Life," NBER Working Paper 35382 (2026), https://doi.org/10.3386/w35382.

We thank Nick Ariniello, Patricia Belden, Deep Biswas, Michael A. Cantalupa, Arion Dako, Brian Golden, Jeffrey Goldstein, Reuben Kantor, Brian Kavoogian, Renee LeFevre, Abe Menzin, Young Park, Jerome L. Rappaport, and Matthew Resseger for providing data and comments. We thank Tim Gray, Stephanie Kestelman, and Jamie Murray for editorial assistance. We are responsible for the analysis and the conclusions provided in the report.