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.

In The Hidden Cost of Stock Market Concentration: When Funds Hit Regulatory Limits (NBER Working Paper 35007), researchers Lubos Pastor, Taisiya Sikorskaya, and Jinrui Wang investigate how a specific regulatory...

What happens when workers get a formal seat at the table in corporate governance? In many European countries, laws require that worker representatives serve on company boards and participate in management decisions, a shared governance system known as codetermination. Germany's version, dating to the postwar era in its current form, is perhaps the most prominent: workers elect representatives to corporate supervisory boards, and establishment-level works councils participate in day-to-day workplace decisions. During its long history, codetermination has regularly attracted attention in countries that typically exclusively rely on shareholder control, such as the United States or the United Kingdom. The central question for economists is whether giving workers formal representation in firm governance meaningfully affects wages,…

Roughly one-quarter of new employer businesses in the United States are started by immigrants. Immigrant inventors have been responsible for approximately 23 percent of US patents produced since 1976 despite making up only 16 percent of the total US-based inventor population. Yet immigrant entrepreneurs usually do not build companies in isolation—many cofound startups alongside US-born entrepreneurs. In Native-Immigrant Entrepreneurial Synergies (NBER Working Paper 33804), Zhao Jin, Amir Kermani, and Timothy McQuade examine whether startups cofounded by immigrant and native entrepreneurs outperform those with founders from exclusively one...

Immune checkpoint inhibitors (ICIs) are immunotherapy drugs that mobilize the patient’s immune system to detect and attack cancer cells. They are considered a breakthrough development in cancer care, but are very expensive, with a full course of treatment costing more than $150,000 per patient. In The Impact of Immunotherapy on Reductions in Cancer Mortality: Evidence from Medicare (NBER Working Paper 34317), Danea Horn, Abby E. Alpert, Mark Duggan, and Mireille Jacobson use Medicare claims data to evaluate the impact of the first ICIs on healthcare use, costs, and mortality among beneficiaries diagnosed with...