Dr. Riccardo Sabbatucci, Associate Professor of Finance at the Stockholm School of Economics, opened his presentation with a clear framing of his and his coauthors’ — Andrea Tamoni and Song Xiao — research agenda: understanding the structural impact of 401(k) plans on U.S. equity markets. As he noted, over half of private-sector American workers now participate in 401(k) plans, making them the dominant retirement savings vehicle in the country. These plans hold a substantial share of their assets in equities—mainly through mutual funds and ETFs—linking the retirement system tightly with the stock market.
The central finding of the study is that the move from active to passive investment is not just a general market phenomenon—it is happening decisively within 401(k) plans as well. Using data from the BrightScope Beacon dataset and Morningstar fund-level holdings, the research team documented that passive equity investments in 401(k)s have increased by roughly 400% over the past 10 to 15 years. By 2020, nearly 40% of all equity assets in these plans were in index funds. Sabbatucci emphasized that this structural shift has profound implications, arguing that “the shift is ongoing… not only in the aggregate market, but also within 401(k) plans.” His empirical findings show that stocks with higher 401(k) ownership tend to perform significantly better, earning about 3–5% higher annual returns than comparable stocks without such ownership.
To explore how this ownership affects equity prices, Sabbatucci and his coauthors combined reduced-form estimation with structural modeling. They introduced a novel variable measuring institutional ownership by 401(k) plans at the stock level, noting that it ranked as “the second, after size, most important characteristic in explaining the average demand for stocks by funds.” His econometric approach revealed that a 10% increase in 401(k) demand can raise stock prices by 3.5–4%. The analysis also showed that demand responses differ across asset categories: large-cap and value stocks tend to benefit from the shift, while mid-cap and growth stocks often depreciate. This heterogeneity, he argued, underscores how passive flows can create uneven pricing pressures and reshape equilibrium in financial markets.
In the latter half of his presentation, Sabbatucci introduced a counterfactual scenario to estimate how equity prices might adjust if all 401(k) plans offered only passive investment options. The model incorporated complex feedback loops between prices, ownership, and flows—what he described as “a recursive cycle” where “price changes affect ownership, which feeds back into equilibrium prices.” The simulations suggested that in such a scenario, stocks with large expected inflows would experience gains of up to 20–30%, while those with outflows would fall by 10–20%. Crucially, the analysis distinguished between cases where investors display a preference for 401(k) ownership and where they do not—showing that the mere perception of stable, long-term 401(k) investors can amplify pricing effects even further.
Sabbatucci concluded by stressing the broader implications of this trend. The transition toward passive management within retirement accounts, he suggested, may be altering the landscape of equity demand and price discovery in ways regulators and asset managers have yet to fully grasp. The study’s findings imply that 401(k) ownership has become an economically significant, yet previously overlooked, form of institutional ownership—one that may contribute to valuation distortions and stability dynamics in equity markets. As Sabbatucci summarized, “we show that 401(k) ownership is a key variable… it matters for the demand of funds for individual stocks, and it contains independent information relative to other forms of institutional ownership.”