When “playing it safe” backfires in financial markets

For decades, institutional investors managing trusts in the United States operated under a paradox: in trying to be prudent, they were often forced to make inefficient investment decisions. Rooted in a legal framework that evaluated risk one asset at a time, fiduciary duties discouraged diversification—despite modern finance clearly showing its benefits. When regulation finally caught up with theory in the late twentieth century, the shift didn’t just improve portfolio performance. It also revealed something deeper about how rules shape investor behavior—and even move markets.

This article was written by Emanuele Rizzo, Assistant Professor in the Department of Economics, Finance and Accounting at Esade, and member of the Group for Research in Economics and Finance (GREF). 

From “prudent man” to modern investor

Modern portfolio theory has advocated diversification for more than 70 years. Yet, for much of that time, a significant share of U.S. institutional capital—trust-managed assets—was constrained by an outdated legal standard.

Under the “prudent man” rule, courts evaluated investment decisions stock by stock rather than at the portfolio level. This discouraged diversification, as each individual holding had to be defensible on its own.

The turning point came in 1985 with the introduction of the Uniform Prudent Investor Act. This reform replaced the old standard with a new one aligned with modern finance, explicitly recognizing diversification as a cornerstone of prudent investing.

A natural experiment in financial regulation

The reform created a unique opportunity to isolate its effects. It applied specifically to trusts, while other institutional investors—such as mutual funds, hedge funds, and insurers—remained unaffected. This distinction provides a natural control group.

Additionally, the staggered adoption of the law across U.S. states introduced variation over time, strengthening the ability to identify causal effects on investment behavior and market outcomes.

When diversification becomes the “prudent” choice

Before the reform, trust portfolios displayed clear biases. They were tilted toward so-called “prudent” stocks—assets that could withstand legal scrutiny individually. These regulatory tilts reflected not just risk-return considerations, but also the need for legal defensibility.

Once the legal framework shifted, these patterns began to unwind. Trusts reduced their exposure to the most “prudent” stocks and reallocated capital toward less conservative assets.

But the shift was not indiscriminate. Rather than moving into highly speculative stocks, trusts targeted assets that improved diversification—particularly those negatively correlated with their existing portfolios.

The real payoff: safer and better portfolios

The most important outcome was not the reallocation itself, but the improvement in performance.

Following the reform, trust portfolios became significantly more efficient. Average returns increased by around two percentage points per year, while annual volatility decreased by approximately 1.8 percentage points.

Ironically, a legal framework designed to protect beneficiaries had been doing the opposite. Only by embracing diversification did portfolios become both safer and more profitable.

When investor behavior moves markets

These changes didn’t just affect portfolios—they also left a mark on asset prices.

In the year following a state’s adoption of the reform, a strategy that buys stocks trusts are expected to purchase and sells those they are expected to offload generates a significant abnormal return—around 0.45% per month, or 5.4% annually.

This suggests that shifts in demand driven by regulatory change can influence prices in a meaningful and persistent way, consistent with the idea of inelastic equity markets.

Measuring how flexible markets really are

Using these regulatory changes as demand shocks, the analysis estimates stock-level price elasticities between 0.10 and 0.36. These figures are far from the theoretical benchmark of perfectly elastic markets.

The findings reinforce a growing body of evidence: markets do not instantly absorb changes in demand. Instead, prices adjust gradually, reflecting frictions and limits to arbitrage.

Rethinking what “prudence” really means

The broader lesson is clear. When well-intentioned rules ignore the fundamentals of financial theory, they can distort investor behavior and undermine market efficiency.

In this case, a mandate to “be prudent” ended up discouraging the very strategy—diversification—that makes portfolios more resilient. Aligning regulation with economic principles not only improved outcomes for investors, but also revealed how deeply rules can shape the functioning of markets.