Institutional investors relying on Environmental, Social, and Governance (ESG) ratings to assess risk may be receiving data optimized for stability rather than accuracy, according to a new working paper from MIT Sloan School of Management. The research, titled "The Traffic Light Effect in ESG Ratings," indicates that the world’s largest ESG ratings provider, MSCI, holds ratings steady even when underlying data suggests a change is warranted. This practice prevents necessary market signals from reaching investors, potentially leading to significant financial mispricing.
The study, led by MIT Sloan principal research scientist Florian Berg alongside Jess Cornaggia, Cristian Foroni, and Francesco Tripoli, analyzed MSCI ESG Ratings monthly from 2014 to 2022. The researchers reverse-engineered MSCI’s internal scoring process to compare published ratings against what the provider’s own methodology would have dictated. They discovered that scores frequently piled up just below the thresholds for higher letter grades and just above the thresholds for lower ones, a phenomenon they termed the "traffic light effect."
Internal Review and Volatility
MSCI’s internal review committees intervene when specific conditions are met, such as a proposed upgrade to AAA, a downgrade to CCC, or a change of two or more notches. The study found that these human interventions, rather than mathematical models, introduced the bunching of scores near rating boundaries. "When scores would cross the threshold, committees keep them below that threshold," said Berg, suggesting that the actions aim to avoid ratings volatility that could trigger costly portfolio reshuffles for clients.
The data reveals that only 24.3% of the 129,702 firm-years undergoing committee reassessment resulted in a letter grade change. Among those that did change, upgrades significantly outnumbered downgrades, totaling 20,588 compared to 10,944. This imbalance suggests an upward bias in the changes that are implemented, further distancing the ratings from the underlying data.
Financial Impact on Investors
The suppression of warranted downgrades has tangible financial consequences. The researchers found that when a ratings downgrade does go through, the affected company’s stock drops an average of 2.8% over the following year. However, when a downgrade is suppressed, this signal never reaches the market. For a company with a $10 billion market cap, the gap between actual stock performance and the performance that would have followed a downgrade represents roughly $300 million to $400 million in avoided market-value loss.
| Metric |
Finding |
| Firm-years reviewed |
129,702 |
| Reassessments leading to grade change |
24.3% |
| Total upgrades |
20,588 |
| Total downgrades |
10,944 |
| Avg. stock drop after downgrade |
2.8% |
| Avoided loss for $10B company |
$300–$400 million |
To rule out the possibility that MSCI and its peers were reacting to the same underlying information, the researchers exploited a methodology overhaul by a competitor in 2020. The results confirmed that MSCI’s adjustments tracked peer ratings that existed at the time of each decision, rather than rewritten historical scores, validating the finding that the adjustments are deliberate rather than reactive to common fundamentals.
"ESG ratings don’t necessarily reflect the most timely information because there’s this incentive to slow down the timeliness to reduce portfolio turnover," said Berg. He cautioned investors that the ratings they pay for are not purely the product of MSCI’s stated methodology, but rather a blend of data and a desire for stability that impacts returns.