When Optimism Fuels Crises: The Hidden Side of Financial Cycles
Economic crises are often portrayed as sudden, almost unpredictable events: a bank failure, a global shock, a collapse in confidence. Yet, looking at recent history—from the 2008 financial crisis to the most recent turmoil—a recurring pattern emerges: periods of greatest instability are almost always preceded by phases of strong expansion.
This sequence raises a fundamental question. If crises were truly caused only by external shocks, why do they tend to occur precisely after periods of sustained growth? One possible answer is that fragility does not come from the outside, but builds up slowly from within the economic system, fueled by the behavior of the market participants themselves.
The role of “diagnostic expectations”
The work of Nicola Gennaioli (Bocconi University, Department of Finance, IGIER) together with Pedro Bordalo (University of Oxford), Andrei Shleifer (Harvard University), and Stephen Terry (University of Michigan) is based on this insight. Their study introduces the idea that the expectations of economic agents are not perfectly rational, but tend to overreact to the most recent information.
This mechanism, defined as “diagnostic expectations”, leads individuals and investors to overestimate the probability that current conditions—especially if positive—will persist into the future. In other words, when the economy is doing well, people tend to believe it will continue to do well.
According to Nicola Gennaioli, in fact,
“Good times produce economic and financial fragility, predicting future disappointment of expectations, low bond returns, and investment declines.”
Optimism, therefore, is not harmless. It becomes a force that pushes the system toward an unstable equilibrium, in which investment and financing decisions are based on overly rosy expectations.
How a credit cycle arises
By incorporating this behavior into a standard macroeconomic model, the paper shows how credit cycles are generated. During expansionary phases, widespread optimism has concrete e al effects: firms increase investment, debt issuance rises, and the cost of credit falls because risk is perceived as low.
This process further fuels growth, creating a virtuous cycle. But it is precisely this dynamic that makes the system vulnerable. When expectations begin to cool (even in the absence of particularly severe shocks), the process rapidly reverses. Credit tightens, investment declines, and returns shrink.
The strength of the model lies in its simplicity: no extreme assumptions are needed to generate instability. A moderate level of overreaction to news is sufficient to produce dynamics very similar to those observed in the data.
As the authors note,
“To generate the size of spread increases observed during 2007–2009, the model requires only moderate negative shocks.”
In other words, deep crises can emerge even without major changes in economic fundamentals. It is the change in expectations that makes the difference.
From theory to data: optimism precedes disappointment
The authors studied corporate executives’ forecasts and found a systematic pattern: during periods of growth, expectations are overly optimistic and are regularly disappointed.
This phenomenon is not confined to forecasts. It has concrete effects on financial markets and the real economy. Phases of optimism are followed by lower bond yields and a slowdown in investment, a sign that decisions made during the boom were based on overly positive assessments.
The interesting point is that these dynamics are observed at the micro level—that is, at the level of individual firms—but aggregate to influence the entire economic system.
Rethinking financial instability
The paper’s contribution goes beyond explaining economic cycles. It offers a different perspective on what “financial shocks” really are. In this framework, they are not necessarily external or unpredictable events, but rather changes in collective expectations.
When the optimism accumulated during boom phases fades, a sudden correction occurs, resulting in a reduction in credit supply and an increase in spreads. The crisis, therefore, is partly the result of an internal, almost inevitable process.
This interpretation also has important implications for economic policy. If expectations play such a central role, then monitoring and understanding them becomes just as crucial as analyzing traditional indicators such as growth, inflation, or debt.
