Stock market downturn: the reflex that separates winners from losers

What is a stock market downturn and why does it happen?

A market downturn refers to a significant and widespread decline in stock prices across one or more stock exchanges. Typically, there are three levels of decline: a correction (a drop of 5% to 10% from a recent high), a bear market (a decline exceeding 15%), and a crash (a sharp and sudden plunge, often over a few sessions).

The causes are varied and often intertwined. An economic slowdown, a rapid increase in interest rates, a geopolitical crisis, the bursting of a speculative bubble, or an unexpected sector-specific shock can trigger a massive sell-off. Sometimes, it is simply the accumulated overvaluation during a period of euphoria that eventually corrects itself mechanically.

An essential point to remember: declines are an integral part of normal market functioning. Since 1928, the S&P 500 index has averaged a correction of more than 10% roughly once a year and a bear market approximately every three to five years. These episodes are not anomalies—they are the price for accessing the long-term return potential of stocks.

What distinguishes a temporary dip from a lasting collapse is generally the strength of the underlying economic fundamentals. When companies continue to generate profits, employment remains strong, and the financial system operates effectively, recoveries historically tend to materialize. The challenge is that this information is never available in real-time with certainty.

The Psychological Traps Costing Investors Dearly

The greatest risk during a market downturn is not the decline itself—it's the reaction it triggers. Behavioral finance has identified several cognitive biases that become active precisely when markets are falling, driving inexperienced investors toward irrational decisions.

The first is loss aversion. Research in behavioral economics, notably by Daniel Kahneman and Amos Tversky, has shown that the pain of a loss is about twice as intense as the pleasure of an equivalent gain. In practical terms, seeing a portfolio drop by 15% generates disproportionate stress, leading to panic selling, often at the worst possible time.

The second pitfall is the recency bias: the natural tendency to give more weight to recent events than to long-term historical data. When markets have been declining for several weeks, the brain extrapolates this trend into a catastrophic scenario, forgetting that every past bear market eventually ended in a recovery. This bias is amplified by the continuous flow of information from the media and social networks, which overrepresent negative news.

The third powerful mechanism is herd behavior. Humans are wired to follow the group, especially in uncertain situations. When everyone is selling, resisting this social pressure requires considerable discipline. However, numerous academic studies show that individual investors tend to buy when markets are at their peak (driven by optimism) and sell when they are at their lowest (driven by collective fear), which mechanically erodes performance.

Knowing these biases is not enough to completely guard against them, but it's an essential first defense. Being aware that a decision made in the heat of emotion is statistically likely to be suboptimal already allows for a beneficial pause before taking action.

Sell, Buy, or Do Nothing: Insights from Historical Data

The temptation to « time the market"—meaning selling before a downturn and buying back at the lowest point—is understandable. In theory, it's the perfect strategy. In practice, it's nearly impossible to execute repeatedly, even for the most seasoned professionals.

A study regularly updated by J.P. Morgan Asset Management strikingly illustrates this point: an investor who remained fully invested in the S&P 500 over a 20-year period would have achieved a significantly higher annualized return than one who missed just the 10 best trading days in that same period. Those best days often occur right after the worst ones—exactly when a panicked investor has just sold.

This doesn't mean you should blindly buy during every downturn. Not all declines are the same, and context matters. However, three approaches have demonstrated their resilience through cycles:

1. Dollar Cost Averaging (DCA). Invest a fixed amount at regular intervals, regardless of market direction. During downturns, this fixed amount allows for the acquisition of more shares or stocks, which mechanically lowers the average cost price. This method largely neutralizes the risk of bad timing.

2. Portfolio rebalancing. If a target allocation has been set (for example, 60% stocks / 40% bonds), a significant drop in stocks throws this balance off. Rebalancing means buying the asset class that has decreased to return to the initial target, which is essentially buying low in a disciplined and methodical manner.

3. Buy and hold. For an investor with a sufficiently long investment horizon (typically more than 8-10 years) and a properly diversified portfolio, doing nothing during a downturn is a perfectly valid strategy. It avoids transaction costs, timing errors, and the taxes associated with capital gains realized during trades.

Key Considerations Before Making Decisions During a Downturn

Instead of searching for a universal answer to the question « should I sell or buy, » the most robust approach is to revert to your own personal parameters. Here are the fundamental questions to consider objectively, ideally before a downturn occurs.

What is my investment horizon? If the invested money is not needed for another 10 or 15 years, a temporary decline, however significant, doesn't have the same impact as it would if those funds were intended for a real estate purchase in six months. The time horizon is the most decisive factor in one's ability to withstand volatility.

Is my diversification adequate? A portfolio concentrated in a single sector, geographic area, or type of asset is inherently more vulnerable. While diversification does not eliminate downturns, it mitigates their magnitude and reduces the risk of permanent capital loss.

Do I have sufficient cash reserves? Investing in the stock market with money that might be needed in the short term is the primary cause of being forced to sell at the worst time. Having a separate emergency fund covering several months of living expenses allows you to navigate through downturns without being compelled to liquidate positions at a loss.

Has my initial investment thesis changed? There are legitimate situations where selling during a downturn is rational: if a company's or sector's fundamentals have structurally deteriorated, if liquidity is needed for a life project, or if it becomes clear that your risk profile was overestimated. The key is that this decision is based on analysis, not emotion.

Markets will inevitably decline again; that is certain. No one can predict when, by how much, or how long the pullback will last. What is manageable, however, is the framework within which decisions are made. The investors who weather crises best are rarely those who accurately predicted market timing — instead, they are the ones who had a plan and stuck to it.