Loss Aversion Bias and Confirmation Bias

One of the most celebrated findings in behavioral finance is Prospect Theory, developed by Daniel Kahneman and Amos Tversky. Humans feel the pain of a loss roughly twice as intensely as the pleasure of an equivalent gain. If the satisfaction from a 10 man-yen profit is rated 1, the distress from a 10 man-yen loss registers at about 2.

For further reading, books on Prospect Theory can help you understand the psychological mechanism behind overweighting losses.

Loss aversion bias causes investors to cling to losing positions thinking 'it will bounce back eventually,' while rushing to lock in profits on winners 'before the gains disappear.' The result is an irrational pattern where losses expand and gains are cut short.

Confirmation bias is the tendency to seek out information that supports your existing beliefs while ignoring contradictory evidence. A classic example is reading only favorable news about a stock you already own and dismissing negative reports.

Anchoring Effect and Herd Mentality

The anchoring effect is the phenomenon where an initial reference number (the anchor) disproportionately influences subsequent judgments. When a stock drops from 1,000 yen to 600 yen, investors anchored to the 1,000 yen price tend to see it as 'cheap' - but if the company's fundamentals have deteriorated, 600 yen may still be overvalued.

Herd mentality, often driven by FOMO (Fear Of Missing Out), is the impulse to invest without adequate analysis because everyone else seems to be profiting. The 2021 cryptocurrency bubble and social-media-driven stock frenzies are textbook examples. Most investors who entered at the tail end of these manias suffered significant losses.

Sunk Cost Fallacy and Overconfidence Bias

The sunk cost fallacy is the tendency to persist with an irrational decision because of the time or money already invested. 'I've held this long, so I can't sell now' or 'I've done so much research on this stock' are classic manifestations. Past costs should not influence forward-looking decisions.

Overconfidence bias is the tendency to overestimate one's own investment skill or the accuracy of one's information. Research shows that individual investors who trade more frequently tend to earn lower returns - those with high annual turnover underperform low-turnover investors by roughly 6-7% per year.

Concrete Strategies to Overcome Psychological Traps

While completely eliminating these biases is difficult, systematic countermeasures can help.

  • Rule-based investing: Set numerical criteria for buying and selling in advance. For example, 'rebalance when any asset class drifts more than 5% from target' or 'cut losses at 20%.' Predefined rules remove emotion from the equation.
  • Automatic contributions: Setting up automatic monthly investments eliminates the temptation to react to market swings and naturally applies dollar-cost averaging.
  • Keep an investment journal: Recording the reasons and emotions behind each trade allows you to objectively review your biases later.
  • Diversify your information sources: Deliberately seeking out perspectives that differ from your own helps counteract confirmation bias.

Books on rule-based investing can teach you how to build investment rules that are immune to emotional interference.