Sunk Cost Fallacy in Stock Ownership

Intermediate | Published: 2025-12-28

Why It Matters

You bought a stock at $350. It drops to $90. Every instinct screams: "Don't sell -- that would lock in the loss." But the loss already happened. Your net worth is already down. Selling just makes the number visible. The real question is whether that $90 of capital is best deployed here or somewhere else -- and your purchase price has nothing to do with the answer.

TL;DR: Sunk cost fallacy causes investors to hold deteriorating positions and throw more money at losers, all to avoid "realizing" a loss that already exists. Research by Andrea Frazzini (2006) found that stocks held by sunk-cost-driven investors underperform by 5-10% over the following year. The antidote: mechanical, forward-looking decision rules that ignore your purchase price entirely.

Definition and Core Concept

Sunk cost fallacy is the tendency to continue an investment because of previously committed resources -- money, time, effort -- even when continuing is no longer optimal. Psychologists Hal Arkes and Catherine Blumer documented this pattern in a landmark 1985 study, showing that people who had already spent money on something were far more likely to continue with it, regardless of the expected outcome.

In stock ownership, your purchase price (the cost basis) is economically irrelevant to future decisions. It tells you what you paid, not what a stock is worth today or where it is headed. Yet cost basis dominates how most people decide whether to hold or sell.

Two predictable distortions follow:

• Holding losers: "I paid $350, I can't sell at $90 -- that would lock in the loss." The cost basis becomes an anchor preventing rational exit.
• Averaging down: "I'll buy more at $50 to lower my average to $60." This is escalation of commitment -- throwing good money after bad to justify the original decision.

Researcher Christopher Heath (1995) showed how mental accounting amplifies this effect. Investors frame a declining stock as an "investment to recover" rather than a "capital allocation decision," which changes the psychology entirely. Instead of asking "where should I put my money?", the investor asks "how do I avoid admitting this was a mistake?"

Barry Staw's (1976) work on escalation of commitment confirmed the mechanism: when an initial decision fails, people often increase their investment to justify the original choice rather than cut losses. Staw found this pattern was strongest when the decision-maker felt personally responsible for the initial investment -- exactly the situation individual stock-pickers face.

KEY INSIGHT: A useful causal chain explains how the fallacy operates: Cost basis anchor (driver) leads to sunk cost thinking (thought pattern), which produces escalation of commitment (behavior), resulting in holding losers too long and averaging down (portfolio damage). Each step feels rational in isolation, but the chain leads reliably to underperformance.

How It Shows Up in Portfolios

Individual Investor: Holding to 'Get Back to Even' (Meta 2022)

You bought Meta at $350/share in September 2021, believing in its dominant ad platform, strong cash generation, and 3 billion monthly users. Then the metaverse pivot, first-ever user decline, and TikTok competition drove the stock to $90 by November 2022 -- a 74% drop.

Sunk cost thinking: "I paid $350. I can't sell at $90. I need to hold until I break even."

Forward-looking analysis (what should have been asked): "If I had $9,000 cash today, would I buy Meta at $90?" At that point, Meta was burning $10 billion or more per year on metaverse development with no clear return, TikTok was stealing engagement from Instagram, and Apple's App Tracking Transparency changes had hammered ad revenue. The honest answer for many investors was: "I'd rather own a diversified index fund."

Meta did recover to $400 by December 2023. But that does not validate the November 2022 decision to hold. Mark Zuckerberg's AI pivot and aggressive cost cuts were unforeseeable events that happened to rescue the stock. The decision process was flawed even though the outcome was lucky. Had Meta continued burning cash on the metaverse without pivoting, the stock could have fallen to $50 while the S&P 500 returned +26% in 2023.

This is the distinction between good process and good outcome. Sunk cost thinking can occasionally produce a winning result, but it remains a bad process that will lose money over hundreds of decisions.

Individual Investor: Averaging Down into Disaster (Enron 2001)

You bought Enron at $70/share in early 2001. By August, it had dropped to $50. Sunk cost thinking kicked in: "I'll buy more to lower my average to $60 -- then I only need a 20% recovery."

Red flags were already public. Short-seller Jim Chanos had flagged fraudulent accounting practices. Bethany McLean's March 2001 Fortune article asked the question no one on Wall Street wanted to answer: "How exactly does Enron make its money?" CFO Andrew Fastow was dumping shares through private partnerships. CEO Jeffrey Skilling resigned after just six months, citing "personal reasons."

Any one of these signals warranted skepticism. Together, they made averaging down reckless. Yet sunk cost thinking reframed the falling price as an opportunity rather than a warning.

The cost of averaging down:

• Selling at $50: $2,857 loss on original $10,000 investment
• Averaging down at $50, then riding to bankruptcy in December 2001: $19,913 loss on $20,000 total invested

The incremental cost of sunk cost fallacy: $17,056. The investor who sold at a loss preserved $7,143 in capital. The investor who averaged down lost everything.

Corporate Case Study: AT&T and Time Warner (2018-2022)

Sunk cost fallacy does not only affect individual investors. It operates at the highest levels of corporate decision-making, often with billions at stake.

In June 2018, AT&T completed its $85 billion acquisition of Time Warner, one of the largest media deals in history. The thesis was "vertical integration" -- combining AT&T's distribution network with Time Warner's content library (HBO, CNN, Warner Bros.) to compete with Netflix and Disney.

Problems emerged almost immediately. Integrating a telecom culture with a creative media company proved far harder than the deal models assumed. AT&T loaded $180 billion in debt onto its balance sheet. Subscriber growth at the new streaming service HBO Max fell short of projections. Meanwhile, AT&T's core wireless business needed billions in 5G investment that the debt burden made difficult.

By 2020, it was clear the acquisition was not delivering. But AT&T had already spent $85 billion. Walking away would mean admitting the deal was a mistake -- exactly the scenario where sunk cost thinking takes hold. So AT&T doubled down, pouring additional billions into HBO Max content spending and reorganizing its corporate structure around the media strategy.

The resolution came in April 2022, when AT&T finally spun off WarnerMedia and merged it with Discovery. AT&T effectively unwound the deal at a substantial loss, receiving roughly $43 billion in value for an asset it had purchased for $85 billion just four years earlier. Shareholders bore roughly $40 billion in destroyed value, plus years of strategic distraction and underinvestment in AT&T's core business.

The AT&T case illustrates how sunk cost thinking scales. The same mental trap that keeps an individual holding 100 shares of a declining stock kept a Fortune 500 board committed to a failing acquisition for years. In both cases, the logic was identical: "We've already invested too much to walk away now."

The Fresh Eyes Test

Before holding any position, ask: "If I had cash equal to this position's value, would I buy this stock today?"

Every day you hold a stock is a fresh decision to allocate capital there. If you would not buy it today with new money, you should not hold it with old money. Your cost basis is irrelevant to this question.

Practical steps:

1. Monthly review: For each position, answer "Would I buy today: Yes / No." If the answer is "No," sell immediately regardless of cost basis.
2. Hide cost basis during reviews: Cover the column in your portfolio tracker. If revealing it afterward changes your decision, sunk cost is influencing you.
3. Averaging down pre-mortem: Before adding to any losing position, confirm the thesis is intact AND that you would buy it as a brand-new position at today's price. If you catch yourself saying "I'll lower my break-even," stop. That is sunk cost talking.

When Holding a Loser Might Be Defensible

Not every decision to hold a declining stock is irrational. There are legitimate, forward-looking reasons to maintain a position that is down from your purchase price. The critical distinction is whether your reasoning references the future or the past.

Holding may be defensible when:

• The original thesis is intact and supported by new evidence. A stock can decline because of temporary market sentiment, sector rotation, or broad selloffs unrelated to the company's fundamentals. If the reasons you bought the stock still hold -- revenue growing, competitive position strengthening, management executing -- a lower price may genuinely represent better value. The key is that you can articulate this case without mentioning your cost basis.

• Tax considerations dominate the next decision. In a taxable account near year-end, holding a loser for a few more weeks to convert a short-term loss (taxed at ordinary income rates) into a long-term loss may be worth the delay. Similarly, if you hold a large unrealized gain alongside the loser, the timing of harvesting losses to offset gains can justify a brief hold. These are forward-looking tax calculations, not sunk cost reasoning.

• Liquidity constraints are real. Thinly traded small-cap stocks or positions in restricted shares may have genuine exit costs (wide bid-ask spreads, lockup periods) that make an immediate sale expensive. In those cases, a planned, gradual exit may be more rational than a fire sale.

• You are following a systematic strategy with predefined rules. A value investor who buys stocks below intrinsic value and holds through volatility is not falling for sunk cost fallacy -- as long as the strategy was defined before the loss occurred and the holding rules are applied consistently, not selectively.

The test in every case: can you justify the position without referencing what you paid? If your best argument starts with "I'm already down so much that..." or "I just need it to get back to..." then the reasoning is backward-looking, and sunk cost fallacy is driving the decision.

Detection Signals

You may be falling prey to sunk cost fallacy if:

• You say "I'll hold until I get back to even" or "I need to recover my investment"
• You cannot articulate a forward-looking thesis for positions you hold -- only "I paid $X and it's now $Y"
• You are averaging down on positions where the original investment thesis has deteriorated
• You feel more pain from realizing a $5,000 loss (selling) than from holding a position down $10,000 (unrealized)
• You check your cost basis before making hold/sell decisions

KEY INSIGHT: Frazzini's research found that across millions of trades, investors systematically hold losing stocks longer than winners, causing stocks with large unrealized losses to underperform by 5-10% annually. On a $100,000 portfolio, that drag amounts to $5,000-$10,000 per year in opportunity cost -- real money lost to a bias that feels like patience.

A 10-Minute Exercise

Run a Fresh Eyes Portfolio Review right now:

1. List all current positions
2. Hide cost basis (cover the column)
3. For each position, answer: "If I had cash equal to this position's value, would I buy today?"
4. Reveal cost basis -- did it change your answer for any position?
5. For every "No," write one sentence explaining why you would not buy today
6. Sell all "No" positions this week, regardless of gain or loss status

If every position passes, your portfolio is driven by forward-looking conviction. If any position fails -- especially with the thought "I'm holding to get back to even" -- sunk cost fallacy is costing you money right now.

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References

Arkes, H. R., & Blumer, C. (1985). The Psychology of Sunk Cost. Organizational Behavior and Human Decision Processes, 35(1), 124-140.

Frazzini, A. (2006). The Disposition Effect and Underreaction to News. The Journal of Finance, 61(4), 2017-2046.

Heath, C. (1995). Escalation and De-escalation of Commitment in Response to Sunk Costs. Organizational Behavior and Human Decision Processes, 62(1), 38-54.

Staw, B. M. (1976). Knee-Deep in the Big Muddy: A Study of Escalating Commitment to a Chosen Course of Action. Organizational Behavior and Human Performance, 16(1), 27-44.