A Good Decision Can Still Ruin You
tail risk, position size and the difference between positive expected value and survival.
People like decisions to come with moral endings.
A careful choice should be rewarded. A reckless choice should be punished. When the opposite happens, we rewrite the decision after seeing the result. The winner becomes intelligent; the loser must have missed something obvious.
Reality is less polite. A decision can have positive expected value and still produce a catastrophic outcome.
Expected value does not guarantee survivability
Expected value averages possible outcomes after weighting them by probability. A proposal can be attractive on average while containing one result large enough to end the game.
Imagine a business investment with a 70% chance of returning €150,000 and a 30% chance of losing €100,000. The expected value is positive:
0.70 × €150,000 − 0.30 × €100,000 = €75,000
That does not make the investment sensible for someone whose entire savings are €100,000. The average is attractive. The position size is lethal.
Sports betting makes the distinction easy to see. A bettor can find a genuinely mispriced selection and still lose. A series of good prices can produce a losing month. The edge describes the quality of the wager, not a promise about the next outcome.
Tail risk lives outside the normal story
Most planning focuses on the middle of the distribution: the expected launch, ordinary demand, normal market conditions. Ruin often arrives from the tail.
A supplier fails, an account is suspended, a legal rule changes, a key person becomes unavailable or a leveraged position moves farther than the model expected. The event may be unlikely, but “unlikely” is not the same as “affordable.”
A rational decision therefore asks two questions:
- Is the opportunity favourable?
- Can I survive the unfavourable version?
People often answer the first and assume it contains the second.
Outcomes corrupt our memory of judgement
Outcome-bias research shows that people evaluate a decision differently after learning whether it succeeded, even when the information available to the decision-maker was identical. [1]
This rewards lucky recklessness and punishes unlucky discipline. A founder who concentrated everything in one launch is praised for conviction if it works. Another who made the same exposure and failed is described as naive.
The lesson should not be “ignore results.” Results update beliefs. The lesson is to preserve the information set and reasoning that existed before the result, so hindsight cannot replace analysis.
Position size is part of the decision
A career move can be attractive while still needing savings. An investment can be promising while deserving a limit. A business experiment can be worth running while not justifying personal guarantees that threaten the founder’s home.
The same idea appears in casino bankroll management: a game can have a low house edge and still be played at stakes that make short-term ruin likely. Mathematics does not protect a person who ignores scale.
Good decisions need room to be wrong.
That means diversification, reserves, insurance, staged commitments and explicit stop conditions. These tools sometimes reduce the maximum upside. They also protect the ability to make another decision later.
A good decision can still ruin you when the loss is larger than your capacity to absorb it. The goal is not to eliminate uncertainty. It is to avoid placing your entire future on the claim that probability owes you a fair ending.
Sources
References
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[1]
Baron and Hershey: Outcome bias in decision evaluation pubmed.ncbi.nlm.nih.gov
Questions readers usually ask next
Can a positive expected-value decision lose?
Yes. Expected value describes an average across possible outcomes, not the result of one attempt.
What is tail risk?
It is exposure to low-probability outcomes that can cause unusually large damage.
What is outcome bias?
It is the tendency to judge a decision more favourably when its result is good, even when the original reasoning and information were unchanged.
How can someone protect against a good decision going badly?
Limit position size, maintain reserves, diversify, insure major risks and define stop conditions before the result is known.