Slight Chance Fallacy
An unlikely event occurring does not by itself prove that its earlier probability estimate was too low. Hindsight can make a lucky success seem more predictable than it was.
THE IDEA
The mistake described here combines dismissing a small probability before an event with treating the outcome as predictable afterward. A low probability is not the same as impossibility, and an event’s occurrence alone does not show that its probability was high.
The Star Trek example quotes Spock saying 'Only a very slight chance it would work,' before Kirk’s plan succeeds. That success is compatible with the plan having had a low chance of working. To assess the estimate, we would need more than the outcome of that attempt.
When to use it
Evaluating risk in business investments where unlikely but catastrophic events (black swans) are often ignored.
Making personal safety decisions (e.g., wearing a seatbelt or buying insurance) despite low probability of accidents.
Analyzing past successes in creative projects to avoid overconfidence in the repeatability of lucky breaks.
Teaching probability and statistical thinking to counteract the bias in educational settings.
FROM THEORY TO EVERYDAY LIFE
In practice
Star Trek Communicator Plan
Kirk proposes using two communicators to create a sonic disruption. Spock describes 'only a very slight chance it would work.' The plan’s success does not by itself show that this estimate was wrong. The supplied account does not establish that the crew later treated success as inevitable.
Lottery Winner's Confidence
A person buys a lottery ticket knowing the odds are minuscule. When they win, they later claim they 'knew it would happen,' ignoring the original low probability. This bias can lead to overconfidence in future risky bets.
A MOMENT TO REFLECT
Put it to work.
Think of a time you discounted a low-probability event that later occurred. How did your perception of its likelihood change after the fact?
How might the slight chance fallacy affect your decisions when planning for rare but high-impact events (e.g., natural disasters, market crashes)?
In what ways can you remind yourself to respect small probabilities without becoming paralyzed by fear of unlikely outcomes?
CONTINUE THE CONVERSATION
Source texts
- The Scout Mindset (Julia Galef)