Teoria Prospectiva — Prospect Theory

A Prospect Theory, desenvolvida por Daniel Kahneman e Amos Tversky (1979), revolucionou understanding de decision-making under risk. Para traders, esta theory explica o most paradoxical e costly behavior: traders take excessive risk para avoid losses enquanto taking insufficient risk para maximize gains — exactly the opposite de what profitability requires.

Conceitos Centrais

1. Reference Dependence

Decisions são made relative a reference point (usually current portfolio value ou recent price), não absolute outcomes:

• Gain de R$5,000 feels different depending se reference é R$0 (great!) ou R$50,000 expected gain (disappointing)
• Loss de R$2,000 feels different se reference é R$0 (terrible) ou R$10,000 expected loss (relief)

Trading implication: entry price becomes reference point → decisions around that entry são distorted by reference dependence, not objective analysis.

2. Loss Aversion

Losses loom approximately 2x larger than equivalent gains:

• Losing R$1,000 feels ~2x worse than gaining R$1,000 feels good
• This asymmetry drives irrational behavior:
  • Holding losing positions too long (avoid realizing loss)
  • Taking profits too early (secure the feel-good gain)
  • Increasing risk on losing positions (double-down to avoid loss)

3. Diminishing Sensitivity

Marginal value decreases as outcome moves further from reference:

• First R$1,000 gain → very valuable
• Next R$1,000 gain → less valuable
• Next R$1,000 → even less
• Same for losses: first R$1,000 loss → very painful, subsequent less so

Trading implication: traders become less careful with larger positions — “already up R$10K, another R$1K doesn’t matter much.”

4. Probability Weighting

Humans don’t weight probabilities linearly:

• Low probabilities are overweighted — 1% chance feels like 5-10%
• High probabilities are underweighted — 95% chance feels like 80%
• Certainty effect — 100% probability is disproportionately valued over 99%

Trading implication:

• Overweighting low-probability big wins → buy lottery-like setups (cheap options, meme stocks)
• Underweighting high-probability modest gains → avoid consistent setups
• Certainty effect → take small sure profits over larger probable profits

The Fourfold Pattern of Risk Attitudes

	Gains	Losses
High Probability	Risk Averse	Risk Seeking
Low Probability	Risk Seeking	Risk Averse

Gains + High Probability → Risk Averse

Example: 95% chance de gain R$1,000

• Most choose sure R$900 over 95% chance de R$1,000
• Trading: take profit early (sure small gain) instead of letting run (probable larger gain)

Gains + Low Probability → Risk Seeking

Example: 5% chance de gain R$20,000

• Most choose 5% chance over sure R$500
• Trading: buy cheap options, meme stocks, lottery-like setups

Losses + High Probability → Risk Seeking

Example: 95% chance de loss R$1,000

• Most choose 95% chance over sure loss de R$900
• Trading: hold losing position (gamble on recovery) instead of accepting small sure loss (stop loss)

Losses + Low Probability → Risk Averse

Example: 5% chance de loss R$10,000

• Most choose sure loss R$500 (insurance) over 5% chance
• Trading: buy protective puts, over-hedge against unlikely crashes

Prospect Theory Manifestations em Trading

1. Early Profit Taking

Trader enters PETR4 at R$28, target R$32 (1:4 R:R)

• Price reaches R$30 (1:2 R:R achieved)
• Prospect theory: “95% chance I’ll keep this R$2 profit if I sell now, vs. maybe R$4 later”
• Risk averse com gains → sell at R$30 (2R sure) instead of R$32 (4R probable)
• Result: average win = 1.5R instead of 3R → expectancy drops dramatically

2. Holding Losing Positions

Trader buys at R$30, stop should be R$28 (1R loss)

• Price drops to R$28 (stop hit)
• Prospect theory: “95% chance I lose R$2 if I sell, vs. maybe 0 if I hold and it recovers”
• Risk seeking com losses → hold instead of selling
• Price drops to R$25 → loss now 5R instead of 1R
• Result: average loss = 2-3R instead of 1R → expectancy drops

3. Doubling Down on Losses

Position losing → trader adds more:

• “If I double position, recovery只需要一半 da distance”
• Risk seeking com losses → increase exposure
• If continues falling → loss doubles → catastrophic
• Result: occasional recovery feels like genius, but statistical outcome é disaster

4. Avoiding Good Setups After Losses

After 3 losses, trader skips next setup:

• “I’ve been losing — another trade probably will lose too”
• Prospect theory: overweight recent losses → underweight system probability
• Result: miss trades que would recover losses

Countermeasures

1. Pre-commitment

Decide exit rules before entry:

• Stop loss: placed before entry, never moved away
• Profit target: predefined, não moved closer
• No exceptions — mechanical execution

2. R-multiple Thinking

Think em R-multiples, not money:

• “This loss is -1R, not -R$1,000”
• “This gain target is +3R, not +R$3,000”
• R language removes monetary emotional weight

3. System Trust

Trust system over feelings:

• “My system has 60% WR over 200 trades”
• “This individual trade may lose — but next 20 trades collectively will profit”
• “Expected losses are costs, not failures”

4. Accountability Partner

Someone who checks your adherence:

• “Did you follow your stop loss rule?”
• “Why did you exit at 1R instead of waiting for 2R target?”
• External accountability overrides internal prospect theory distortion

Conclusão

Prospect Theory explains why traders consistently make the same irrational decisions — taking profits too early, holding losses too long, doubling down, avoiding good setups after losses. These aren’t random errors — they’re systematic biases rooted em human psychology. Para investidores brasileiros, countermeasures são clear: pre-commitment (stops e targets before entry), R-multiple thinking, system trust e accountability. Prospect Theory não pode be eliminated, mas can be managed through disciplined systems that override intuitive bias.

Teoria Prospectiva