Equation 6 · Part 2 · Ecological Rationality: When Biases Are Adaptations

Symbol D

\frac{P(D \mid H)}{P(D \mid \lnot H)} \; > \; \frac{P(\lnot H)}{P(H)} \cdot \frac{c_{fa}}{c_{miss}}

D

What this part means

D is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

Its job in the formula

D is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

Full expressionSymbol DArticle meaning

The passage around this formula

This is a rule about optimal thresholds, not merely an intuition, and it has a compact formal statement. A decision-maker observes ambiguous evidence D bearing on whether a state H holds — a predator is present, a partner is unfaithful, an approach signals romantic interest — against the alternative that it does not. Where $c_{fa}$ is the fitness cost of a false alarm and $c_{miss}$ is the fitness cost of a miss, the threshold that minimizes total…

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