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The passage around this formula

The evidence assembled here fits inside a formulation more modest than “an evolved fear module” and, for that reason, more resilient to the objections just described. Treat ordinary associative learning as a single general mechanism, and treat what evolution supplies not as a separate circuit but as a prior on that mechanism’s own parameters. A standard error-correction rule for associative strength states the update on each learning trial as $\Delta V = \alpha \,(\lambda - V)$. where V is the current associative strength between a stimulus and an outcome, $\lambda$ is the asymptotic strength that outcome supports, and $\alpha$ is a learning-rate parameter governing how much of the gap between the two is closed…

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