Fig 2. Factual and counterfactual learning biases.

(A) Predicted results. Based on previous studies we expected that in Experiment 1 factual learning would display a “positivity” bias (i.e. the learning rate for the chosen positive outcomes would be relatively higher than that of the chosen negative outcomes (${\alpha }_c^{+}>{\alpha }_c^{-}$; note that in Experiment 1 the “positivity” and the “confirmation” bias are not discernible). In Experiment 2, one possibility was that this “positivity” bias would extend to counterfactual learning, whereby positive outcomes would be over-weighted regardless of whether the outcome was chosen or unchosen (“valence” bias) (${\alpha }_u^{+}>{\alpha }_u^{-}$). Another possibility was that counterfactual learning would present an opposite bias, whereby the learning rate for unchosen negative outcomes was higher than the learning rate of unchosen positive outcomes (${\alpha }_u^{+}<{\alpha }_u^{-}$) (“confirmation” bias). (B) Actual results. Learning rate analysis of Experiment 1 data replicated previous findings, demonstrating that factual learning presents a “positivity” bias. Learning rate analysis of Experiment 2 indicated that counterfactual learning was also biased, in a direction that was consistent with a “confirmation” bias. ***P<0.001 and *P<0.05, two-tailed paired t-test.