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Statistically inaccurate and morally unfair judgements via base rate intrusion


From a statistical standpoint, judgements about an individual are more accurate if base rates about the individual’s social group are taken into account1,2,3,4. But from a moral standpoint, using these base rates is considered unfair and can even be illegal5,6,7,8,9. Thus, the imperative to be statistically accurate is directly at odds with the imperative to be morally fair. This conflict was resolved by creating tasks in which Bayesian rationality and moral fairness were aligned, thereby allowing social judgements to be both accurate and fair. Despite this alignment, we show that social judgements were inaccurate and unfair. Instead of appropriately setting aside social group differences, participants erroneously relied on them when making judgements about specific individuals. This bias—which we call base rate intrusion—was robust, generalized across various social groups (gender, race, nationality and age), and differed from analogous non-social judgements. Results also demonstrate how social judgements can be corrected to achieve both statistical accuracy and moral fairness. Overall, these data (total N = 5,138) highlight the pernicious effects of social base rates: under conditions that closely approximate those of everyday life10,11,12, these base rates can undermine the rationality and fairness of human judgements.

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Fig. 1: Two Bayesian networks that are identical in structure but differ in whether the base rates are social or non-social.
Fig. 2: The erroneous influence of base rates in experiment 1 (N = 399).
Fig. 3: Social base rates no longer influenced judgments in experiment 4 (N = 411).


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This work was supported by NSF Graduate Research Fellowships to J.C. and M.K.W.; an Inequality and Social Policy fellowship from Harvard University Kennedy School of Government to J.C.; a Hertz Fellowship to M.K.W. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. We are grateful to B. Rehder and S. Gershman for helpful comments and to K. Morehouse for research assistance.

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J.C., M.K.W. and M.R.B. designed research. J.C. performed research and analysed data. J.C., M.K.W. and M.R.B. wrote the paper.

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Correspondence to Jack Cao.

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Supplementary Methods, Supplementary Discussion, Supplementary Experiments, Supplementary Figures 1–13, Supplementary Tables 1–7

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Cao, J., Kleiman-Weiner, M. & Banaji, M.R. Statistically inaccurate and morally unfair judgements via base rate intrusion. Nat Hum Behav 1, 738–742 (2017).

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