Naturally nested, but why dual process?

Why so low? A striking feature of the data reviewed by Barney & Sloman (B&S) is that the percentage of participants achieving the correct answer in the Medical Diagnosis problem rarely exceeds 50% (e.g., Table 3 of the target article). Thus, whether it is presenting information as natural frequencies or making nested set relations apparent that leads to improvements, overall the levels of performance remain remarkably low.

Potential reasons for this low level of overall performance are not discussed adequately in the target article. Although acknowledging in section 2.2 that “wide variability in the size of the effects makes it clear that in no sense do natural frequencies eliminate base-rate neglect” (para. 2), B&S fail to apply the same standard to their own proposal that “set-relation inducing formats” (be they natural frequencies or otherwise) facilitate a shift to a qualitatively different system of reasoning. The clear message of the article is that by presenting information appropriately, participants can “overcome their natural associative tendencies” (sect. 4, para. 3) and employ a reasoning system which applies rules to solve problems. Why does this system remain inaccessible for half of the participants in the studies reviewed? Is the rule system engaged, but the wrong rules are applied (e.g., Brase et al. 2006, Experiment 1)? Or do these participants remain oblivious to the nested sets relations and persevere with “inferior” associative strategies?

B&S cite evidence from...