Knowledge that Transforms

Psychological theories of subjective probability judgments assume that accumulated evidence (support) mediates the relation between the description of a to-be-judged event (hypothesis) and the judgment. To scale a probability from the support for a hypothesis, these psychological theories make a strong independence assumption. This assumption is stated in the form of the product rule, in which the support garnered for a particular hypothesis is independent of the support for the alternative hypothesis. In the study reported here, I asked participants to judge the likelihood of a bicyclist winning a simulated race. Results showed that the independence assumption was systematically violated. Observed judgments suggest that when a probability judgment is made, the comparability or similarity of the hypotheses on one dimension increases the weight that judges allocate to differences on the other dimensions. These results speak against the simple scalability-processing assumption of support theory, and they illustrate the need for a theory of judgment processes that describes how the similarity between hypotheses shapes judgment.

A recent research report (Brandt, 2011) suggests that sexist ideologies predict increases in gender inequality at the country level. Using advanced multilevel modeling methods, the author combined aggregated individual-level data on sexism (N > 80,000) from the World Values Survey (World Values Survey Association, 2009) with country-level data on gender inequality (N < 60) from United Nations Human Development Reports. We were curious and plotted the sexism effect based on the data printed in the article. Figure 1 reveals that the effect was driven by only a few influential cases. When we replicated Brandt’s (2011) analysis in Mplus 6.1 (Muthén & Muthén, 2010), the effect dropped to nonsignificance after we excluded Switzerland, and was reduced to virtually zero after we excluded three further outliers. Our reanalysis casts serious doubts on Brandt’s (2011) conclusion. We are not suggesting that it would have been better for these influential cases to have been deleted. However, they should have been discussed so that readers could assess the robustness of the effect. Furthermore, unusual cases often tell their own important stories, which may critically inform future research (McClelland, 2002).

A central tenet of current memory theories is the independence of data-driven and conceptually driven priming. Although these types of repetition priming have different functional properties, it has remained unclear whether they are independent and additive or interact with each other. Therefore, we orthogonally combined these priming types in two experiments requiring occupation judgments for celebrities, represented by their faces or names. In both experiments, the sum of the effects of separate data-driven priming and conceptually driven priming equaled the effect of the combination of these priming types. These findings demonstrate for the first time the independence of data-driven and conceptually driven priming and so confirm a central tenet of current memory theories. This independence may thus be an important element for the development of future overarching memory theories.

Galton (1907) first demonstrated the "wisdom of crowds" phenomenon by averaging independent estimates of unknown quantities given by many individuals. Herzog and Hertwig (2009; hereafter H&H in Psychological Science) showed that individuals' own estimates can be improved by asking them to make two estimates at separate times and averaging them. H&H claimed to observe far greater improvement in accuracy when participants received "dialectical" instructions to consider why their first estimate might be wrong before making their second estimates than when they received standard instructions. We reanalyzed H&H's data using measures of accuracy that are unrelated to the frequency of identical first and second responses and found that participants in both conditions improved their accuracy to an equal degree.

People are quick to perceive meaningful patterns in the co-occurrence of events. We report two studies exploring the effects of streaks in symptom checklists on perceived personal disease risk. In the context of these studies, a streak is a sequence of consecutive items on a list that share the characteristic of being either general or specific. We identify a psychological mechanism underlying the effect of streaks in a list of symptoms and show that the effect of streaks on perceived risk varies with the length of the symptom list. Our findings reveal a tendency to infer meaning from streaks in medical and health decision making. Participants perceived a higher personal risk of having an illness when presented with a checklist in which common symptoms were grouped together than when presented with a checklist in which these same symptoms were separated by rare symptoms. This research demonstrates that something as arbitrary as the order in which symptoms are presented in a checklist can affect perceived risk of disease.