The diversity principle and the evaluation of evidence

The diversity principle—the intuitive notion that diverse evidence is, all else equal , more persuasive, suggestive, confirmatory, or otherwise better than less varied sets of evidence—is a clear component of scientific practice and endorsed by scientists and philosophers alike. A great body of psychological research on people’s understanding and application of the diversity principle exists, yet it remains divided into multiple, distinct research communities, which often come to conflicting conclusions. One reason for this is that the range of tasks and domains investigated is appropriately wide. Without a common understanding of what it means for evidence to be diverse, however, it is hard to compare what are at times diverging results. To address this, I review three perspectives from philosophy on what makes diverse evidence valuable. I will use the perspectives to frame results from psychology and assess whether people understand the value of diverse evidence on both an intuitive and explicit level. My conclusions have a leveled optimism: While people are generally aware of the value of diverse evidence, they often struggle to apply what they know. I argue this is because people do not assess the diversity of their evidence as a matter of course but rely on its intuitive diversity as a cue to its evidential diversity. When this cue is absent, people can overlook otherwise obvious problems with their evidence. This has potential consequences for how people seek out, evaluate, and understand evidence from a variety of domains, but leaves open the possibility that various interventions—such as education or reminders to attend to the quality of evidence—may increase people’s application of what they know.’