I am currently a postdoctoral scholar at Stanford University, working under Ellen Markman. I earned my PhD in Cognitive Psychology at the University of California, Los Angeles under advisor Keith Holyoak. I was born and raised in Wauwatosa, WI and previously educated in the desert at Arizona State University. I love coffee and beer and have recently become an amateur coffee roaster and home brewer (emphasis on amateur). I also dabble in photography.

Professional Education

  • Doctor of Philosophy, University of California Los Angeles (2016)

Stanford Advisors

All Publications

  • Deontological coherence: A framework for commonsense moral reasoning. Psychological bulletin Holyoak, K. J., Powell, D. 2016; 142 (11): 1179-1203


    We review a broad range of work, primarily in cognitive and social psychology, that provides insight into the processes of moral judgment. In particular, we consider research on pragmatic reasoning about regulations and on coherence in decision making, both areas in which psychological theories have been guided by work in legal philosophy. Armed with these essential prerequisites, we sketch a psychological framework for how ordinary people make judgments about moral issues. Based on a literature review, we show how the framework of deontological coherence unifies findings in moral psychology that have often been explained in terms of a grab-bag of heuristics and biases. (PsycINFO Database Record

    View details for PubMedID 27709981

  • Countering antivaccination attitudes PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Horne, Z., Powell, D., Hummel, J. E., Holyoak, K. J. 2015; 112 (33): 10321-10324


    Three times as many cases of measles were reported in the United States in 2014 as in 2013. The reemergence of measles has been linked to a dangerous trend: parents refusing vaccinations for their children. Efforts have been made to counter people's antivaccination attitudes by providing scientific evidence refuting vaccination myths, but these interventions have proven ineffective. This study shows that highlighting factual information about the dangers of communicable diseases can positively impact people's attitudes to vaccination. This method outperformed alternative interventions aimed at undercutting vaccination myths.

    View details for DOI 10.1073/pnas.1504019112

    View details for Web of Science ID 000359738300060

    View details for PubMedID 26240325

  • Moral Severity is Represented as a Domain-General Magnitude EXPERIMENTAL PSYCHOLOGY Powell, D., Horne, Z. 2017; 64 (2): 142-147


    The severity of moral violations can vary by degree. For instance, although both are immoral, murder is a more severe violation than lying. Though this point is well established in Ethics and the law, relatively little research has been directed at examining how moral severity is represented psychologically. Most prominent moral psychological theories are aimed at explaining first-order moral judgments and are silent on second-order metaethical judgments, such as comparisons of severity. Here, the relative severity of 20 moral violations was established in a preliminary study. Then, a second group of participants were asked to decide which of two moral violations was more severe for all possible combinations of these 20 violations. Participant's response times exhibited two signatures of domain-general magnitude comparisons: we observed both a distance effect and a semantic congruity effect. These findings suggest that moral severity is represented in a similar fashion as other continuous magnitudes.

    View details for DOI 10.1027/1618-3169/a000354

    View details for Web of Science ID 000401140400007

    View details for PubMedID 28497722

  • How Large Is the Role of Emotion in Judgments of Moral Dilemmas? PLOS ONE Horne, Z., Powell, D. 2016; 11 (7)


    Moral dilemmas often pose dramatic and gut-wrenching emotional choices. It is now widely accepted that emotions are not simply experienced alongside people's judgments about moral dilemmas, but that our affective processes play a central role in determining those judgments. However, much of the evidence purporting to demonstrate the connection between people's emotional responses and their judgments about moral dilemmas has recently been called into question. In the present studies, we reexamined the role of emotion in people's judgments about moral dilemmas using a validated self-report measure of emotion. We measured participants' specific emotional responses to moral dilemmas and, although we found that moral dilemmas evoked strong emotional responses, we found that these responses were only weakly correlated with participants' moral judgments. We argue that the purportedly strong connection between emotion and judgments of moral dilemmas may have been overestimated.

    View details for DOI 10.1371/journal.pone.0154780

    View details for Web of Science ID 000379809400001

    View details for PubMedID 27385365

  • Marginally Significant Effects as Evidence for Hypotheses: Changing Attitudes Over Four Decades PSYCHOLOGICAL SCIENCE Pritschet, L., Powell, D., Horne, Z. 2016; 27 (7): 1036-1042


    Some effects are statistically significant. Other effects do not reach the threshold of statistical significance and are sometimes described as "marginally significant" or as "approaching significance." Although the concept of marginal significance is widely deployed in academic psychology, there has been very little systematic examination of psychologists' attitudes toward these effects. Here, we report an observational study in which we investigated psychologists' attitudes concerning marginal significance by examining their language in over 1,500 articles published in top-tier cognitive, developmental, and social psychology journals. We observed a large change over the course of four decades in psychologists' tendency to describe a p value as marginally significant, and overall rates of use appear to differ across subfields. We discuss possible explanations for these findings, as well as their implications for psychological research.

    View details for DOI 10.1177/0956797616645672

    View details for Web of Science ID 000380937800010

    View details for PubMedID 27207874

  • Causal competition based on generic priors COGNITIVE PSYCHOLOGY Powell, D., Merrick, M. A., Lu, H., Holyoak, K. J. 2016; 86: 62-86


    Although we live in a complex and multi-causal world, learners often lack sufficient data and/or cognitive resources to acquire a fully veridical causal model. The general goal of making precise predictions with energy-efficient representations suggests a generic prior favoring causal models that include a relatively small number of strong causes. Such "sparse and strong" priors make it possible to quickly identify the most potent individual causes, relegating weaker causes to secondary status or eliminating them from consideration altogether. Sparse-and-strong priors predict that competition will be observed between candidate causes of the same polarity (i.e., generative or else preventive) even if they occur independently. For instance, the strength of a moderately strong cause should be underestimated when an uncorrelated strong cause also occurs in the general learning environment, relative to when a weaker cause also occurs. We report three experiments investigating whether independently-occurring causes (either generative or preventive) compete when people make judgments of causal strength. Cue competition was indeed observed for both generative and preventive causes. The data were used to assess alternative computational models of human learning in complex multi-causal situations.

    View details for DOI 10.1016/j.cogpsych.2016.02.001

    View details for Web of Science ID 000374354700003

    View details for PubMedID 26896879

  • A Single Counterexample Leads to Moral Belief Revision COGNITIVE SCIENCE Horne, Z., Powell, D., Hummel, J. 2015; 39 (8): 1950-1964


    What kind of evidence will lead people to revise their moral beliefs? Moral beliefs are often strongly held convictions, and existing research has shown that morality is rooted in emotion and socialization rather than deliberative reasoning. In addition, more general issues-such as confirmation bias-further impede coherent belief revision. Here, we explored a unique means for inducing belief revision. In two experiments, participants considered a moral dilemma in which an overwhelming majority of people judged that it was inappropriate to take action to maximize utility. Their judgments contradicted a utilitarian principle they otherwise strongly endorsed. Exposure to this scenario led participants to revise their belief in the utilitarian principle, and this revision persisted over several hours. This method provides a new avenue for inducing belief revision.

    View details for DOI 10.1111/cogs.12223

    View details for Web of Science ID 000368273300010

    View details for PubMedID 25810137

  • A Bayesian framework for knowledge attribution: Evidence from semantic integration COGNITION Powell, D., Horne, Z., Pinillos, N. A., Holyoak, K. J. 2015; 139: 92-104


    We propose a Bayesian framework for the attribution of knowledge, and apply this framework to generate novel predictions about knowledge attribution for different types of "Gettier cases", in which an agent is led to a justified true belief yet has made erroneous assumptions. We tested these predictions using a paradigm based on semantic integration. We coded the frequencies with which participants falsely recalled the word "thought" as "knew" (or a near synonym), yielding an implicit measure of conceptual activation. Our experiments confirmed the predictions of our Bayesian account of knowledge attribution across three experiments. We found that Gettier cases due to counterfeit objects were not treated as knowledge (Experiment 1), but those due to intentionally-replaced evidence were (Experiment 2). Our findings are not well explained by an alternative account focused only on luck, because accidentally-replaced evidence activated the knowledge concept more strongly than did similar false belief cases (Experiment 3). We observed a consistent pattern of results across a number of different vignettes that varied the quality and type of evidence available to agents, the relative stakes involved, and surface details of content. Accordingly, the present findings establish basic phenomena surrounding people's knowledge attributions in Gettier cases, and provide explanations of these phenomena within a Bayesian framework.

    View details for DOI 10.1016/j.cognition.2015.03.002

    View details for Web of Science ID 000353854100008

    View details for PubMedID 25813346

  • Array training in a categorization task QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY Homa, D., Powell, D., Ferguson, R. 2014; 67 (1): 45-59


    Two components of categorization, within-category commonalities and between-category distinctiveness, were investigated in a categorization task. Subjects learned three prototype categories composed of moderately high distortions, by observing arrays containing patterns that belonged either to a common prototype category or to three different categories; a third group learned patterns presented one at a time, mirroring the standard paradigm. Following 6 learning blocks, subjects transferred to old patterns and new patterns at low-, medium-, and high-level distortions of the category prototype. The results showed that array training facilitated learning, especially when patterns in the array belonged to the same category. Transfer results showed a strong gradient effect across pattern distortion level for all conditions, with the highest performance obtained following array training on different category patterns and worst in the control condition. Interestingly, the old training patterns were classified worse than new low and no better than medium distortions. Neither this ordering nor the steepness of the gradient across prototype similarity for each condition could be predicted by the generalized context model. A prototype model better captured the steep gradient and ordinal pattern of results, although the overall fits were only slightly better than the exemplar model. The crucial role played by category commonalities and distinctiveness on categorical representations is addressed.

    View details for DOI 10.1080/17470218.2013.790909

    View details for Web of Science ID 000329779400005

    View details for PubMedID 23713978