About

Professor Peter H. Ditto is the Principal Investigator at the UCI Hot Cognition Lab, affiliated with the UCI School of Social Ecology. His research interests encompass Social Psychology, Human Judgment and Decision-making, Political Psychology, and Moral Reasoning. The lab page lists his role and research focus but does not provide further detailed biographical information or descriptions of his background and key contributions.

Research topics

• Political Science
• Computer Science
• Psychology
• Law
• Social psychology
• Sociology
• Epistemology
• Internet privacy
• Ecology
• Mathematics education
• Business
• Advertising
• Medicine
• Public relations
• Economics

Selected publications

• Political breakups: Interpersonal consequences of polarization

  PNAS Nexus · 2026-04-30

  articleOpen accessSenior author

  Abstract We explored the phenomenon of “political breakups,” i.e. losing relationships with friends, family members, romantic partners, or others due to political differences. We studied the conditions surrounding political breakups in the United States across four datasets (combined n = 3,791) and supplemented our findings using a proxy measure from the American National Election Studies. In a survey we conducted in April 2025, we found that 37% of Americans reported having had a political breakup, mostly with friends. The prevalence of breakups in the United States appears to have increased since 2016, although evidence is limited. Across all datasets, Democrats were more likely to have experienced a breakup compared with Republicans and were usually the initiator. Political breakups were associated with increased partisan hostility, above and beyond strength of partisanship: Those who reported breakups had more negative feelings toward their political opponents (especially voters rather than party elites), perceived them as more extreme, and ascribed selfish views to them. We discuss potential causes and call for more research on this phenomenon.

  PublisherDOI

• True, false, or in between? Examining the role of partisanship in judging political news of varying degrees of truthfulness

  2025-07-21

  preprintOpen accessSenior author

  Research conducted to understand belief in misinformation often dichotomizes stimuli into true and false information. Yet news in the real world exists in varying shades of truthfulness. In three studies (N = 4046), we presented participants with statements from Politifact.com that were selected to be liberal- or conservative-leaning and varied in truthfulness, rated by Politifact on a scale from True to Pants on Fire (72 total statements tested). Political conservatism was positively associated with believing conservative-friendly statements (ORs = 1.26 to 1.41) and disbelieving liberal-friendly statements (ORs = 0.75 to 0.87), a pattern which persisted across the scale of statement truthfulness. Additional analyses did not reveal consistent strong predictors of belief in politically congenial or uncongenial statements, though ideological extremity tended to be associated with belief in congenial statements and disbelief in uncongenial ones. These real-world claims, across a range of truthfulness, help illustrate patterns of partisan (mis)information belief.

  PublisherOA PDFDOI

• True, false, or in between? Examining the role of partisanship in judging political news of varying degrees of truthfulness

  2025-07-15

  preprintOpen accessSenior author

  Research conducted to understand belief in misinformation often dichotomizes stimuli into true and false information. Yet news in the real world exists in varying shades of truthfulness. In three studies (N = 4046), we presented participants with statements from Politifact.com that were selected to be liberal- or conservative-leaning and varied in truthfulness, rated by Politifact on a scale from True to Pants on Fire (72 total statements tested). Political conservatism was positively associated with believing conservative-friendly statements (ORs = 1.26 to 1.41) and disbelieving liberal-friendly statements (ORs = 0.75 to 0.87), a pattern which persisted across the scale of statement truthfulness. Additional analyses did not reveal consistent strong predictors of belief in politically congenial or uncongenial statements, though ideological extremity tended to be associated with belief in congenial statements and disbelief in uncongenial ones. These real-world claims, across a range of truthfulness, help illustrate patterns of partisan (mis)information belief.

  PublisherOA PDFDOI

• Chapter 9 The Social Psychology of Political Polarization

  2024-04-08 · 1 citations

  book-chapter1st authorCorresponding

  A key contributor to political conflict in the United States is the different factual beliefs held by liberals and conservatives about matters such as taxes, gun violence, and climate change. We propose an account of how differential beliefs arise, describing how prescriptive beliefs (moral-based beliefs regarding how the world should be) shape descriptive ones ("factual" beliefs regarding how the world really is). Three contributing processes are identified: moralization (the infusion of issues with moral significance), factualization (the construction of pseudo-descriptive justifications for moral evaluations), and socialization (the reinforcement of morally palatable beliefs by selective exposure to ideologically sympathetic people and media sources). These normal processes of intergroup conflict have been exacerbated by technological advances and exploited by political actors interested in promoting partisan animosities for political gain. The shortest part of the chapter addresses the hardest part of the problem: what can be done to promote more civil and more rational political discourse.

  PublisherDOI

• Of preferences and priors: Motivated reasoning in partisans’ evaluations of scientific evidence.

  Journal of Personality and Social Psychology · 2024-11-01 · 6 citations

  articleOpen accessSenior author

  Despite decades of research, it has been difficult to resolve debates about the existence and nature of partisan bias-the tendency to evaluate information more positively when it supports, rather than challenges, one's political views. Whether partisans display partisan biases, and whether any such biases reflect motivated reasoning, remains contested. We conducted four studies (total N = 4,010) in which participants who made unblinded evaluations of politically relevant science were compared to participants who made blinded evaluations of the same study. The blinded evaluations-judgments of a study's quality given before knowing whether its results were politically congenial-served as impartial benchmarks against which unblinded participants' potentially biased evaluations were compared. We also modeled the influence of partisans' preferences and prior beliefs to test accounts of partisan judgment more stringently than past research. Across our studies, we found evidence of politically motivated reasoning, as unblinded partisans' preferences and prior beliefs independently biased their evaluations. We contend that conceptual confusion between descriptive and normative (e.g., Bayesian) models of political cognition has impeded the resolution of long-standing theoretical debates, and we discuss how our results may help advance more integrative theorizing. We also consider how the blinding paradigm can help researchers address further theoretical disputes (e.g., whether liberals and conservatives are similarly biased), and we discuss the implications of our results for addressing partisan biases within and beyond social science. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

  PublisherDOI

• Of preferences and priors: Motivated reasoning in partisans’ evaluations of scientific evidence

  2024-09-24 · 2 citations

  preprintOpen accessSenior author

  Despite decades of research, it has been difficult to resolve debates about the existence andnature of partisan bias—the tendency to evaluate information more positively when it supports, rather than challenges, one’s political views. Whether partisans display partisan biases, and whether any such biases reflect motivated reasoning, remains contested. We conducted four studies (total N = 4,010) in which participants who made unblinded evaluations of politically- relevant science were compared to participants who made blinded evaluations of the same study. The blinded evaluations—judgments of a study’s quality given before knowing whether its results were politically-congenial—served as impartial benchmarks against which unblinded participants’ potentially biased evaluations were compared. We also modeled the influence of partisans’ preferences and prior beliefs to test accounts of partisan judgment more stringently than past research. Across our studies, we found evidence of politically motivated reasoning, as unblinded partisans’ preferences and prior beliefs independently biased their evaluations. We contend that conceptual confusion between descriptive and normative (e.g., Bayesian) models of political cognition has impeded the resolution of longstanding theoretical debates, and we discuss how our results may help advance more integrative theorizing. We also consider how the blinding paradigm can help researchers address further theoretical disputes (e.g., whether liberals and conservatives are similarly biased), and we discuss the implications of our results for addressing partisan biases within and beyond social science.

  PublisherDOI

• Partisan Bias in Political Judgment

  Annual Review of Psychology · 2024-09-05 · 18 citations

  reviewOpen access1st authorCorresponding

  This article reviews empirical data demonstrating robust ingroup favoritism in political judgment. Partisans display systematic tendencies to seek out, believe, and remember information that supports their political beliefs and affinities. However, the psychological drivers of partisan favoritism have been vigorously debated, as has its consistency with rational inference. We characterize decades-long debates over whether such tendencies violate normative standards of rationality, focusing on the phenomenon of motivated reasoning. In light of evidence that both motivational and cognitive factors contribute to partisan bias, we advocate for a descriptive approach to partisan bias research. Rather than adjudicating the (ir)rationality of partisan favoritism, future research should prioritize the identification and measurement of its predictors and clarify the cognitive mechanisms underlying motivated political reasoning. Ultimately, we argue that political judgment is best evaluated by a standard of ecological rationality based on its practical implications for individual well-being and functional democratic governance.

  PublisherDOI

• The challenging task of demonstrating motivated tribalism: Review of Bernstein et al. (2023)

  Journal of Open Inquiry in the Behavioral Sciences · 2023-04-11

  article1st authorCorresponding

  The contribution of Bernstein et al reports a series of studies demonstrating partisan bias: the tendency to evaluate otherwise identical information more favorably when it supports one’s political beliefs or allegiances than when it challenges those beliefs or allegiances. The write-up is clear and concise, and the studies are interesting with a number of nice empirical touches, but the novelty and quality of the data need to be considered, especially the difficulty of ruling out rational counter explanations for data ostensibly showing motivated partisan bias.

  PublisherDOI

• Moral Judgments Impact Perceived Risks From COVID-19 Exposure

  Collabra Psychology · 2023-01-01

  articleOpen access

  The COVID-19 pandemic created enormously difficult decisions for individuals trying to navigate both the risks of the pandemic and the demands of everyday life. Good decision making in such scenarios can have life and death consequences. For this reason, it is important to understand what drives risk assessments during a pandemic, and to investigate the ways that these assessments might deviate from ideal risk assessments. In a preregistered online study of U.S. residents (N = 841) using two blocks of vignettes about potential COVID exposure scenarios, we investigated the effects of moral judgment, importance, and intentionality on COVID infection risk assessments. Results demonstrate that risk judgments are sensitive to factors unrelated to the objective risks of infection. Specifically, activities that are morally justified are perceived as safer while those that might subject people to blame or culpability, are seen as riskier, even when holding objective risk fixed. Similarly, unintentional COVID exposures are judged as safer than intentional COVID exposures. While the effect sizes are small, these findings may have implications for public health and risk communications, particularly if public health officials are themselves subject to these biases.

  PublisherOA PDFDOI

• Politicization of a Pathogen: A Prospective Longitudinal Study of <scp>COVID</scp>‐19 Responses in a Nationally Representative U.S. Sample

  Political Psychology · 2023-05-03 · 12 citations

  articleOpen access

  Understanding population‐level variability in responses to pathogens over time is important for developing effective health‐based messages targeted at ideologically diverse populations. Research from psychological and political sciences suggests that party and elite cues shape how people respond to major threats like climate change. Research on responses to the COVID‐19 pandemic suggests similar variability across party identities; however, prior work has methodological limitations. This prospective, longitudinal study of a large probability‐based nationally representative U.S. sample assessed in March–April 2020 ( N = 6,514) and then 6 months later in September–October 2020 ( N = 5,661) demonstrates that COVID‐19 fear, perceived COVID‐19 death risk, and reported health‐protective behaviors became increasingly polarized over the first 6 months of the pandemic. Initial differences between Democrats and Republicans failed to converge over time and became more pronounced. Responses among Republicans were further polarized by support for former President Donald Trump: Trump Republicans initially reported weaker responses to COVID‐19 than non‐Trump Republicans, and these differences became more pronounced over time. Importantly, political identity and Trump support were not linked to perceived infection risk of a nonpoliticized pathogen, the flu. Finally, political identity and Republican Trump support prospectively predicted COVID‐19 vaccine intentions 6 months into the pandemic.

  PublisherOA PDFDOI

Recent grants

• NIH Grant R01HS008180

  NIH · $1.6M · 2002

• NIH Grant R03HS007660

  NIH · $69k · 1995

Frequent coauthors

• Joseph H. Danks

  118 shared

• William D. Smucker

  Summa Health System

  117 shared

• Kristen M. Coppola

  Johnson University

  106 shared

• Angela Fagerlin

  96 shared

• Lisa K. Lockhart

  Texas A&M University

  90 shared

• Renate Houts

  Duke University

  88 shared

• R. Mitchell Gready

  Kent State University

  85 shared

• Jamila Bookwala

  Lafayette College

  83 shared

Labs

• UCI Hot Cognition LabPI

Education

• Ph.D.

  Princeton University