Disgust, a common and important emotion in daily life, is of great interest in cognitive science, inspiring important debates in psychology, philosophy, and moral theory—for example, whether disgust emotion justifies moral judgments. In Yuck! The Nature and Moral Significance of Disgust, Daniel Kelly seeks to develop proximate and ultimate explanations of the nature of disgust through a synthesis of theoretical and empirical evidence. Since its publication in 2011, the book has attracted great attention (it has been cited more than 300 times). Yuck! can be considered a rare breed because of its highly interdisciplinary nature; it integrates evidence and perspectives from evolutionary biology, psychology, anthropology, and philosophy. Its examination of whether disgust should be treated as a “trustworthy” guide to justifiable moral judgments is both thought-provoking and timely.
The book encompasses five chapters, which discuss and answer several main questions: What is the nature of disgust? What is the evolutionary function of disgust? Why is the expression and recognition of disgust mostly unconscious and automatic? What role does disgust play in social interaction? Should disgust guide moral judgments?
To enhance discussion and understanding of the nature and role of disgust in human behavior, in Chapter 1, Kelly systematically reviews related empirical research results. He uses large amounts of empirical data to build an integrated psychological model to capture the disgust system’s proximate mechanisms, including its three main parts: (1) the acquisition subsystem, which explains the mechanisms for acquiring various disgust elicitors; (2) the execution subsystem, which maintains a database of elicitors and physiological, psychological, and behavioral disgust responses; and (3) the various psychological and behavioral downstream effects of disgust. Based on this model, the author proposes an entanglement thesis, explaining the evolutionary function of disgust for human beings in Chapter 2. Evolutionarily, the function of affective disgust responses, such as gape, nausea, and withdrawal, is to avoid toxic substances, while the function of the set of core disgust responses, such as oral association, offensiveness, and contamination sensitivity, is to avoid the presence of diseases and parasites.
In Chapter 3, Kelly further explains disgust’s sentimental signaling system—expression and recognition of disgust. Why is expression and recognition of disgust largely automatic and unconscious? In this chapter, Kelly introduces the classic commitment model (CCM) and the cultural transmission model (CTM). After comparing these two models, the author argues that CTM better explains disgust because of its role in the sentimental signaling system through acquiring and transmitting important cultural information by natural pedagogy and social learning. Finally, the author discusses the relationship between disgust and morality in Chapters 4 and 5. Here, to better understand this relationship, Kelly proposes the co-opt thesis as a supplement to the entanglement theory—that is, the cognitive system is co-opted to play an important role in representing social norms and group boundary markers when individuals feel disgusted. Based on the entanglement thesis and the co-opt thesis, Kelly argues that “disgust is neither necessary nor sufficient to justify a moral judgment” (p. 147).
The main contribution of Yuck! The Nature and Moral Significance of Disgust lies in putting forward several models and theories to explain the nature and functions, both proximate and ultimate, of disgust based on empirical evidence and rigorous argumentation. Specifically, Kelly proposes a functional-level model that represents how people acquire various disgust elicitors from the environment, how people respond to those disgust elicitors, and what downstream effects disgust induces. What is impressive is that Kelly goes back to the core nature of disgust when coming up with the model. As he writes, “to avoid becoming entangled in the vagaries of those speculations and begin by returning to the ground floor of what is known, to the facts” (p. 13). The entanglement thesis solves the puzzle about disgust—that is, are there similarities or differences in disgust between humans and animals from an evolutionary perspective? Similar to other animals, disgust in humans plays the role of avoiding toxic food and diseases or parasites. However, these two evolutionary functions of disgust are integrated only within human beings; they remain separate in other animals, making the function of disgust uniquely “entangled” in humans. Furthermore, the co-opt thesis specifies the novel and important function of disgust for human beings—cognition of social norms and ethnic boundaries, such as table manners, food taboos, interpersonal communication norms, and in-group membership judgments.
Overall, this is a remarkable contribution; it is rigorous, detailed, targeted, and communicative. For example, Chapter 3 begins with the puzzle of why expression and recognition of disgust are largely automatic and unconscious. To resolve this puzzle, Kelly compares two different models (CCM and CTM) that are used to understand the relevant features of disgust in detail, discussing how they explain the puzzle and then proposing his own arguments. It is no exaggeration to say that readers will feel like communicating with the author in person and will be amazed by the author’s rigorous thinking, detailed descriptions, and intensive discussion when reading the book. It is also commendable that each of the first four chapters ends with a functional model of the disgust system, letting readers understand Kelly’s core ideas more intuitively.
The central debate of this book—whether disgust emotion “can” and/or “should” justify moral judgments—is carefully examined in Chapter 5. This issue is prominent, as it is still at the core of researchers’ debate on disgust (e.g., Tybur et al., Reference Tybur, Lieberman, Kurzban and DeScioli2013). In this book, Kelly presents arguments from advocates and skeptics and uses his own theories and models proposed in the first four chapters to analyze the arguments in detail. On one hand, the nature of disgust is to avoid toxins and diseases based on the entanglement thesis. Disgust is sensitive to various cues and easily activated, and, importantly, it follows a “better safe than sorry” logic (p. 143). Thus, many disgust responses are false positives, making disgust unreliable for making moral judgments. On the other hand, according to his co-opt thesis, disgust has been co-opted by social norms and group membership cognition, and thus it has become a vehicle of cultural transmission. Kelly writes that “the extensions of ‘disgusting’ and ‘morally wrong’ simply crosscut each other” (p. 148), and thus disgust cannot be used to justify moral judgments. This chapter is an integration and sublimation of all previous discussions, pointing to the key conclusion that disgust should not guide moral judgments.
Perhaps the weakest aspect of the book, which Kelly admits, is that “the discussion has made the reportedly illegitimate leap from ‘is’ to ‘ought’” (p. 152) in the final chapter. Kelly jumps from descriptive and explanatory theories to the conclusion that disgust should not guide moral judgment, which is injunctive. This leaves room for the author to extend his work to shed light on the long-debated naturalistic fallacy (inferring value, “ought,” from fact, “is”).
Another point that the book misses is a discussion of emergent adaptive problems. For example, in proposing the co-opt thesis, Kelly argues that “the feedback loop generates a variety of new adaptive problems, involving especially social interactions, and the disgust system lends itself to being co-opted to deal with new adaptive problems, especially those involving social interactions” (p. 119). Without much elaboration, this may leave readers wondering how some emergent norm violations could elicit disgust or not. For example, in the face of climate change, many countries have anticipated food supply shortages. To adapt to this environmental pressure, people are becoming more flexible in their food choices, eating some previously disgusting food such as worms, which is a good source of protein. Take globalization as another example. In the face of frequent intercultural contacts, crossing ethnic boundaries and mixing cultures could elicit lesser disgust (at least for some people; see Cheon et al., Reference Cheon, Christopoulos and Hong2016). These are examples of how emergent pressure from the physical and social environments could exert pressure on humans to adapt and how it can be reflected through the updating of the database of elicitors in the disgust system, borrowing terminology from Kelly’s model.
In sum, Kelly’s book provides a comprehensive introduction to and scholarly discussion of the nature and moral significance of disgust. This book is written to reach a wide interdisciplinary audience. Based on the level of the discourse throughout, especially the first three chapters, this book is ideally suited for those unfamiliar with disgust as a starting point to learn about what disgust is, how it is evolved, and what its function is. As such, it should be illuminating for students of moral philosophy. The book is also recommended for researchers in social cognition, culture, identity, and intergroup relations, because it can be a springboard for contemplating how disgust takes on new forms or functions as human beings adapt to growing pressure from physical environments (e.g., scarcer food sources in the face of climate change) and social environments (e.g., more intercultural mixing in the face of globalization) in our time.
