Debra Lieberman

Microbes, mating, and morality: Individual differences in three functional domains of disgust

What is the function of disgust? Whereas traditional models have suggested that disgust serves to protect the self or neutralize reminders of our animal nature, an evolutionary perspective suggests that disgust functions to solve 3 qualitatively different adaptive problems related to pathogen avoidance, mate choice, and social interaction. The authors investigated this 3-domain model of disgust across 4 studies and examined how sensitivity to these functional domains relates to individual differences in other psychological constructs. Consistent with their predictions, factor analyses demonstrated that disgust sensitivity partitions into domains related to pathogens, sexuality, and morality. Further, sensitivity to the 3 domains showed predictable differentiation based on sex, perceived vulnerability to disease, psychopathic tendencies, and Big 5 personality traits. In exploring these 3 domains of disgust, the authors introduce a new measure of disgust sensitivity. Appreciation of the functional heterogeneity of disgust has important implications for research on individual differences in disgust sensitivity, emotion, clinical impairments, and neuroscience.

The architecture of human kin detection

Evolved mechanisms for assessing genetic relatedness have been found in many species, but their existence in humans has been a matter of controversy. Here we report three converging lines of evidence, drawn from siblings, that support the hypothesis that kin detection mechanisms exist in humans. These operate by computing, for each familiar individual, a unitary regulatory variable (the kinship index) that corresponds to a pairwise estimate of genetic relatedness between self and other. The cues that the system uses were identified by quantitatively matching individual exposure to potential cues of relatedness to variation in three outputs relevant to the system’s evolved functions: sibling altruism, aversion to personally engaging in sibling incest, and moral opposition to third party sibling incest. As predicted, the kin detection system uses two distinct, ancestrally valid cues to compute relatedness: the familiar other’s perinatal association with the individual’s biological mother, and duration of sibling coresidence.

Disgust: Evolved Function and Structure

Interest in and research on disgust has surged over the past few decades. The field, however, still lacks a coherent theoretical framework for understanding the evolved function or functions of disgust. Here we present such a framework, emphasizing 2 levels of analysis: that of evolved function and that of information processing. Although there is widespread agreement that disgust evolved to motivate the avoidance of contact with disease-causing organisms, there is no consensus about the functions disgust serves when evoked by acts unrelated to pathogen avoidance. Here we suggest that in addition to motivating pathogen avoidance, disgust evolved to regulate decisions in the domains of mate choice and morality. For each proposed evolved function, we posit distinct information processing systems that integrate function-relevant information and account for the trade-offs required of each disgust system. By refocusing the discussion of disgust on computational mechanisms, we recast prior theorizing on disgust into a framework that can generate new lines of empirical and theoretical inquiry.

Infection, incest, and iniquity: Investigating the neural correlates of disgust and morality

Disgust, an emotion related to avoiding harmful substances, has been linked to moral judgments in many behavioral studies. However, the fact that participants report feelings of disgust when thinking about feces and a heinous crime does not necessarily indicate that the same mechanisms mediate these reactions. Humans might instead have separate neural and physiological systems guiding aversive behaviors and judgments across different domains. The present interdisciplinary study used functional magnetic resonance imaging (n = 50) and behavioral assessment to investigate the biological homology of pathogen-related and moral disgust. We provide evidence that pathogen-related and sociomoral acts entrain many common as well as unique brain networks. We also investigated whether morality itself is composed of distinct neural and behavioral subdomains. We provide evidence that, despite their tendency to elicit similar ratings of moral wrongness, incestuous and nonsexual immoral acts entrain dramatically separate, while still overlapping, brain networks. These results (i) provide support for the view that the biological response of disgust is intimately tied to immorality, (ii) demonstrate that there are at least three separate domains of disgust, and (iii) suggest strongly that morality, like disgust, is not a unified psychological or neurological phenomenon.

The Effect of Social Category on Third Party Punishment

Motivations to punish should depend on a number of factors including the nature of the interaction (e.g., collective action versus dyadic exchange) and the social category of the interactants. Here we focus on social category and investigate whether the relationship to a perpetrator and, separately, a victim of a moral transgression affects the magnitude of third party punishment, moral judgment, attribution, and emotional response. Participants read scenarios describing a moral violation in which the perpetrator (Experiment 1) or victim (Experiment 2) of an offense was described as kin, a schoolmate, or a foreigner. Penalties and attributions of remorse varied according to the social category of the perpetrator as well as the victim. However, moral judgments did not. In a third experiment, which also varied the relationship to the victim of a moral transgression, participants reported their willingness to expend time and energy to bring a perpetrator to justice as well as their emotional responses to the crime. As predicted, participants reported a greater willingness to sacrifice their weekends and a days pay to search for a perpetrator victimizing kin followed by a schoolmate and then foreign visitor. These and other results including emotional reactions are discussed in the context of motivations to punish third party violators of a social norm.

A feel for disgust: Tactile cues to pathogen presence

One function of disgust is to act as a pathogen-avoidance system preventing contact with substances harbouring disease-causing organisms. Avoiding pathogens, however, requires systems for their detection. Whereas previous research on disgust has focused on visual and olfactory detection cues, one largely overlooked modality is touch. Here we examine whether tactile cues play a role in pathogen detection and activate the disgust response. Participants briefly touched and then rated stimuli varying along dimensions predicted to correlate with pathogen presence: moisture, temperature, and consistency. Results show that participants rated wet stimuli and stimuli resembling biological consistencies as more disgusting than dry stimuli and stimuli resembling inanimate consistencies, respectively. No main effect for temperature was found. We report on predicted interactions, the relationship between disgust ratings and perceived infection risk, and individual differences. Taken together, these data suggest that touch is an important modality providing information for disgust-related processes.

Kin Affiliation Across the Ovulatory Cycle Females Avoid Fathers When Fertile

A commonplace observation in humans is that close genetic relatives tend to avoid one another as sexual partners. Despite the growing psychological research on how antierotic attitudes develop toward relatives, few studies have focused on actual behavior. One prediction, stemming from parental investment theory, is that women should be more vigilant of reproduction-compromising behaviors, such as inbreeding, during times of peak fertility than during times of low fertility. Indeed, females of other species avoid interactions with male kin when fertile—but the corollary behavior in humans has yet to be explored. Here we fill this gap. Using duration and frequency of cell-phone calls, an objective behavioral measure that reflects motivations to interact socially, we show that women selectively avoid interactions with their fathers during peak fertility. Avoidance specifically targeted fathers, which rules out alternative explanations. These data suggest that psychological mechanisms underlying mating psychology regulate sexual avoidance behaviors, and in women they fluctuate according to fertility status.

Fitness Costs Predict Inbreeding Aversion Irrespective of Self-Involvement: Support for Hypotheses Derived from Evolutionary Theory

It is expected that in humans, the lowered fitness of inbred offspring has produced a sexual aversion between close relatives. Generally, the strength of this aversion depends on the degree of relatedness between two individuals, with closer relatives inciting greater aversion than more distant relatives. Individuals are also expected to oppose acts of inbreeding that do not include the self, as inbreeding between two individuals posits fitness costs not only to the individuals involved in the sexual act, but also to their biological relatives. Thus, the strength of inbreeding aversion should be predicted by the fitness costs an inbred child posits to a given individual, irrespective of this individual’s actual involvement in the sexual act. To test this prediction, we obtained information about the family structures of 663 participants, who reported the number of same-sex siblings, opposite-sex siblings, opposite-sex half siblings and opposite-sex cousins. Each participant was presented with three different types of inbreeding scenarios: 1) Participant descriptions, in which participants themselves were described as having sex with an actual opposite-sex relative (sibling, half sibling, or cousin); 2) Related third-party descriptions, in which participants’ actual same-sex siblings were described as having sex with their actual opposite-sex relatives; 3) Unrelated third-party descriptions, in which individuals of the same sex as the participants but unrelated to them were described as having sex with opposite-sex relatives. Participants rated each description on the strength of sexual aversion (i.e., disgust-reaction). We found that unrelated third-party descriptions elicited less disgust than related third-party and participant descriptions. Related third-party and participant descriptions elicited similar levels of disgust suggesting that the strength of inbreeding aversion is predicted by inclusive fitness costs. Further, in the related and unrelated conditions alike, the strength of inbreeding aversion was positively associated with the degree of relatedness between those described in the descriptions.

Hemispheric Asymmetries during Processing of Immoral Stimuli

Evolutionary approaches to dissecting our psychological architecture underscore the importance of both function and structure. Here we focus on both the function and structure of our neural circuitry and report a functional bilateral asymmetry associated with the processing of immoral stimuli. Many processes in the human brain are associated with functional specialization unique to one hemisphere. With respect to emotions, most research points to right-hemispheric lateralization. Here we provide evidence that not all emotional stimuli share right-hemispheric lateralization. Across three studies employing different paradigms, the processing of negative morally laden stimuli was found to be highly left-lateralized. Regions of engagement common to the three studies include the left medial prefrontal cortex, left temporoparietal junction, and left posterior cingulate. These data support the hypothesis that processing of immoral stimuli preferentially engages left hemispheric processes and sheds light on our evolved neural architecture.

Coresidence duration and cues of maternal investment regulate sibling altruism across cultures.

Genetic relatedness is a fundamental determinant of social behavior across species. Over the last few decades, researchers have been investigating the proximate psychological mechanisms that enable humans to assess their genetic relatedness to others. Much of this work has focused on identifying cues that predicted relatedness in ancestral environments and examining how they regulate kin-directed behaviors. Despite progress, many basic questions remain unanswered. Here we address three of these questions. First, we examine the replicability of the effect of two association-based cues to relatedness-maternal perinatal association (MPA) and coresidence duration-on sibling-directed altruism. MPA, the observation of a newborn being cared for by ones mother, strongly signals relatedness, but is only available to the older sibling in a sib-pair. Younger siblings, to whom the MPA cue is not available, appear to fall back on the duration of their coresidence with an older sibling. Second, we determine whether the effects of MPA and coresidence duration on sibling-directed altruism obtain across cultures. Last, we explore whether paternal perinatal association (PPA) informs sibship. Data from six studies conducted in California, Hawaii, Dominica, Belgium, and Argentina support past findings regarding the role of MPA and coresidence duration as cues to siblingship. By contrast, PPA had no effect on altruism. We report on levels of altruism toward full, half, and step siblings, and discuss the role alternate cues might play in discriminating among these types of siblings. (PsycINFO Database Record