A Functional and Neuroanatomical Model of Dehumanization

Abstract

The dehumanization of others is a major scourge of mankind; however, despite its significance, physicians have little understanding of the neurobiological mechanisms for this behavior. We can learn much about dehumanization from its brain–behavior localization and its manifestations in people with brain disorders. Dehumanization as an act of denying to others human qualities includes two major forms. Animalistic dehumanization (also called infrahumanization) results from increased inhibition of prepotent tendencies for emotional feelings and empathy for others. The mechanism may be increased activity in the inferior frontal gyrus. In contrast, mechanistic dehumanization results from a loss of perception of basic human nature and decreased mind-attribution. The mechanism may be hypofunction of a mentalization network centered in the ventromedial prefrontal cortex and adjacent subgenual anterior cingulate cortex. Whereas developmental factors may promote animalistic dehumanization, brain disorders, such as frontotemporal dementia, primarily promote mechanistic dehumanization. The consideration of these two processes as distinct, with different neurobiological origins, could help guide efforts to mitigate expression of this behavior.

Understanding dehumanization is important. Dehumanization is the act of depriving others of human qualities or demeaning their humanity. Excesses of dehumanization have had profoundly harmful consequences for targeted groups or individuals (^{Jack et al, 2013}). People do not automatically attribute humanness to the same degree to all human beings (^{Haslam and Loughnan, 2014; Kteily et al, 2015}), and subtle and unconscious dehumanization can lead to the debasement of groups or individuals (^{Harris and Fiske, 2006, 2007}).

Otherwise normal people may deny positive human qualities, such as emotional warmth and competence, to certain individuals while experiencing these individuals as disgusting, less typically human, and more like animals (^{Harris and Fiske, 2006}). At its most profound level, this type of dehumanization involves withholding empathy and moral protection from others, with the potential mistreatment of outgroups (^{Kteily et al, 2015}). In addition to this form of dehumanization, normal people can also withhold humanness from others and treat them as indifferent objects through a failure to consider that they have minds containing thoughts and feelings, that is, mentalization (^{Harris and Fiske, 2011; Harris et al, 2007}).

Neuroscience can contribute to understanding this important psychosocial phenomenon. Neurologists have long observed that dehumanization among people with frontally predominant neurologic dysfunction results in indifference to the humanness of others without experiencing them as disgusting or “animalistic.” The differences in dehumanization are a major clue to their neurobiological processes and, potentially, to mechanisms that can be used to modify this behavior (Figure 1).

FIGURE 1: Model of dehumanization and the brain. gACC = pregenual or subgenual anterior cingulate cortex. IFG = inferior frontal gyrus. VLPFC = ventrolateral prefrontal cortex. VMPFC = ventromedial prefrontal cortex.

“ANIMALISTIC” DEHUMANIZATION

Theoretical Background

Non–brain-injured or neurotypical subjects can manifest “infrahumanization,” or the perception of outgroup individuals as lacking unique human traits or emotions (^{Leyens et al, 2000}). In addition to a perceived absence of positive human traits, such as social manners and cleanliness, infrahumanization theory states that primary and basic emotions shared with animals, such as fear and anger, are intact in outgroups as well as ingroups, but secondary or self-conscious emotions, such as guilt or shame, are less common in outgroups than in ingroups (^{Demoulin et al, 2004; Wilson and Haslam, 2013}). Infrahumanization—by selectively denying others uniquely human secondary emotions but not basic ones—conceptualizes humanization in comparison with animals (^{Vaes et al, 2002}). ^{Haslam’s (2006)} dual model of dehumanization further applies the term “animalistic dehumanization” to describe the disavowal of uniquely human traits, such as language and culture, and the devaluing of others as subhuman or nonhuman animals, resulting in feelings of disgust, use of animal epithets, and disdain toward outgroups (^{Goff et al, 2008}).

Functional Mechanism

This form of dehumanization restrains empathy and emotional contagion when a person chooses, for example, to keep from empathizing with a destitute beggar or from considering another’s feelings in social hierarchies and dominance (^{Berthoz et al, 2006; Haidt, 2001, 2007; Ligneul et al, 2017; Moll et al, 2005}). Dehumanization may result from the inhibition of tendencies to empathize when perceiving individuals of decreased physical or social status, including unattractiveness, facial disfigurement (^{Alaei et al, 2021; Cassidy et al, 2022; Hartung et al, 2019}), and low socioeconomic status (^{Sainz et al, 2019}).

An individual’s tendency to use animalistic dehumanization decreases the more the individual’s spatial and temporal features resemble the self (^{Hasson and Frith, 2016; Northoff and Huang, 2017; Scalabrini et al, 2019}) and the more dissimilar those features are to those of animals, as suggested by electroencephalographic μ suppression in face-to-face dyadic interactions (^{Simon and Gutsell, 2021}). In contrast, increased anthropomorphism, or extending human features to other animals or objects, may potentially increase animalistic dehumanization by blurring the human–animal boundary (^{Hortensius and Cross, 2018; Rottman et al, 2021}). The “animalistic” nature of this dehumanization may relate to the status of animals as being lower than humans in the social order (^{Haslam and Loughnan, 2014}).

Implicated Brain Regions

Several studies have implicated the ventrolateral prefrontal cortex (VLPFC), specifically the inferior frontal gyrus (IFG), as the source of this form of dehumanization. Neurobiological data suggest that the main role of the IFG in animalistic dehumanization is to inhibit prepotent positive emotional reactions to humans but not to animals (^{Aron et al, 2014a, 2014b; Swick et al, 2008}). The IFG, especially on the right side, is involved in the “stop signal” inhibition of emotional contagion, the mirror neuron network, empathy, and risk aversion (^{Aron et al, 2003, 2014a, 2014b; Christopoulos et al, 2009; Iacoboni et al, 1999; Swick et al, 2008}). This inhibition of positive, prosocial emotions may trigger the experience of disgust in the insula and amygdala. Furthermore, the VLPFC responds to social dominance cues and is engaged in rank perception (^{Qu et al, 2017}): The left VLPFC/dorsal IFG responds to social hierarchy judgments (^{Farrow et al, 2011}), and the right VLPFC responds to tasks of cooperation and competition and to cues of high social status (^{Liu et al, 2015; Marsh et al, 2009}).

There may also be increased left IFG activity with animalistic and lower social status assessments (^{Bruneau et al, 2018}). In contrast to its reaction to humans, the IFG may promote the anthropomorphism of animals and objects, suggesting a shift in positive emotions from humans (^{Gobbini et al, 2007}). Increased right IFG activity occurs when resolving animate aspects of shapes (^{Cross et al, 2016}). Viewing animals and humans engaging in animal-like behavior, as opposed to human behavior, results in greater activity in the left IFC as well as in other left lateralized brain regions (^{Jack et al, 2013}).

“MECHANISTIC” DEHUMANIZATION

Theoretical Background

^{Haslam’s (2006)} dual model of dehumanization distinguishes a second form of dehumanization that involves a perceived lack of a “typical” or essential human nature, such as having a mind with intentionality and agency (^{Harris et al, 2007; Haslam, 2006}). This form of “mechanistic dehumanization,” by denying a mental life with thoughts, intentions, and an autonomous agency to others, devalues others as nonliving machines, resulting in feelings of indifference while seeing others as object-like, robotic, or alien (^{Haslam, 2006}).

Functional Mechanism

The central characteristic of mechanical dehumanization is the failure to attribute mentalization to other human beings (^{Harris and Fiske, 2011; Haslam, 2006; Hortensius and Cross, 2018; Jack et al, 2013}), resulting in the perception that they lack thoughts, feelings, and beliefs and, thus, do not warrant moral consideration (^{Harris and Fiske, 2006, 2011; Krumhuber et al, 2015; Waytz et al, 2010}). People are viewed as human through the attribution of minds to them (^{Contini et al, 2020}), and they reserve social cognitive resources for faces that have “attached” minds (^{Gray et al, 2007}). Although humans can extend the capacity for mentalization to animals or to nonliving entities via anthropomorphism (^{Williams et al, 2020}), people normally attribute full minds to humans.

Mechanistic dehumanization is a failure to perceive minds, perceiving them instead as just objects or machines. Mechanistic dehumanization thus objectifies others (^{Sainz et al, 2019})—a process that occurs among normal people in certain social encounters. For example, physicians may think of patients as “mechanical systems made up of interacting parts” that need repair (^{Haque and Waytz, 2012}), and any corresponding lack of warmth and personal care of them reflects mechanistic dehumanization (^{Haque and Waytz, 2012; Haslam, 2006}).

Implicated Brain Regions

Mechanistic dehumanization is associated with decreased mesial prefrontal activity involving the ventromedial prefrontal cortex (VMPFC) and the pregenual anterior cingulate cortex (gACC) (^{Harris and Fiske, 2006; Scalabrini et al, 2019}), in addition to decreased parietal and default mode network function (^{Bruneau et al, 2018; Jack et al, 2013}). The mesial frontal cortex appears to be the critical area for mentalization and for detecting valence and value from people but not objects. Decreased activity in the mesial frontal cortex, with a presumed denial of mentalization, has been associated with passively viewing images of people who are perceived as being low in warmth and competence (^{Harris and Fiske, 2006, 2007}). Investigators have also reported decreased activity in brain areas for mentalizing and the use of fewer mental state verbs when considering people of low socioeconomic status (^{Harris and Fiske, 2006}).

In addition to increased activity in the VMPFC during animate versus inanimate interactions, there is increased activity in the gACC, suggesting that this area is also critical for assessing the state of “livingness” in another human being (^{Scalabrini et al, 2019}). In summary, dysfunction in both the VMPFC and the gACC may be responsible for mechanistic dehumanization and seeing people as objects or cold robots rather than humans.

EVIDENCE FROM BRAIN DISORDERS

Brain disorders, such as autism, frontal lobe trauma, frontal-predominant dementias, and Huntington disease, can cause mechanistic dehumanization but do not usually lead to animalistic dehumanization. Among these disorders, behavioral variant frontotemporal dementia (bvFTD) may best illustrate the effect of brain disease on dehumanization. Patients with bvFTD experience mechanistic dehumanization and are generally unemotional, unempathic, and indifferent to others, including the feelings of their family and friends (^{Rascovsky et al, 2011}). However, these patients do not overtly devalue others or view them as disgusting.

The neuropathology of bvFTD is centered in the mesial frontal cortex (VMPFC, anterior cingulate cortex), frontal anterior insula, and anterior temporal lobe structures (^{Rosen et al, 2005; Seeley, 2008})—regions that are involved in mentalization. The affective component of mentalization is impaired early in bvFTD (^{Adenzato et al, 2010; Bora et al, 2015; Dodich et al, 2016; Henry et al, 2014; Orso et al, 2020; Pardini et al, 2013; Poletti et al, 2012, 2013; Torralva et al, 2015}), and individuals with bvFTD exhibit decreased performance on moral dilemmas, where they do not foresee negative emotional reactions when contemplating directly harming others (^{Greene and Haidt, 2002; Greene et al, 2001}). Furthermore, when viewing moving geometric shapes (^{Fong et al, 2017}), individuals with bvFTD who have right IFG atrophy show significantly decreased anthropomorphization compared with controls. Additional evidence for impaired “humanness” perception is that individuals with frontotemporal dementia who are artists, specifically caricaturists, may draw others in a progressively distorted, nonhuman, or “alien” fashion as their dementia progresses (^{Mendez and Perryman, 2003}). Finally, individuals with bvFTD express a seemingly paradoxical increased “presence” and interaction with avatars on virtual reality (^{Mendez et al, 2015}), suggesting a blurred human–robot distinction.

CONCLUSION

There are two main mechanisms for dehumanization: restraint of prosocial emotions toward other humans in the IFG and impaired mentalization in the VMPFC and gACC. Both occur normally; however, only normal, non–brain-injured subjects demonstrate increased inhibition of prosocial emotions due to increased IFG activity, leading to animalistic dehumanization and disgust. People are normally responsive to others’ emotional experiences, and the IFG functions to restrain prepotent emotional contagion as well as complex emotions such as empathy for self-protection and hierarchical social ranking. It is this form of dehumanization, which promotes disgust of groups or individuals that are “flawed” and likens them to animals, that is linked to intergroup hostility and risk of violence (^{Kteily and Landry, 2022}).

On the other hand, individuals with a brain injury may have disordered mentalization with a denial of full mind-attribution to others and mechanistic dehumanization with indifference. In contrast to individuals with animalistic dehumanization, individuals with mechanistic dehumanization do not show a tendency to blatantly dehumanize in an animalistic or infrahuman fashion. Mechanistic dehumanization may result from a decrease in the anterior network for mind-attribution or mentalization that is centered in the mesial frontal regions of the VMPFC and adjacent gACC.

These conclusions are a proposed model for dehumanization that is not definitively established and requires further research. Indeed, most of this paper involves the interpretation of neuroimaging studies that include relatively small numbers of subjects, and not all studies agree with the underlying cognitive processes involved and the role of the different frontal regions. Nevertheless, the profound importance of dehumanization behavior cries out for further theorizing and investigation of this concept.

Although dehumanized perception and biases are a fundamental part of our social cognition system and cannot be completely overcome, they can be moderated. The considerations described here should stimulate future research exploring these neurobiological mechanisms because they have the potential for behavioral and other interventions and preventative measures targeted at these proposed neurobiological mechanisms underlying dehumanization.