The behavioral immune system is a phrase coined by the psychological scientist Mark Schaller to refer to a suite of psychological mechanisms that allow individual organisms to detect the potential presence of infectious parasites or pathogens in their immediate environment, and to engage in behaviors that prevent contact with those objects and individuals. [1] [2] [3]
The existence of a behavioral immune system has been documented across many animal species, including humans. It is theorized that the mechanisms that comprise the behavioral immune system evolved as a crude first line of defense against disease-causing pathogens. [2]
In humans and animals, activating a physiological immune response to pathogens is effective, but metabolically costly. Immune responses are activated at the expense of other fitness enhancing activities. [4] [5] Inflammation after infection can also be harmful to the body (e.g., contribute to diseases of aging). [6] [7] In addition to cultural adaptations to avoid pathogens, the behavioral immune system acts as a set of defense mechanisms to protect against pathogens before infection occurs [8] [9] [10]
Mechanisms for the behavioral immune system include sensory processes through which cues connoting the presence of parasitic infections are perceived (e.g., the smell of a foul odor, the sight of pox or pustules), as well as stimulus–response systems through which these sensory cues trigger a cascade of aversive affective, cognitive, and behavioral reactions (e.g., arousal of disgust, automatic activation of cognitions that connote the threat of disease, behavioral avoidance). [2] [11]
Early and current research on behavioral immune system activation has been focused on visual cues or triggers that elicit responses. However, recent work suggests that other sensory modalities may be at work for disease detection. [12]
Studies show that olfactory cues of disease elicit disgust and predict pathogen avoidance behaviors. [12] In humans, body odors from diseased individuals are rated less desirable and likeable, [13] and perceived as unhealthier, more intense, and less pleasant. [14] Disgust of body odors is also influenced by the closeness of the source of odor. Odors from family members are rated as less disgusting than body odors from strangers. [15] Experimental studies showed that the presence of aversive odors leads to more prophylactic behaviors, such as more willing condom use [16] and less-direct prophylactic behaviors, like stricter punishment for moral violations. [17]
A new body of work has linked olfactory acuity to disgust and behavioral immune system responses. Given that olfactory cues for pathogen threat are often ambiguous and overgeneralized, a better ability to detect smells would lead to better pathogen avoidance. [12] Recent work suggests that higher olfactory acuity may be associated with higher avoidance motivation. [18]
Gustatory stimuli, particularly of spoiled foods, elicit feels of disgust and motivate pathogen avoidance behavior. [19] People with more disgust sensitivity are predicted to have more aversion to novel or foreign foods. [20]
Visual cues of pathogen threat have been linked to increases in tactile sensitivity and lead to perceived people with accents as more foreign, especially among individuals with higher disgust sensitivity. [21] [22] Disgust sensitivity among individuals is also predictive of preferred amount of personal space. [23]
The “smoke detector principle” [24] [25] of evolved systems that regulate protective responses has also been used to describe the behavioral immune systems tendency to overgeneralize. [26] Evolved responses to signals of pathogen threat cannot be perfect and rely on liberal identification criteria. [26] This makes the behavioral immune system susceptible to activating when pathogens are absent. Noninfectious physical and mental abnormalities including elderly appearance, [27] [28] disabilities, [29] obesity, [30] and disfigurement [31] [32] can act as cues of pathogen presence, when none are present. Even though many false alarms may be triggered in response to these nonharmful cues, the costs associated with behavioral immune activation may be relatively small to the costs of missing a true disease threat. [26]
Respond "Absent" | Respond "Present" | |
---|---|---|
Stimulus Present | Miss | Hit |
Stimulus Absent | Correct Rejection | False Alarm |
The pathogen disgust system and the behavioral immune system have been studied separately, but there is recognition that they are functionally the same. [33] Darwin first recognized that the emotion of disgust aided in avoiding "tainted" food. [34] The emotion of disgust has now been recognized as an adaptive function for avoiding pathogen exposure in response to cues of potential pathogen threat. However, disgust to pathogen-related cues should be context-dependent to function adaptively. [35] In one study on the Shuar, an indigenous subsistence-based population with high-pathogen stress, pathogen disgust sensitivity (PDS), measured with a disgust questionnaire, was used to predict pathogen infection. The study found that individual PDS was negatively correlated with pathogen infection predictive of the hypothesis that disgust acts as a pathogen defense mechanism sensitive to local costs and benefits of avoidance and infection. [35]
Functional flexibility is a term used to describe the ability of the behavioral immune system to adjust responses to pathogens depending on the individual’s infection-related threat and infection-relating vulnerability. Like many evolved threat management systems, the behavioral immune system is sensitive to the costs and benefits of pathogen avoidance. [36] [9] [37]
Two categories of outputs can be generated by the behavioral immune system; reactive and proactive responses [26] Reactive responses occur in response to the presence of cues connoting an immediate infection risk. The responses generally take the form of avoidant or prophylactic behaviors. [38] [39] These can include restricted sexual attitudes, [40] [41] [42] positivity towards condom use, [43] and avoidance of people with cues associated with illness. [29] [44]
Proactive responses occur in response to the long-term and persistent threat of pathogens. [26] Across species, proactive management of pathogen threat can be seen in hygiene behaviors to mitigate reoccurring bacterial and viral threats. [45] Additionally, proactive responses can be seen in the importance placed on a potential mate's physical attractiveness, symmetry, and secondary sex-characteristics which are all indicators of health, and healthy people are less likely to carry disease. [46] [47] [48] [49] [50]
Two social consequences of activating the behavioral immune system according to Ackerman et al., are "1. increased aversion and avoidance of unfamiliar and outgroup targets and 2. strengthened cohesion with familiar and ingroup targets." [26]
Interpersonal perception is influenced by the behavioral immune system due to the early stage cognitive processes recruited for identifying pathogenic threat and the risks associated with interpersonal contagion.
Visual attention is one these early cognitive process recruited toward cues of pathogen threat and perceivers find it more difficult to visually disengage from faces possessing these cues, like physical abnormalities, even if no threat is present . [31]
The behavioral immune system may also cause people who perceive greater pathogen threat to report greater distinctions between ingroup and outgroup members, and classify strangers as more threatening. [51] [28] [21]
Judgments, inferences, and decisions about people and objects are other cognitive processes recruited by the behavioral immune system that are affected by pathogen threat. These processes motivative choices that help perceivers avoid unfamiliar stimuli that may include contamination dangers. [52] [53]
Examples of the influence perceived pathogen threat has on these processes include: avoidance of controllable risk, [54] reduced desire for social affiliation, [55] and devaluation of consumer products previously handled by strangers. [56]
Additional lines of research on the behavioral immune system have shown that people engage in more reticent and conservative forms of behavior under conditions in which they feel more vulnerable to disease transmission. For instance, when the potential threat of disease is made salient, people tend to be less extraverted or sociable. [57]
The behavioral immune system influences potential mating choices and sexual behavior. High concern for chronic pathogen threats and environments with greater pathogen stress increase the value an individual places on physical attractiveness, [46] [49] [58] and lessens preference for mates with physical abnormality or sex-divergent features. [59] [60] Among perceivers, cues of pathogen prevalence (real or perceived) [26] are associated with more restricted sexual pursuits and attitudes to avoid infection. [40] [42] Opposite, more sexual opportunism is found in individuals with perceived insufficient immune systems for survival in environments with higher pathogen stress. [47]
Another outcome of behavioral immune system activity is prejudice and stereotyping of outgroup members. Individuals at higher risk of pathogen infection are more likely to stigmatize other individuals possessing cues of disease, real or perceived. [44] [61] [62] [63] [64] [28] [65]
The disease–avoidant processes that characterize the behavioral immune system have been shown to contribute to prejudices against obese individuals, elderly individuals, and people with physical disfigurements or disabilities. [66] [67] [27] In addition, the behavioral immune system appears to contribute to xenophobia and ethnocentrism. [64] [68] This can be seen among pregnant women, which face higher infection-related vulnerability, that express an increase ethnocentric views. [68] These attitudes might function as a way to avoid people with new pathogens or practices that local practices are unsuited to manage. [26] One implication is that these prejudices tend to be exaggerated under conditions in which people feel especially vulnerable to the potential transmission of infectious diseases.
The behavioral immune system has the ability to impact group intragroup attitudes and behaviors. Research shows that pathogen stress is associated with higher social conformity [69] [70] and higher levels of disgust, which aids in pathogen avoidance [56] [11] [71] [39] and predictive of greater sensitivity to moral violations. [26]
Some studies have used the behavioral immune system to explain the root of more fundamental dimensions of culture including the variance in: individualism/collectivism, [72] social and political orientation, [73] [74] [58] and religious beliefs, [75] [76] in response to levels of pathogen stress.
Collectivist cultures defined by behavioral manifestations such as ethnocentrism and social conformity which aid in pathogen avoidance, have been correlated with higher historical pathogen stress compared to individualistic cultures. [72] Given that there might be benefits associated with individualistic cultures in the societies they create, individualist cultures also confer greater pathogen exposure. In environments with greater pathogen stress, the behavioral manifestations of collectivism that help avoid pathogens may serve an adaptive advantage. [72]
In-group preferences defined by the degree to which people prefer interacting with and investing in family, friends, and in group members has been suggested to be a function of the behavioral immune system in order to defend against pathogens. [72] [10] [76] However, these studies on cross-population level data have been criticized for not incorporating non-independence variables and alternative hypotheses. [77] [78] [79] In a follow up study, using measures the same measure for in-group preference, Hofstede's collectivism, Van de Vliert's in-group favoritism, and Fincher and Thornhill's strength of family ties, find that less government effectiveness is a better predictor of in-group preference than pathogen stress. [80]
Some research suggests that the behavioral immune system has implications for the functioning of the physiological immune system (PIS) too. One study found that the mere visual perception of diseased-looking people stimulated white blood cells to respond more aggressively to infection (as indicated by the production of the proinflammatory cytokine Interleukin 6 in response to a bacterial stimulus). [81]
In other studies, exposure to visual environmental pathogen cues, in addition to increased feelings of disgust and prejudice responses associated with the behavioral immune system (BIS), upregulate oral and blood immune inflammatory biomarkers. [82] [83] [84] This body of literature suggests that visual cues connected to the BIS may invoke PIS responses when pathogen threat is immediate. [12]
In the absence of an immediate pathogen threat, the PIS and BIS may not be complimentary, but compensatory. For example, in an experiment that stimulated release of proinflammatory cytokines (IL-6, IL-1b, TNF-alpha) collected from healthy individuals, were not related to self-reported germ aversion, but in-vivo IL-6 levels were negatively correlated to germ aversion and perceived longevity. This research may highlight the function of the BIS for long-term health by decreasing proinflammatory responses (function of IL-6), linked to diseases of aging. [85]
Research also indicates that immune-relevant interventions which target pathogen transmission can interrupt behavioral responses. For example, receiving a flu vaccination or washing one's hands can reduce the extent of negative interpersonal and intergroup attitudes elicited by disease cues and concerns. [86]
The large body of literature on the behavioral immune system is focused on behaviors triggered by pathogen cues in the environment and the role disgust plays in mitigating exposure before infection. However, the emotion associated with being sick, lassitude, that is triggered by an active infection may also act as behavioral defense mechanism against pathogens. One study suggests that lassitude may help fight against an active infection by:
The behavioral immune system's prediction of ingroup favoritism and ethnocentric beliefs has been applied to individual beliefs during the global pandemic, COVID-19. In several studies, individuals who scored higher in dispositional worry about disease reported stronger preference for restrictive travel bans on several regions of Asia. One study found that as COVID-19 cases increased, individuals reported higher concern about contracting the virus and showed greater levels of Right-Wing Authoritarianism, including conservatism and traditionalism. [88] Experimental increase of pandemic salience also increased support of travel bans on high risk nation (China and Italy), but not on low risk nations like Canada and Mexico. [89] Another study measuring individual perceived vulnerability to disease (PVD) and responses to COVID-19 found that higher PVD was positively associated with stronger reactions to the threat of COVID-19, including increased anxiety, perceptions that people should alter their typical behavior, and self-reported importance of social distancing. [90]
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Sociosexuality, sometimes called sociosexual orientation, is the individual difference in the willingness to engage in sexual activity outside of a committed relationship. Individuals who are more restricted sociosexually are less willing to engage in casual sex; they prefer greater love, commitment and emotional closeness before having sex with romantic partners. Individuals who are more unrestricted sociosexually are more willing to have casual sex and are more comfortable engaging in sex without love, commitment or closeness.
Fluctuating asymmetry (FA), is a form of biological asymmetry, along with anti-symmetry and direction asymmetry. Fluctuating asymmetry refers to small, random deviations away from perfect bilateral symmetry. This deviation from perfection is thought to reflect the genetic and environmental pressures experienced throughout development, with greater pressures resulting in higher levels of asymmetry. Examples of FA in the human body include unequal sizes (asymmetry) of bilateral features in the face and body, such as left and right eyes, ears, wrists, breasts, testicles, and thighs.
A psychological adaptation is a functional, cognitive or behavioral trait that benefits an organism in its environment. Psychological adaptations fall under the scope of evolved psychological mechanisms (EPMs), however, EPMs refer to a less restricted set. Psychological adaptations include only the functional traits that increase the fitness of an organism, while EPMs refer to any psychological mechanism that developed through the processes of evolution. These additional EPMs are the by-product traits of a species’ evolutionary development, as well as the vestigial traits that no longer benefit the species’ fitness. It can be difficult to tell whether a trait is vestigial or not, so some literature is more lenient and refers to vestigial traits as adaptations, even though they may no longer have adaptive functionality. For example, xenophobic attitudes and behaviors, some have claimed, appear to have certain EPM influences relating to disease aversion, however, in many environments these behaviors will have a detrimental effect on a person's fitness. The principles of psychological adaptation rely on Darwin's theory of evolution and are important to the fields of evolutionary psychology, biology, and cognitive science.
Affective neuroscience is the study of how the brain processes emotions. This field combines neuroscience with the psychological study of personality, emotion, and mood. The basis of emotions and what emotions are remains an issue of debate within the field of affective neuroscience.
Chronic stress is the physiological or psychological response induced by a long-term internal or external stressor. The stressor, either physically present or recollected, will produce the same effect and trigger a chronic stress response. There is a wide range of chronic stressors, but most entail relatively prolonged problems, conflicts and threats that people encounter on a daily basis. Several chronic stressors have been identified as associated with disease and mortality including "neighbourhood environment, financial strain, interpersonal stress, work stress and caregiving."
Evolutionary approaches to depression are attempts by evolutionary psychologists to use the theory of evolution to shed light on the problem of mood disorders within the perspective of evolutionary psychiatry. Depression is generally thought of as dysfunction or a mental disorder, but its prevalence does not increase with age the way dementia and other organic dysfunction commonly does. Some researchers have surmised that the disorder may have evolutionary roots, in the same way that others suggest evolutionary contributions to schizophrenia, sickle cell anemia, psychopathy and other disorders. The proposed explanations for the evolution of depression remain controversial.
Social anxiety is the anxiety and fear specifically linked to being in social settings. Some categories of disorders associated with social anxiety include anxiety disorders, mood disorders, autism spectrum disorders, eating disorders, and substance use disorders. Individuals with higher levels of social anxiety often avert their gazes, show fewer facial expressions, and show difficulty with initiating and maintaining a conversation. Social anxiety commonly manifests itself in the teenage years and can be persistent throughout life; however, people who experience problems in their daily functioning for an extended period of time can develop social anxiety disorder. Trait social anxiety, the stable tendency to experience this anxiety, can be distinguished from state anxiety, the momentary response to a particular social stimulus. Half of the individuals with any social fears meet the criteria for social anxiety disorder. Age, culture, and gender impact the severity of this disorder. The function of social anxiety is to increase arousal and attention to social interactions, inhibit unwanted social behavior, and motivate preparation for future social situations.
Disgust is an emotional response of rejection or revulsion to something potentially contagious or something considered offensive, distasteful or unpleasant. In The Expression of the Emotions in Man and Animals, Charles Darwin wrote that disgust is a sensation that refers to something revolting. Disgust is experienced primarily in relation to the sense of taste, and secondarily to anything which causes a similar feeling by sense of smell, touch, or vision. Musically sensitive people may even be disgusted by the cacophony of inharmonious sounds. Research has continually proven a relationship between disgust and anxiety disorders such as arachnophobia, blood-injection-injury type phobias, and contamination fear related obsessive–compulsive disorder.
Mark Schaller is an American psychological scientist who has made many contributions to the study of human psychology, particularly in areas of social cognition, stereotyping, evolutionary psychology, and cultural psychology. He is a professor of psychology at the University of British Columbia.
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Emotion perception refers to the capacities and abilities of recognizing and identifying emotions in others, in addition to biological and physiological processes involved. Emotions are typically viewed as having three components: subjective experience, physical changes, and cognitive appraisal; emotion perception is the ability to make accurate decisions about another's subjective experience by interpreting their physical changes through sensory systems responsible for converting these observed changes into mental representations. The ability to perceive emotion is believed to be both innate and subject to environmental influence and is also a critical component in social interactions. How emotion is experienced and interpreted depends on how it is perceived. Likewise, how emotion is perceived is dependent on past experiences and interpretations. Emotion can be accurately perceived in humans. Emotions can be perceived visually, audibly, through smell and also through bodily sensations and this process is believed to be different from the perception of non-emotional material.
In evolutionary psychology and behavioral ecology, human mating strategies are a set of behaviors used by individuals to select, attract, and retain mates. Mating strategies overlap with reproductive strategies, which encompass a broader set of behaviors involving the timing of reproduction and the trade-off between quantity and quality of offspring.
Parasite-stress theory, or pathogen-stress theory, is a theory of human evolution proposing that parasites and diseases encountered by a species shape the development of species' values and qualities, proposed by researchers Corey Fincher and Randy Thornhill.
Some evolutionary theorists consider prejudice as having functional utility in evolutionary process. A number of evolutionary psychologists in particular posit that human psychology, including emotion and cognition, is influenced by evolutionary processes. These theorists argue that although psychological variation appears between individuals, the majority of our psychological mechanisms are adapted specifically to solve recurrent problems in our evolutionary history, including social problems.
Groups of animals and humans that live in places with high population density have an increased risk of disease prevalence. In looking at sociality and disease transmission, an examination of how social grouping strategies may reduce or increase the spread of disease is critical for the health of large groups of people. Social groups, community structures, and cultures affect the use of different strategies and behaviors to reduce the spread of disease.
Major depression is often associated or correlated with immune function dysregulation, and the two are thought to share similar physiological pathways and risk factors. Primarily seen through increased inflammation, this relationship is bidirectional with depression often resulting in increased immune response and illness resulting in prolonged sadness and lack of activity. This association is seen both long-term and short-term, with the presence of one often being accompanied by the other and both inflammation and depression often being co-morbid with other conditions.
Costly signaling theory in evolutionary psychology refers to uses of costly signaling theory and adaptationism in explanations for psychological traits and states. Often informed by the closely related fields of human behavioral ecology and cultural evolution, such explanations are predominantly focused on humans and emphasize the benefits of altering the perceptions of others and the need to do so in ways that are difficult to fake due to the widespread existence of adaptations which demand reliable information to avoid manipulation through dishonest signals.
Pathogen avoidance, also referred to as, parasite avoidance or pathogen disgust, refers to the theory that the disgust response, in humans, is an adaptive system that guides behavior to avoid infection caused by parasites such as viruses, bacteria, fungi, protozoa, helminth worms, arthropods and social parasites. Pathogen avoidance is a psychological mechanism associated with the behavioral immune system. Pathogen avoidance has been discussed as one of the three domains of disgust which also include sexual and moral disgust.