Why inflammation and the activities of the immune system matter for social and personality psychology (and not only for those who study health)

Homo temporus: Seasonal Cycles as a Fundamental Source of Variation in Human Psychology

Many animal species exhibit seasonal changes in their physiology and behavior. Yet despite ample evidence that humans are also responsive to seasons, the impact of seasonal changes on human psychology is underappreciated relative to other sources of variation (e.g., personality, culture, development). This is unfortunate because seasonal variation has potentially profound conceptual, empirical, methodological, and practical implications. Here, we encourage a more systematic and comprehensive collective effort to document and understand the many ways in which seasons influence human psychology. We provide an illustrative summary of empirical evidence showing that seasons impact a wide range of affective, cognitive, and behavioral phenomena. We then articulate a conceptual framework that outlines a set of causal mechanisms through which seasons can influence human psychology-mechanisms that reflect seasonal changes not only in meteorological variables but also in ecological and sociocultural variables. This framework may be useful for integrating many different seasonal effects that have already been empirically documented and for generating new hypotheses about additional seasonal effects that have not yet received empirical attention. The article closes with a section that provides practical suggestions to facilitate greater appreciation for, and systematic study of, seasons as a fundamental source of variation in human psychology.

Personality and aging-related immune phenotype

An aging-related immune phenotype (ARIP) has been defined as a decrease in naïve T cells (T_N) relative to the accumulation of memory T cells (T_M). Recent research implicates ARIP measures, such as CD4 +T_N/T_M and CD8 +T_N/T_M ratios, in multimorbidity and mortality. This study examined whether psychological dispositions that assess how people think, feel, and behave are related to CD4 +T_N/T_M and CD8 +T_N/T_M. Participants were adults aged 50-104 years (N = 4798; 58% women, Mean Age= 67.95, SD= 9.56) from the Health and Retirement Study. Data on CD4 +T_N/T_M and CD8 +T_N/T_M were obtained in 2016. Data on personality, demographic factors, and potential clinical (body mass index, disease burden), behavioral (smoking, alcohol, physical activity), psychological (depressive symptoms, stress), and biological (cytomegalovirus IgG antibodies) mediating factors were obtained in 2014/2016. Controlling for demographic factors, higher conscientiousness was related to higher CD4 +T_N/T_M and CD8 +T_N/T_M. To a lesser extent, higher neuroticism and lower extraversion were associated with lower CD4 +T_N/T_M. Physical activity, and to a lesser extent BMI and disease burden, were the most robust mediators between personality and ARIP measures. Cytomegalovirus IgG level mediated the association between conscientiousness and both CD4 +T_N/T_M and CD8 +T_N/T_M. This study provides novel evidence that personality is related to ARIP. Higher conscientiousness and, to a lesser extent, higher extraversion may be protective against age-related immunophenotype change, whereas neuroticism may be a risk factor.

Heightened inflammation in bipolar disorder occurs independent of symptom severity and is explained by body mass index

Inflammation is hypothesized to be a key component of bipolar disorder (BP) development and progression. However, findings linking BP prevalence and symptomology to immune functioning have been mixed, with some work suggesting that obesity may play an important role in BP-relevant inflammation. Here we investigate differences in biomarkers of inflammation [C-reactive protein (CRP), interleukin (IL)-1β, IL-6, IL-8, IL-10] between healthy controls (HC) and individuals with BP or other mental illness (MI). Adults with BP, MI, or HC (n = 545, 70% BP, 21% HC, 9% MI) self-reported depressive and manic symptoms close to a blood draw and physical exam that included measurement of height and weight. A composite score was calculated from the four cytokines measured in plasma; follow-up analyses explored a pro-inflammatory composite and IL-10, individually. BP individuals had elevated cytokine concentrations compared to HC (B = 0.197, [0.062, 0.333], t (542) = 2.855, p = .004); this difference was also evident for the pro-inflammatory composite and for IL-10. Cytokine concentrations were not associated with BP mood states. Body mass index (BMI), an indicator of obesity, was significantly higher in BP compared to HC (B = 3.780, [2.118, 5.443], t (479) = 4.457, p < .001) and differences in cytokines between the two groups was no longer significant after controlling for BMI. No differences in CRP were evident between BP and HC. These results suggest that cytokine concentrations are elevated in BP and this difference from HC is associated with obesity. Interventions targeting immune modulators in BP must carefully consider the complex relationships within the BP-inflammation-obesity triangle.

Exploring links between pathogen avoidance motivation, COVID-19 case counts, and immune function

OBJECTIVES

The selection pressures exerted by pathogens have played important roles in shaping the biology and behavior of animals, including humans. Immune systems recognize and respond to cues of infection or damage by coordinating cellular, humoral, and metabolic shifts that promote recovery. Moreover, animals also possess a repertoire of behavioral tools to help combat the threat of pathogens, often referred to as the behavioral immune system. Recently, researchers have begun to examine how cognitive, affective, and behavioral disease avoidance mechanisms interact with the biological immune system.

METHODS

The present study explored relationships among individual differences in behavioral immune system activity (e.g., pathogen disgust), shifts in SARS-CoV-2 infection risk (i.e., 7-day case averages), and immune function in a community cohort from McLennan County, Texas, USA (n = 387).

RESULTS

Levels of disease concern were not consistently associated with immune markers. However, serum levels of IFN-γ, TNF-α, IL-2, and IL-8, as well as serum killing ability of Escherichia coli, each varied with case counts. Additional analyses found that case counts also predicted changes in stress physiology, but not subjective measures of distress. However, follow-up mediation models did not provide evidence that relationships between case counts and immunological outcomes were mediated through levels of stress.

CONCLUSIONS

The present project provides initial evidence that markers of immune function may be sensitive to changes in infection risk during the COVID-19 pandemic. This adds to the growing body of research finding relationships among behavioral and biological pathogen management mechanisms.

Le Petit Machiavellian Prince: Effects of Latent Toxoplasmosis on Political Beliefs and Values

Humans infected by Toxoplasma gondii express no specific symptoms but manifest higher incidence of many diseases, disorders and differences in personality and behavior. The aim of this study was to compare the political beliefs and values of Toxoplasma-infected and Toxoplasma-free participants. We measured beliefs and values of 2315 responders via an online survey (477 Toxoplasma-infected) using the Political Beliefs and Values Inventory (PI34). This study showed Toxoplasma-infected and Toxoplasma-free participants of our cross-sectional study differed in three of four factors of PI34, scoring higher in Tribalism and lower in Cultural liberalism and Anti-Authoritarianism. We found sex differences in political beliefs associated with Toxoplasma infection. Infected women scored higher in tribalism and lower in cultural liberalism, compared with the Toxoplasma-free control group, while infected men scored higher in economic equity. These results fit with sexual differences in behavior and attitude observed after toxoplasmosis infection. Controlling for the effect of worse physical health and mental health had little impact, suggesting that impaired health did not cause these changes. Rather than adaptation to prevalence of parasites, as suggested by parasite-stress theory, the differences might be side-effects of long-term mild inflammatory reaction. However, to get clear picture of the mild inflammation effects, more research focused on different infectious diseases is needed.

Exploring the Links between Personality and Immune Function

Decades of research finds associations between personality traits and health. In recent years, it has become clear that the activities of the immune system play a key role in linking these variables. In the current work, we add to this research by exploring the relationship between Big Five personality traits and (Study 1) polymorphisms known to impact cytokine release and (Study 2) immunological parameters measured in vivo (differential white blood cell counts, plasma proinflammatory cytokine levels) and in vitro (proinflammatory cytokine release by peripheral blood mononuclear cells, Staphylococcus aureus growth in plasma). Results provide insights into potential mechanistic drivers of the link between personality and immune function and the possibility that, in some cases, relationships between personality and immune function may be sex differentiated.

"Touch Me If You Can!": Individual Differences in Disease Avoidance and Social Touch

The threat of diseases varies considerably among individuals, and it has been found to be linked to various proactive or reactive behaviors. In the present studies, we investigated the impact of individual differences in the perceived vulnerability to disease (PVD) on social touch before (Study 1) or during the coronavirus disease 2019 (COVID-19) pandemic (Study 2). We also investigated the influence of personality traits in the covariation between these two dimensions. We found that people who are the most disease-avoidant are also the most reluctant to touching or being touched by others (and this relationship holds when personality traits are taken into account). Interestingly, the association between PVD and social touch increased during the COVID-19 pandemic compared with a few months before. By showing that the fear of contamination has an association with social touch, the findings provide further evidence for the behavioral immune system ( Schaller and Park, 2011), a psychological system acting as a first line of defense against pathogens.

Beyond social withdrawal: New perspectives on the effects of inflammation on social behavior

Decades of research in animals and humans show that inflammation is an important regulator of social behavior. While much research in this area has concluded that inflammation causes a withdrawal from social interaction, closer examination of the literature reveals that the effects of inflammation on social behavior are much more nuanced. Indeed, while many studies do show that increases in inflammation lead to social withdrawal, other studies show the exact opposite, finding that inflammation leads to an increase in social approach behavior. Critically, whether an organism withdraws or approaches when inflamed may depend on the whether the target of the behavior is a close other or a stranger. In the present paper, we review both animal research and our initial research in humans that has utilized experimental manipulations of inflammation and examined their effects on social approach behavior. We argue, based on complementary theoretical perspectives and supporting evidence from the literature, that there are three critical next steps for translational work examining the effects of inflammation on social behavior: (1) We need to study actual social behavior, as expressed toward both close others and strangers; (2) We should examine not just the social behavior of the inflamed individual, but also the behavior of others interacting with an inflamed individual; and (3) We must consider the relative increases in inflammation (i.e., higher vs. lower) as a contributor to social withdrawal vs. approach. Ultimately, we urge the field to move beyond a singular focus on inflammation and social withdrawal so that we can develop a more comprehensive understanding of the effects of inflammation on a variety of social behaviors.

Fundamental Motives Illuminate a Broad Range of Individual and Cultural Variations in Thought and Behavior

An article published in Current Directions a decade ago introduced the fundamental-motives framework and reviewed initial promising findings using this general approach. According to this framework, a recurring set of challenges and opportunities during human evolution gave rise to overarching motivational systems in the domains of self-protection, disease avoidance, social affiliation, status seeking, mate acquisition, mate retention, and kin care. When activated, fundamental motives influence psychological processes by directing cognition and behavior in distinct and functionally relevant ways. In the intervening years, the approach has been expanded to a broader range of motives, individual and cultural variations in those motives, and the physiological correlates of activating different motives. In this article, we review a decade of research applying the fundamental-motives framework and point to promising new research directions.

Explaining socioeconomic disparities in health behaviours: A review of biopsychological pathways involving stress and inflammation

The purpose of this article was to explore how individuals' position in a socioeconomic hierarchy is related to health behaviours that are related to socioeconomic disparities in health. We identified research which shows that: (a) low socioeconomic status (SES) is associated with living in harsh environments, (b) harsh environments are related to increased levels of stress and inflammation, (c) stress and inflammation impact neural systems involved in self-control by sensitising the impulsive system and desensitising the reflective system, (d) the effects are inflated valuations of small immediate rewards and deflated valuations of larger delayed rewards, (e) these effects are observed as increased delay discounting, and (f) delay discounting is positively associated with practicing more unhealthy behaviours. The results are discussed within an adaptive evolutionary framework which lays out how the stress response system, and its interaction with the immune system and brain systems for decision-making and behaviours, provides the biopsychological mechanisms and regulatory shifts that make widespread conditional adaptability possible. Consequences for policy work, interventions, and future research are discussed.

Infection threat shapes our social instincts

We social animals must balance the need to avoid infections with the need to interact with conspecifics. To that end we have evolved, alongside our physiological immune system, a suite of behaviors devised to deal with potentially contagious individuals. Focusing mostly on humans, the current review describes the design and biological innards of this behavioral immune system, laying out how infection threat shapes sociality and sociality shapes infection threat. The paper shows how the danger of contagion is detected and posted to the brain; how it affects individuals’ mate choice and sex life; why it strengthens ties within groups but severs those between them, leading to hostility toward anyone who looks, smells, or behaves unusually; and how it permeates the foundation of our moral and political views. This system was already in place when agriculture and animal domestication set off a massive increase in our population density, personal connections, and interaction with other species, amplifying enormously the spread of disease. Alas, pandemics such as COVID-19 not only are a disaster for public health, but, by rousing millions of behavioral immune systems, could prove a threat to harmonious cohabitation too.

Does the Punishment Fit the Crime (and Immune System)? A Potential Role for the Immune System in Regulating Punishment Sensitivity

Although the criminal justice system is designed around the idea that individuals are invariant in their responses to punishment, research indicates that individuals exhibit a tremendous amount of variability in their punishment sensitivity. This raises the question of why; what are the individual- and situation-level variables that impact a person’s sensitivity to punishment? In the current research, we synthesize theory and research on inflammation, learning, and evolutionary biology to examine the relationship between inflammatory activity and sensitivity to punishment. These theories combine to predict that inflammatory activity – which is metabolically costly and reflects a context in which the net payoff associated with future oriented behaviors is diminished – will decrease sensitivity to punishment, but not rewards. Consistent with this hypothesis, Study 1 found that in U.S. states with a higher infectious disease burden (a proxy for average levels of inflammatory activity) exhibit harsher sentencing in their criminal justice systems. Studies 2 and 3 experimentally manipulated variables known to impact bodily inflammatory activity and measured subsequent punishment and reward sensitivity using a probabilistic selection task. Results revealed that (a) increasing inflammation (i.e., completing the study in a dirty vs. clean room) diminished punishment sensitivity (Study 2), whereby (b) administering a non-steroidal anti-inflammatory drug, suppressing inflammatory activity, enhanced it. No such changes were found for reward sensitivity. Together, these results provide evidence of a link between the activities of the immune system and punishment sensitivity, which may have implications for criminal justice outcomes.

The aetiology of social deficits within mental health disorders: The role of the immune system and endogenous opioids

The American National Institute for Mental Health (NIMH) has put out a set of research goals that include a long-term plan to identify more reliable endogenous explanations for a wide variety of mental health disorders (Insel, 2013). In response to this, we have identified a major symptom that underlies multiple mental health disorders - social bonding dysfunction. We suggest that endogenous opioid abnormalities can lead to altered social bonding, which is a symptom of various mental health disorders, including depression, schizophrenia and ASD. This article first outlines how endogenous opioids play a role in social bonding. Then we show their association with the body's inflammation immune function, and review recent literature linking inflammation to mental health 'immunophenotypes'. We finish by explaining how these immunophenotypes may be caused by alterations in the endogenous opioid system. This is the first overview of the role of inflammation across multiple disorders where we provide a biochemical explanation for why immunophenotypes might exist across diagnoses. We propose a novel mechanism of how the immune system may be causing 'sickness-type' behaviours (fatigue, appetite change, social withdrawal and inhibited motivation) in those who have these immunophenotypes. We hope that this novel aetiology can be used as a basis for future research in mental health.