The majority of studies, & especially the higher quality studies, showed that those who avoided meat consumption had significantly higher rates or risk of depression, anxiety, or self-harm behaviors

Meat and mental health: a systematic review of meat abstention and depression, anxiety, and related phenomena. Urska Dobersek et al. Critical Reviews in Food Science and Nutrition, Apr 20 2020. https://doi.org/10.1080/10408398.2020.1741505

Abstract
Objective: To examine the relation between the consumption or avoidance of meat and psychological health and well-being.

Methods: A systematic search of online databases (PubMed, PsycINFO, CINAHL Plus, Medline, and Cochrane Library) was conducted for primary research examining psychological health in meat-consumers and meat-abstainers. Inclusion criteria were the provision of a clear distinction between meat-consumers and meat-abstainers, and data on factors related to psychological health. Studies examining meat consumption as a continuous or multi-level variable were excluded. Summary data were compiled, and qualitative analyses of methodologic rigor were conducted. The main outcome was the disparity in the prevalence of depression, anxiety, and related conditions in meat-consumers versus meat-abstainers. Secondary outcomes included mood and self-harm behaviors.

Results: Eighteen studies met the inclusion/exclusion criteria; representing 160,257 participants (85,843 females and 73,232 males) with 149,559 meat-consumers and 8584 meat-abstainers (11 to 96 years) from multiple geographic regions. Analysis of methodologic rigor revealed that the studies ranged from low to severe risk of bias with high to very low confidence in results. Eleven of the 18 studies demonstrated that meat-abstention was associated with poorer psychological health, four studies were equivocal, and three showed that meat-abstainers had better outcomes. The most rigorous studies demonstrated that the prevalence or risk of depression and/or anxiety were significantly greater in participants who avoided meat consumption.

Conclusion: Studies examining the relation between the consumption or avoidance of meat and psychological health varied substantially in methodologic rigor, validity of interpretation, and confidence in results. The majority of studies, and especially the higher quality studies, showed that those who avoided meat consumption had significantly higher rates or risk of depression, anxiety, and/or self-harm behaviors. There was mixed evidence for temporal relations, but study designs and a lack of rigor precluded inferences of causal relations. Our study does not support meat avoidance as a strategy to benefit psychological health.

Keywords: Anxiety, depression, meat, mental health, self-harm, vegan, vegetarianism

Discussion

Based on this systematic review comprising 160,257 participants from varied geographic regions, including Europe, Asia, North America, and Oceania, aged 11 to 96 years, there is clear evidence that meat-abstention is associated with higher rates or risk of depression, anxiety, and self-harm. The results with respect to mood states, affective well-being, stress perception and QoL are less clear and the majority of studies do not support unequivocal inferences.

Across all studies, there was no evidence to support a causal relation between the consumption or avoidance of meat and any psychological outcomes. However, three studies provided evidence suggesting (contradictory) temporal relations between meat-abstention and depression and anxiety. Michalak, Zhang, and Jacobi (2012) demonstrated that the mean age at the adoption of meat-abstention (30.58 years) was substantially older than the mean age of the onset of metal disorder (24.69 years). These authors posited that mental disorders may lead to the adoption of a meat-less diet. The authors stated that individuals with mental disorders may “choose a vegetarian diet as a form of safety or self-protective behavior” (Michalak, Zhang, and Jacobi 2012, 6) due to the perception that plant-based diets are more healthful or because individuals with mental disorders may be “more aware of suffering of animals” (Michalak, Zhang, and Jacobi 2012, 2). Interestingly, these investigators also found that people with a lifetime diagnosis of psychological disorders consumed less fish and fast food. While these results conflict with previous research on fast food and mental health (Crawford et al. 2011), they support Matta et al.’s results and hypothesis that the exclusion of any food group, and especially meat and poultry, is associated with increased odds of having symptoms of psychological disorders (Matta et al. 2018).

Conversely, in their longitudinal analysis, Lavallee et al. (2019) found that meat-abstention was linked to “slight increases over time” (Lavallee et al. 2019, 153) in depression and anxiety in Chinese students. One important caveat when considering these disparate results on temporal relations may be differences in the factors that led to meat-abstention (e.g., religious practices, health and ethical considerations, or socio-economic status). For example, economically disadvantaged individuals who do not consume meat due to its relative cost may be at risk for ill-health for myriad reasons independent of their lack of meat consumption. Thus, future research examining temporal relations should establish clear distinctions between individuals and populations that abstain from meat consumption due to ethical, religious, and health-related perceptions, or those who do not consume meat for economic reasons.

Strengths, limitations, and flaws of the extant literature

Psychological outcomes

The most rigorous studies used objective and/or multiple assessments of psychological outcomes. For example, Michalak, Zhang, and Jacobi (2012) were the first investigators to use standardized and comprehensive assessment of mental disorders based on DSM criteria (APA 2013). Similarly, Baines, Powers, and Brown (2007) provided multiple assessments of psychological outcomes that included reported physician-diagnosed conditions, and the use of prescription medication for depression and anxiety in concert with self-reported symptoms and behaviors (e.g., self-harm; Baines, Powers, and Brown 2007). Comparably, Hibbeln et al. (2018) included detailed participant histories, including childhood contact with psychiatric services and family history of depression. Methodologically weaker studies employed a single questionnaire and, in some cases, employed a single-item examining psychological symptoms over a limited timeframe (e.g., the previous week).

Sampling

The most rigorous studies examined large, representative and/or matched samples (Baines, Powers, and Brown 2007; Matta et al. 2018; Michalak, Zhang, and Jacobi 2012), whereas the least rigorous used biased recruitment strategies and biased convenience sampling. For example, Michalak, Zhang, and Jacobi (2012) employed both a large representative sample and a socio-demographically matched subsample for comparison. The strength of this approach cannot be understated. Similarly, Neumark-Sztainer et al. (1997) employed a matched sample drawn from a much larger study population. Conversely, a number of studies attempted to over-sample vegans and vegetarians by directly targeting these groups via internet sources such as “social websites geared to VG and VEG [vegan and vegetarians]”(Beezhold et al. 2015) and vegan chat-rooms and/or magazines and vegetarian “fairs” (Boldt et al. 2018; Wirnitzer et al. 2018). As discussed below, these sampling strategies in concert with self-reported data are a major design flaw.

Investigators who seek to over-sample groups that are highly invested (e.g., ethically, socially, intellectually or emotionally) in their “lifestyle” or dietary choices should acquaint themselves with the large body of research on cognitive dissonance (Festinger 1962), social-desirability (Fisher 1993), and observer-expectancy effects (e.g., reactivity). This body of work is especially relevant when applied to the errors and biases of self-reported dietary intake data; for details, please see (Archer, Hand, and Blair 2013; Archer, Lavie, and Hill 2018b; Archer, Marlow, and Lavie 2018c, 2018d; Archer, Pavela, and Lavie 2015; Hebert et al. 1995; Schoeller et al. 2013). This large and well-established body of research suggests that for individuals who maintain a strong group identity or affinity, meat consumption or avoidance may represent a significant ethical, intellectual, emotional, behavioral, social and/or spiritual investment that extends well-beyond a simple dietary choice. As such, many individuals (e.g., Paleo and “meat-only” dieters, vegans, vegetarians, Seventh Day Adventists) will be pre-disposed to report significantly higher levels of physical and psychological health to avoid cognitive dissonance and remain consistent with self- and/or group-appraisals.

These non-intentional biases in concert with the potential for a participant to intentionally misreport outcomes to support his or her ideological stances or religious beliefs may induce systematic and non-quantifiable errors when employing self-report protocols. As such, the oversampling of groups that are highly invested in their dietary regimes for health, religious, or ideologic concerns (e.g., animals rights) will lead to biased recruitment and extremely unreliable data. In fact, research on cognitive dissonance and social desirability suggests that the greater the motivation for adhering to one’s dietary or lifestyle pattern (or self-conception), the larger the potential error induced via the use of self-reports (Archer, Marlow, and Lavie 2018c; Festinger 1962). Future studies should employ objective data collection protocols when over-sampling groups that may be prone to intentional and/or non-intentional misreporting.

Assessment of dietary status (exposure)

One major limitation of all studies in this review was the use of self-reported dietary status. Currently, there is an escalating and contentious debate on the validity of self-reported dietary data and the use of food frequency questionnaires in nutrition (Archer and Lavie 2019b; Archer, Lavie, and Hill 2018b; Archer, Marlow, and Lavie 2018d; Archer, Pavela, and Lavie 2015; Ioannidis 2018; Martín-Calvo and Martínez-González 2018; Satija et al. 2015; Schoeller et al. 2013; Subar et al. 2015; Trepanowski and Ioannidis 2018). The debate revolves around two major criticisms. First, critics of self-reported data state that without objective corroboration of dietary self-reports, it is impossible to quantify measurement error due to intentional and nonintentional distorting factors, such as deliberate misreporting (i.e., deception/lying), social desirability, reactivity, misestimation, and false memories of dietary intake (Archer, Pavela, and Lavie 2015). Second, critics argue that pseudo-quantification (i.e., the transformation of reported foods and beverages into estimates of nutrient and caloric intake) created a fictional discourse on diet-disease relations (Archer, Hand, and Blair 2013; Archer, Lavie, and Hill 2018b). This latter argument is based on the fact that ∼65% of self-reported dietary data have been shown to be physiologically implausible [i.e., respondents cannot survive on the amount of foods and beverages reported (Archer, Hand, and Blair 2013; Archer, Pavela, and Lavie 2015; Ferrari et al. 2002; Goldberg et al. 1991)].

The first criticism is potentially applicable to our analyses. Nevertheless, the dichotomous nature of our classifications (i.e., meat-consumers versus meat-abstainers) reduces both its importance and impact. The second critique regarding pseudo-quantification is not relevant to our review, nor is it relevant to qualitative assessments of dietary intake.

Duration of dietary patterns

A number of studies failed to include information of the age at which meat-abstention began or the duration of non-consumption. If a relationship exists between the length of time an individual has abstained from meat consumption and physical or psychological health, these data are essential to future investigations. This is especially true since the nutritional deficiencies that are sometimes associated with veganism and vegetarianism may be more detrimental in children and adolescents (Cofnas 2019) and may take years to develop (Craig 2010; Dwyer 1991).

Other potential confounders

Clearly, diet is not the only determinant of psychological health (Archer 2018a, 2018b). Nevertheless, a number of the studies failed to include important potential confounders and effect modifiers. These include race, ethnic, or religious affiliation, social norms, as well as lifestyle behaviors that directly affect health (e.g., smoking, and alcohol use) and the physiologic determinants of dietary energy intake (e.g., physical activity, body cellularity; Archer 2018b; Archer, Lavie, and Hill 2018a; Archer et al. 2018e). Given that when compared to the general population, individuals who follow a vegetarian diet tend to be more health-conscious, more physically active, more highly educated, consume less alcohol, be nonsmokers and have higher socio-economic status (Appleby et al. 2016; Appleby and Key 2016; Chang-Claude et al. 2005), it is essential for future studies to include detailed information on participants’ health and behavioral histories and current characteristics.

Strengths and limitations of this review

This systematic review had several strengths, including our a priori decision to select only studies that provided a clear distinction between meat-consumers and meat-abstainers. This decision allowed for a clear and yet rigorous assessment. While myriad studies examined vegetarianism along a continuum, these were excluded simply because the lack of a clear distinction rendered inferences equivocal.

A second strength was our decision to limit our primary outcomes to well-defined mental disorders (i.e., depression, anxiety, and related symptoms) and a limited number of secondary outcomes (e.g., self-harm). This focus allowed for a concise yet rigorous review and ameliorated the effects of poorly operationalized psychological phenomena. For example, by excluding results on disordered eating, dietary restraint, orthorexia, and personality (e.g., neuroticism), we avoided the potential misclassification and concomitant pathologizing of those who simply wish to avoid specific foods or food groups (e.g., vegans).

Our study also had limitations. First, we excluded non-English language studies, that could potentially bias our results in favor of “Western” norms which include meat consumption. For example, our selection criteria excluded papers published in languages other than English. (e.g., Japanese, Hindi) and in non-English databases. Thus, our review may have omitted studies from geographic regions that follow predominantly vegetarian or plant-based dietary patterns. In these areas, the relation between meat-avoidance and psychological health may differ from “Western” nations. Nevertheless, our review included a large sample from China; so, this limitation may be trivial.

Second, while our search was clearly defined and comprehensive, our criteria excluded a large number of papers that provided data on this topic [e.g., see (Anderson et al. 2019; Barthels, Meyer, and Pietrowsky 2018; Burkert et al. 2014a; Cooper, Wise, and Mann 1985; Jacka et al. 2012; Larsson et al. 2002; Li et al. 2019; Northstone, Joinson, and Emmett 2018)]. Nevertheless, we think that a highly focused review has the potential to provide stronger evidence and, as such is more informative to the medical, research, and lay communities. Third, despite the high confidence we place in our finding that meat-abstention is linked to psychological disorders, study designs and lack of rigor precluded valid inferences of temporality and causality.

Fourth, meat consumption is often inconsistently classified in research and national surveillance settings (Gifford et al. 2017; O’Connor et al. 2020) as well as across languages. For example, in English, the broad category of “meat” subsumes both “red” and “white” meat (e.g., beef and poultry). However, in German, the term “meat” excludes poultry. As such, the results for the studies employing German samples were potentially more restrictive. Finally, inferences from our results are only as accurate as the data collected by the included studies. Given that all studies relied on self-reported dietary status, there is the possibility of misclassification because self-reported dietary consumption is not the equivalent of actual dietary consumption (Archer, Hand, and Blair 2013; Archer, Lavie, and Hill 2018b; Archer, Marlow, and Lavie 2018c, 2018d; Archer, Pavela, and Lavie 2015). In other words, there is an obvious and important distinction between merely reporting that one avoids meat and actual meat-abstention; and research supports the fact that self-defined vegetarians and meat-abstainers may consume meat (Haddad and Tanzman 2003).

Suggestions for future direction

Future investigators should avoid the most common flaws detailed herein (e.g., uncorroborated self-reported data, biased sampling, confounding, etc.). First, the limitations of self-reported dietary data may be partially overcome with point-of-purchase (barcode) data (Ng and Popkin 2012). Nevertheless, while purchase data may be more objective, it is not necessarily an accurate proxy for actual consumption. Second, investigators must acknowledge and address the effects of biased sampling with the use of self-reported data. Individuals highly invested in specific dietary patterns may be predisposed to intentional and non-intentional misreporting. Third, detailed behavioral and health-related histories and current lifestyles should be considered essential. The use of physician-diagnosed disorders based on criteria from the DSM-V (APA 2013) is preferable to self-reported symptoms and would assist in producing more definitive results.

Given the results of this review, an interesting future direction would be to examine if meat consumption per se has psychological benefits. For example, there is evidence that a significant number of vegans and vegetarians return to meat consumption over time and that former vegetarians and vegans in the U.S. outnumber current meat-abstainers (Faunalytics 2016). As such, one research question that can be answered empirically is whether it is the nutritional properties of meat (as measured via serum biochemical analyses), the reduced social burden or stigma associated with omnivory, or other physiologic or social factors that drive the transition from meat-abstainer back to meat-consumer.

Bilingualism Affords No General Cognitive Advantages: A Population Study of Executive Function in 11,000 People

Bilingualism Affords No General Cognitive Advantages: A Population Study of Executive Function in 11,000 People. Emily S. Nichols et al. Psychological Science, April 20, 2020. https://doi.org/10.1177/0956797620903113

Abstract: Whether acquiring a second language affords any general advantages to executive function has been a matter of fierce scientific debate for decades. If being bilingual does have benefits over and above the broader social, employment, and lifestyle gains that are available to speakers of a second language, then it should manifest as a cognitive advantage in the general population of bilinguals. We assessed 11,041 participants on a broad battery of 12 executive tasks whose functional and neural properties have been well described. Bilinguals showed an advantage over monolinguals on only one test (whereas monolinguals performed better on four tests), and these effects all disappeared when the groups were matched to remove potentially confounding factors. In any case, the size of the positive bilingual effect in the unmatched groups was so small that it would likely have a negligible impact on the cognitive performance of any individual.

Keywords: bilingualism, executive function, cognition, aging, null-hypothesis testing

Discrimination in Harvard Admissions: Asian Americans would be admitted at a rate 19% higher absent discrimination

Asian American Discrimination in Harvard Admissions. Peter Arcidiacono Josh Kinsler Tyler Ransom. April 20, 2020. http://public.econ.duke.edu/~psarcidi/realpenalty.pdf

Abstract: Detecting racial discrimination using observational data is challenging because of the presence of unobservables that may be correlated with race. Using data made public in the SFFA v. Harvard case, we estimate discrimination in a setting where this concern is mitigated. Namely, we show that there is a substantial penalty against Asian Americans in admissions with limited scope for omitted variables to overturn the result. This is because (i) Asian Americans are substantially stronger than whites on the observables associated with admissions and (ii) the richness of the data yields a model that predicts admissions extremely well. Our preferred model shows that Asian Americans would be admitted at a rate 19% higher absent this penalty. Controlling for one of the primary channels through which Asian American applicants are discriminated against—the personal rating—cuts the Asian American penalty by less than half, still leaving a substantial penalty.

Cognitive sophistication (indexed by analytic thinking, numeracy, basic science knowledge, and bullshit skepticism) was a negative predictor of COVID-19 misperceptions

Pennycook, Gordon, Jonathon McPhetres, Bence Bago, and David G. Rand. 2020. “Predictors of Attitudes and Misperceptions About COVID-19 in Canada, the U.K., and the U.S.A.” PsyArXiv. April 14. doi:10.31234/osf.io/zhjkp

Abstract: The COVID-19 pandemic presents an unprecedented challenge to humanity. Yet there seems to be substantial variation across individuals in knowledge and concern about COVID-19, as well as in the willingness to change behaviors in the face of the pandemic. Here, we investigated the roles of political ideology and cognitive sophistication in explaining these differences across the U.S.A. (N = 689), the U.K. (N = 642), and Canada (N = 644) using preregistered surveys conducted in late March, 2020. We found evidence that political polarization around COVID-19 risk perceptions, behavior change intentions, and misperceptions was greater in the U.S. than in the U.K.. However, Canada and the U.S. did not strongly differ in their level of polarization. Furthermore, in all three countries, cognitive sophistication (indexed by analytic thinking, numeracy, basic science knowledge, and bullshit skepticism) was a negative predictor of COVID-19 misperceptions – and in fact was a stronger predictor of misperceptions than political ideology (despite being unrelated to risk perceptions or behavior change intentions). Finally, we found no evidence that cognitive sophistication was associated with increased polarization for any of our COVID-19 measures. Thus, although there is some evidence for political polarization of COVID-19 in the U.S. and Canada (but not the U.K.), accurate beliefs about COVID-19 (albeit not intentions to act) are broadly associated with the quality of one’s reasoning skill regardless of political ideology or background polarization.

We tend to overstate the improvement in well-being over time and to understate past happiness; it thus seems that feeling happy today implies feeling better than yesterday

Alberto Prati, Claudia Senik. Feeling good or feeling better?. 2020. ffhalshs-02545228f

Abstract: Can people remember correctly their past well-being? We study three national surveys of the British, German and French population, where more than 50,000 European citizens were asked questions about their current and past life satisfaction. We uncover systematic biases in recalled subjective well-being: on average, people tend to overstate the improvement in their well-being over time and to understate their past happiness. But this aggregate figure hides a deep asymmetry: while happy people recall the evolution of their life to be better than it was, unhappy ones tend to exaggerate its worsening. It thus seems that feeling happy today implies feeling better than yesterday. These results offer an explanation of why happy people are more optimistic, perceive risks to be lower and are more open to new experiences.

Keywords: life satisfaction; remembered utility, memory biases; intra-personal comparisons.
JEL: I31, D91

Gullibility impressions are based on cues linked to low levels of perceived threat, like babyfacedness & smiles, making judgments based on perceived harmlessness (i.e., positive intentions & low capabilities)

Jaeger, Bastian, and Erdem O. Meral. 2020. “Who Can Be Fooled? Modeling Perceptions of Gullibility from Facial Appearance.” PsyArXiv. April 14. doi:10.31234/osf.io/vw79y

Abstract: In many situations, ranging from cooperative exchange to fraud, people are faced with the challenge to judge how trusting or naïve (i.e., gullible) others are. In three studies, using both theory-driven and data-driven methods, we examine how people form gullibility judgments based on a person’s facial appearance. People have a shared representation of what a gullible person looks like. Gullibility impressions are positively related to trustworthiness impressions, but negatively related to dominance impressions (Study 1, n = 254). Examining the influence of a wide range of facial characteristics, we find that gullibility impressions are based on cues that have been linked to low levels of perceived threat, such as babyfacedness (Study 2, n = 403) and smiles (Study 3, n = 209). Together, these findings show that people form gullibility judgments based on facial cues that are seen as indicators of relative harmlessness (i.e., positive intentions and low capabilities).

The presence of sex differences in social cognition is controversial; these authors found no significant sex differences in most of social cognition tasks; conversely, women reported higher scores on empathy

Social cognition and sex: Are men and women really different? Marialaura Di Tella, Francesca Miti, Rita B. Ardito, Mauro Adenzato. Personality and Individual Differences, Volume 162, 1 August 2020, 110045. https://doi.org/10.1016/j.paid.2020.110045

Highlights
• The presence of sex differences in social cognition is controversial.
• Several social cognition measures were administrated to 210 healthy participants.
• Participants were equally divided between men and women.
• No significant sex differences were found in most of social cognition tasks.
• Conversely, women reported higher scores on empathy than men.

Abstract: Social cognition includes the ability to represent other people's intentions and beliefs, and the ability to share and recognise the emotions of others. Here, the main aim was to assess the possible presence of sex differences across four aspects of social cognition: (1) recognition of dynamic facial expressions; (2) representation of other people's mental states (both affective and cognitive Theory of Mind, ToM); (3) empathy; (4) identification and regulation of one's own emotions. Measures assessing social cognition were administrated to two hundred ten participants equally divided between men and women. Results showed no significant sex differences in affective and cognitive ToM, in the recognition of emotional facial expressions (with the exception of anger: women were more accurate than men), and in the ability to identify and regulate one's own emotions. A different result was found for empathy, with women reporting higher scores than men. No significant differences between women during follicular vs. luteal phase of menstrual cycle for all the social cognition measures were found. These results are discussed in light of the existing literature. To our knowledge, this study represents one of the few attempts to analyse in a single work sex differences across multiple areas of social cognition.

Keywords: Emotion recognitionEmotional functioningEmpathySex differencesSocial cognitionTheory of Mind

Disgust-sensitive people find little similarity between themselves and strangers; pathogen disgust predicts similarity perceptions beyond sexual and moral disgust

We're not of the same feather: Disgust sensitivity and reduced perceived similarity to unknown others. Sari Mentser, Ravit Nussinson. Personality and Individual Differences, Volume 163, 1 September 2020, 110039. https://doi.org/10.1016/j.paid.2020.110039

Highlights
• Disgust-sensitive people find little similarity between themselves and strangers
• Pathogen disgust predicts similarity perceptions beyond sexual and moral disgust
• The threat of pathogens is linked with increased perceived social distance

Abstract: Perceptions of interpersonal similarity are accompanied by attraction and bonding, often leading to physical contact. Given that physical proximity to social beings increases the odds of catching infectious diseases, we propose a reverse relationship, whereby sensitivity to the presence of pathogens results in perceiving unfamiliar others as less similar to oneself. Four studies involving 980 participants and operationalizing others in three different ways confirm that individual differences in propensity to feel disgust (i.e., react emotionally to potential sources of pathogens in the environment) are associated with perceptions of interpersonal similarity to strangers. Study 1 showed that individuals who score higher in disgust sensitivity perceive themselves as less psychologically similar to visually displayed social targets. Study 2, using vague descriptions of hypothetical figures, found that high-disgust-sensitivity participants tend to assume that others' personal preferences contrast with their own. Study 3 demonstrated that the disgust–dissimilarity association holds for prototypical members of social groups. Finally, Study 4 confirmed that this link reflects pathogen-related (above and beyond sexual or moral) disgust. In all studies, controlling for participants' gender, religiosity, and illness recency did not change the results. We discuss our findings and propose novel directions for future research.

Keywords: Disgust sensitivityPathogen threatBehavioral immune systemInterpersonal similarity

Losing stinks! The effect of competition outcome on body odour quality

Losing stinks! The effect of competition outcome on body odour quality. Jitka Fialová, Vít Třebický, Radim Kuba, David Stella, Jakub Binter and Jan Havlíček. Philosophical Transactions of the Royal Society B: Biological Sciences, Apr 20 2020. https://doi.org/10.1098/rstb.2019.0267

Abstract: Dominance hierarchy is often established via repeated agonistic encounters where consistent winners are considered dominant. Human body odour contains cues to psychological dominance and competition, but it is not known whether competition outcome (a marker of a change in dominance hierarchy) affects the hedonic quality of human axillary odour. Therefore, we investigated the effect of winning and losing on odour quality. We collected odour samples from Mixed Martial Arts fighters approximately 1 h before and immediately after a match. Raters then assessed samples for pleasantness, attractiveness, masculinity and intensity. We also obtained data on donors' affective state and cortisol and testosterone levels, since these are known to be associated with competition and body odour quality. Perceived body odour pleasantness, attractiveness and intensity significantly decreased while masculinity increased after a match irrespective of the outcome. Nonetheless, losing a match affected the pleasantness of body odour more profoundly, though bordering formal level of significance. Moreover, a path analysis revealed that match loss led to a decrease in odour attractiveness, which was mediated by participants’ negative affective states. Our study suggests that physical competition and to some extent also its outcome affect the perceived quality of human body odour in specific real-life settings, thus providing cues to dominance-related characteristics.

5. Discussion

In this study, we tested the effect of winning and losing a physical confrontation on human body odour quality and investigated factors that may mediate observed changes. We expected that winning would have a positive effect on body odour quality and losing a negative effect on it. We also predicted that winning would be associated with positive affective states, a rise in T levels and decrease in C levels, while losing would be characterized by the opposite pattern. Moreover, we hypothesized that these factors would mediate a change in body odour quality.

We found that after a match, perceived odour pleasantness, attractiveness and intensity significantly decreased, while masculinity increased in both winners and losers. Nevertheless, the decrease in pleasantness was more pronounced in losers, which suggests that losing has a more negative effect on body odour quality. We found no statistically significant differences in body odour ratings between winners and losers either before or after a match. Path analysis showed that a loss led to a decrease in odour attractiveness and this effect was mediated by participants' negative emotions.

Winning a match may entail a rise in the dominance hierarchy, while losing lowers the rank: in our case, the match score was added to a personal profile, which affects the league rank. Both losing and winning have consequences for both competitors. Nonetheless, the relatively more profound odour changes in losers suggest that losing may be perceived as having more serious consequences because it may be linked not only to lower league ranking but also to more or less serious injuries. In ring-tailed lemurs (Lemur catta), injury affects the chemical composition of secretion and injury status can be discerned by other individuals [86,87]. Similarly, it could be expected that fighters who lost suffered more injuries during a match and that might be reflected in their body odour. In our study, however, that does not seem to be the case since self-assessed pain did not significantly differ between winners and losers and it was not associated with any of the rated odour characteristics.

Our results thus indicate that competitive situations per se can have an effect on body odour, because after a match, while masculinity increased, both pleasantness and attractiveness decreased. The influence of competition on body odour has been demonstrated already by Adolph et al. [37], although they found that the tested odour samples affected exposed individuals on a subliminal level without any perceived differences between competition-linked and control odours. In our study, the perceived odour quality changed, which could be linked to increased sweating owing to the physical effort during the match. Thermoregulation involves mainly eccrine glands [88], which during exertion produce more diluted, watery, and less oily secretion consisting mostly of water and electrolytes derived from blood plasma and sodium chloride [89]. The change in odour quality we observed could thus be owing to the different quality of sweat. This finding should raise caution with respect to using sports odours as control stimuli (e.g. [90,91]), because they may have somewhat different properties than for example odours collected during sleep (e.g. [83]).

(a) Link between dominance and body odour

The results of some animal studies suggest dominant individuals may have specific intrinsic odour qualities [27] that are probably androgen-dependent [43]. In our study, however, ratings of pre-match body odour were similar for winners and losers, suggesting no prior differences in fighters’ body odour quality. Still, we obtained data on the result of only one match and have no information about the fighters' current rank position. We cannot, therefore, draw stronger conclusions about possible links between various qualities of body odour and rank in the dominance hierarchy. In other words, we do not know whether there exists any connection between dominance rank and specific odour qualities, although previous studies did find some olfactory cues to psychological dominance [31,33].

(b) Sex-related differences in rating

The absence of significant sex-linked differences in raters’ assessments in most rated characteristics suggests that these odour cues have no specific role in intra- or intersexual selection. The sole exception was masculinity ratings. Overall, losers' post-match body odour was rated as more masculine than their odour before a match. This effect was driven by male raters who evaluated losers’ body odour as more masculine after a match. Females did not perceive any difference between pre- and post-match quality. Comparison between the calculated POS values and the actual number of male raters, however, indicates a lower stability in masculinity rating by men, which is why this result should be interpreted rather cautiously. Future studies should further investigate these findings to assess their robustness.

(c) The mediating effect of affective states and hormone levels on body odour quality

If we view winning and losing a fight as a way of change in an individual's rank in dominance hierarchy, then a change of position can be reflected in body odour and others can use it to assess the result of competition. This effect could be owing to changes in hormone levels (T and/or C) associated with victory or defeat. This effect has been repeatedly shown in previous studies (e.g. [60,62]). T stimulates the proliferation of sebocytes and affects the function of apocrine sweat glands [92]. Similarly, C has an either direct or indirect link (via adrenaline) with the activity of apocrine glands [93], which can, in turn, affect the quality of body odour. Two studies that investigated the association between T and C levels and body odour quality produced rather contradictory results. Rantala et al. [72] found no significant correlation between T levels and either intensity or attractiveness of men's body odour. Interestingly, however, they found that C concentrations were positively associated with odour attractiveness, though not its intensity. Using a larger sample and a slightly different methodology (a longer sampling period and elimination of samples from men who reported violations of instructions on the use of fragranced cosmetics), Thornhill et al. [94] observed that women in the fertile phase of their menstrual cycle preferred body odours of men with higher T levels than women in the non-fertile phase. This study did not, however, find any significant relation between odour preferences and C levels.

Our study showed no significant effect of competition outcome on T levels. C levels, however, were higher after a match, which suggests that a match is a stressful event for both winners and losers. The positive relationship between C levels and perceived odour attractiveness, shown by mediation analysis, is in line with a previous study by Rantala et al. [72]. Similarly, we found that C levels did not significantly correlate with intensity ratings (a marker of the amount of perspiration), which indicates a qualitative, not quantitative effect. Glucocorticoids possess certain immunosuppressive qualities [95,96] and it has been hypothesized that only immunocompetent individuals can maintain high C levels [97], which is why such preference would be adaptive.

Another set of variables that have previously been linked to both competition outcomes and body odour are affective states. Body odour can contain cues to affective states such as happiness [98], disgust [99], fear [100], anxiety [90], stress [91], but also aggression [38] and contexts such as competition [37]. This is why we investigated possible mediating effects of changes in hormone levels and affective states. A path analysis showed not only a direct effect of competition outcome on perceived odour attractiveness, where a loss led to decrease in odour attractiveness, but also a mediating indirect effect of negative affective state, where the more negative emotions participants experienced, the less attractive was their perceived body odour. Exploratory correlation analysis showed further relationships between some odour characteristics, affective states and hormone levels, which are in line with the evidence summarized above. These findings, however, should be interpreted with caution owing to our relatively small donor sample.

(d) Study limitations

It is a common procedure in body odour studies to ask odour donors to avoid activities that could affect body odour quality, such as eating spicy food, using soaps, deodorants, demanding physical activities, smoking etc. the day before and on the day of sampling (e.g. [83,31]). Owing to logistical reasons, we were able to contact study participants only a few hours before the match. They did not, therefore, receive any such list of restrictions and followed their habitual regime without any standardization as to diet, smoking, shaving, the use of fragranced cosmetics or dietary supplements. This may have introduced some noise to the data and obscured some effects. Nevertheless, we were still able to detect some significant changes and the use of a within-subject experimental design should diminish inter-individual differences. Such an approach has a higher ecological validity, although future studies should investigate the issue under more standardized settings as well.

We explored the possible effect of injuries on body odour quality using a single question regarding participants' self-perceived pain as a proxy for suffered injuries. Admittedly, a record of actual injuries would be a more objective measure, because participants’ answers may be biased by competitiveness and excitement induced by the competition. These data were not, however, available and we did not want to overload participants with additional tasks and screenings.

In the present study, we used hedonic ratings to assess changes in body odour quality induced by winning or losing a match. Apart from explicit subjective ratings, future studies could employ some objective psychophysiological measures such as skin conductance, heart rate, possible activation of different brain areas by fMRI or variance in time activation by EEG.

Finally, future studies should collect data from the same individual both after a victory and a loss in a competition to assess the effect of outcome on body odour directly. In our field study conditions, such balanced design could not be implemented. It therefore remains as a challenge for future research.

Major histocompatibility complex-associated odour preferences and human mate choice: near and far horizons

Major histocompatibility complex-associated odour preferences and human mate choice: near and far horizons. Jan Havlíček, Jamie Winternitz and S. Craig Roberts. Philosophical Transactions of the Royal Society B: Biological Sciences, April 20 2020. https://doi.org/10.1098/rstb.2019.0260

Abstract: The major histocompatibility complex (MHC) is a core part of the adaptive immune system. As in other vertebrate taxa, it may also affect human chemical communication via odour-based mate preferences, with greater attraction towards MHC-dissimilar partners. However, despite some well-known findings, the available evidence is equivocal and made complicated by varied approaches to quantifying human mate choice. To address this, we here conduct comprehensive meta-analyses focusing on studies assessing: (i) genomic mate selection, (ii) relationship satisfaction, (iii) odour preference, and (iv) all studies combined. Analysis of genomic studies reveals no association between MHC-dissimilarity and mate choice in actual couples; however, MHC effects appear to be independent of the genomic background. The effect of MHC-dissimilarity on relationship satisfaction was not significant, and we found evidence for publication bias in studies on this area. There was also no significant association between MHC-dissimilarity and odour preferences. Finally, combining effect sizes from all genomic, relationship satisfaction, odour preference and previous mate choice studies into an overall estimate showed no overall significant effect of MHC-similarity on human mate selection. Based on these findings, we make a set of recommendations for future studies, focusing both on aspects that should be implemented immediately and those that lurk on the far horizon. We need larger samples with greater geographical and cultural diversity that control for genome-wide similarity. We also need more focus on mechanisms of MHC-associated odour preferences and on MHC-associated pregnancy loss.

4. Discussion

(a) Meta-analyses

A recent meta-analysis on MHC-associated mate choice concluded that there is a consistent preference for MHC-heterozygous individuals [20]. By contrast, there was no systematic preference for MHC-dissimilarity. Here, we provide the results of further meta-analyses primarily focusing on genomic studies and relationship satisfaction, together with updated meta-analyses on odour preferences and human mate selection studies. Overall, the genomic studies show no significant association between MHC-similarity and mate choice in actual couples nor in mate preferences. However, we also found that the effect of MHC-similarity is independent of the genomic background. The overall effect of MHC-similarity on sexual satisfaction was not significant, but we found a negative association between MHC-similarity and sexual satisfaction in non-HC using women. Nevertheless, several lines of evidence for publication bias in studies investigating MHC-similarity and sexual satisfaction suggest that these results should be interpreted with caution. Furthermore, we found no significant effect of MHC-similarity on odour preferences among currently available studies. Finally, combining each of the effect sizes analysed above with previously extracted effect sizes for mate choice among couples into an overall estimate, showed no overall significant effect of MHC-similarity on human mate selection.

(b) Near horizons: issues arising from the meta-analyses

Our meta-analyses raise a number of pressing outstanding issues that should, and can be, addressed in future studies. Perhaps the strongest conclusion one can draw from the available data is that our knowledge is patchy across different populations. Even a brief inspection of figure 1 shows that most studies are based on populations of European ancestry; there is a notable absence or near-absence of data from two of the largest populations, China and India, from smaller populations in Oceania and Sub-Saharan Africa, and from small-scale societies. Why is this important? First, individual populations vary considerably in cultural norms regarding the level of consanguinity [52]. In addition, while all populations show some amount of admixture, this tends to be higher in large-scale populations such as those from Western European or Eastern Asian complex societies [53]. Owing to high MHC polymorphism, mating with almost any unrelated individual would probably lead to a sufficient level of dissimilarity. It is thus possible that humans, as in other species [54–56], tend to avoid individuals with high MHC-similarity, but show no systematic preference beyond a certain threshold (see [28] for a similar suggestion). However, large-scale populations are a relative novelty in human evolutionary history [57]; it is therefore of key importance to focus on small-scale societies with comparatively higher levels of inbreeding, which better reflect likely population structure during most of human evolution. To our knowledge, the only available study from small-scale societies comes from South Amerindian couples [58], which showed they were not significantly MHC dissimilar compared to random pairing. In that study, however, MHC typing was of relatively low sensitivity (serotyping of HLA-A and -B loci to the level of two-digit allele groups, no class II loci were recorded), sample size was too small to detect selection below a selection coefficient s = 0.45, and there is cultural promotion of cross-cousin marriages in some tribes [58].

Most previous studies have specifically targeted the MHC region, assuming that their findings are a consequence of selective processes in that region. While this is a reasonable assumption in view of MHC polymorphism and allele-specific associations with some diseases [8–11], apparent MHC-similar mate selection might be an epiphenomenon of more general population stratification (e.g. positive assortment [59]). In support of this, a recent meta-analysis found that MHC-similarity in couples was observed in ethnically heterogeneous, but not homogeneous, populations [20]. However, our new analysis of studies that control for genomic similarity shows that MHC-dissimilarity among couples is independent of genome-wide similarity (although the association is positive). In addition, the positive relationship detected between MHC effects (spouses versus permuted pairs) and the extremeness of the MHC within spouses indicates that observed MHC effects are relatively independent of socio-demographic processes that would affect spouses genome-wide. For example, if spouses were highly dissimilar at the MHC compared to randomly assigned mates, but had levels of MHC-similarity in line with the rest of the genome, we may conclude that the MHC does not play an independent role in mate choice and mate choice may be for inbreeding avoidance. But this was not what we observed.

The overall effect of MHC-associated mate selection was not significant but was restricted to some populations. In other words, we may observe MHC-associated preferences in some populations but not in others. For example, we found that Israeli individuals showed a significant preference for mates with higher levels of MHC-similarity. Dandine-Roulland et al. [29] contributed one of the three effect sizes to this result, and using principal component analysis detected genetic stratification, with clusters of samples lying between European and Middle Eastern populations. The two other effect sizes contributed by Israeli et al. [51] came from unmarried couples to determine paternity status and from married couples undergoing infertility treatment. The study did not specifically detail testing for population stratification, and it is likely that a random sample of the population would capture multiple ethnic groups, as Dandine-Roulland et al. [29] demonstrated. Thus, MHC-similarity preferences most likely reflect social homogamy in a genetically heterogeneous population. The Swiss individuals' significant preference for MHC-dissimilar odours was observed in the dataset without HC-using individual effects and included both female and male odour preferences. These MHC-dissimilar preferences might be related to relatively low levels of genetic variation and were specifically present in German-speaking cantons, perhaps as a consequence of geographical isolation in Alpine valleys [60]. By contrast, studies based on other European populations (such as in neighbouring Germany) did not report MHC-dissimilar preferences, emphasizing the need for investigations across diverse populations which differ in levels of genetic variation. For example, cultural practices vary related to body care. If body odour is a primary source of information about one's MHC profile, then practices such as armpit hair shaving and use of extrinsic fragrances or deodorants may impact perceptibility of MHC-associated odours. Although there is conjecture that fragrance selection may be linked to a wearer's own MHC [61,62], perhaps as a mechanism to complement body odour rather than cover it [63], we do not yet know how such cultural effects influence odour perceptibility and MHC-associated preference. Further, in cultures which idealize an ‘odourless human body’, it is considered inappropriate to overtly smell other people; under such circumstances, the effect of MHC-associated preferences might go unrealized. Clearly, our understanding of the interplay between cultural and biological evolution is far from complete, and MHC-associated mate choice is no exception.

Many cultures also practice various types of positive assortment such as ethnic, socio-economic, religious, and caste-based endogamy. Even within a single culture, mate choice is a multidimensional process based on a set of preferences for various traits which might not be linked to MHC, such as physical appearance, socio-economic status, personality, attitudes, age and many others [64,65]. Each of these may be prioritized over genotypic factors [66], including MHC. Furthermore, if positive assortment occurs for any trait with a genetic component, even subtle assortment on such traits might interfere with MHC-associated preferences.

Beyond actual mate choice, it remains possible that MHC-associated preferences exert effects on the quality of resulting relationships. Indeed, in a study of 48 couples, Garver-Apgar et al. [22] found that more MHC-similar couples report relatively lower sexual satisfaction. Subsequent investigations have recorded considerably larger sample sets [67,68]. Here, we quantitatively assessed these studies for a possible link between MHC-similarity and sexual satisfaction. The overall effect was not significant. However, in the subset of women not using HC, there was a negative association between MHC-similarity and sexual satisfaction: couples sharing fewer HLA alleles experienced greater sexual satisfaction. This pattern of results is consistent with the studies by Wedekind et al. [18] who found odour preferences for MHC-dissimilarity only in women not using HC, and by Roberts et al. [69,70] who report higher sexual satisfaction in women who did not use HC when they met their current partner. Nevertheless, the robustness of the HC-associated preferences was neither confirmed by a previous meta-analysis [20] nor in our updated analysis. There is another reason why the link between MHC-similarity and sexual satisfaction should be interpreted with extreme caution. The meta-analysis on relationship satisfaction found three different types of evidence for publication bias. First, there was a significant asymmetry in a funnel plot suggesting missing studies with a negative outcome, particularly those with small effect sizes. Second, there was a temporal effect suggesting the unequal distribution of the effect sizes over time; specifically, the initial study [22] found a considerably stronger effect than subsequent studies. Finally, studies with larger samples (i.e. having a higher power to detect possible effects) show significantly smaller effect sizes.

(c) Far horizons on major histocompatibility complex-associated mate choice

Beyond those issues raised above, we believe there are two further matters that require significant attention in the longer-term. The first of these concerns the generation of MHC-associated odours. Understanding this may be of fundamental interest in itself, but a clearer picture of the underlying mechanisms may also clarify how some cultural and contextual factors (e.g. fragrance use) affect odour variability. Several hypotheses have been proposed relating to interactions between MHC molecules and skin microflora, which produces volatile compounds that can subsequently be perceived. However, most evidence supports an idea that body odour is affected by antigen peptides bound by specific MHC molecules. It was first shown in mice that these peptides can be perceived by the vomeronasal organ [71]; however, subsequent research shows that the main olfactory system can perceive MHC peptide ligands via the olfactory epithelium [72]. MHC peptide ligands can be detected in mouse urine, although at very low concentration [73]. Evidence extends beyond mice, as sticklebacks prefer water enriched with MHC-dissimilar peptides [74]. So far, only one study addressed this mechanism in humans [75]. Two commercially available peptides were added to body odour samples, and neurophysiological responses were recorded using functional magnetic resonance imaging while participants attempted to recognize their own odour. The results showed a higher preference for odour samples enriched with peptides corresponding to the MHC of the smeller and activity in brain areas related to self-recognition. However, it is not clear whether the self-recognition paradigm can be simply generalized to mate preferences. More importantly, the study was criticized for not providing an explanation for the transduction mechanism, as peptide molecules are involatile and considerably larger than molecules usually perceived by smell [73,76,77]. Furthermore, it is also not clear whether the MHC-associated peptides are commonly present in human axillae or more generally on human skin.

A second area which requires more attention is the nature of potential selective benefits arising from MHC-associated mate choice. While it is usually assumed that MHC-preferences are a consequence of infection-driven selection, it might be alternatively (or additionally) driven by the probability of successful pregnancy. A foetus expresses paternal alloantigens which must be tolerated by the maternal immune system. It has been proposed that MHC allele-sharing between father and mother may lead to insufficient stimulation of the maternal immune system by paternal antigens—a factor that was expected to be important for maternal tolerance and inflammatory immune response—and thus decrease the chance of successful implantation [78]. Several studies suggest that MHC allele-sharing is associated with recurrent pregnancy loss (RPL) [79,80], with a recent meta-analysis indicating that HLA-B and -DR are especially important [81]. However, these results should be viewed with caution because many studies used serological genotyping resolving only to allele groups, which may miss related alleles that are functionally different [82]. More critically, classical MHC class I and II proteins (except for HLA-C) are not expressed on the trophoblast, a part of conceptus which subsequently develops into the embryonic part of the placenta and is in direct contact with the maternal immune system. Researchers have, therefore, recently focused on classical HLA-C and non-classical MHC class Ib, which are expressed on the trophoblast. In contrast to previous studies, it was reported that a mismatch, i.e. not sharing, at HLA-C*07 between mother and father was related to a higher risk of RPL [83]. These authors also observed a higher incidence of HLA-C antibodies in RPL patients than in the controls. There is a growing body of evidence showing that HLA-E, -G, and to some extent also HLA-F, all play a key role in immunotolerance of the foetus by the maternal immune system in general and uterine NK cells in particular (for a review, see [3]). Some studies report higher RPL in women with the HLA-E*101 allele [84], although others find no difference in HLA-E polymorphism between controls and couples with RPL [85,86]. Most studies on non-classical MHC Ib polymorphism and its role in pregnancy disorders focused on HLA-G polymorphism. For example, it was reported that a 14 bp insertion HLA-G allele is associated with a smaller placenta and higher probability of RPL [87], although this may be restricted to cases with three and more abortions [88]. In summary, there appears to be some evidence that couples sharing alleles at HLA-B and -DR loci are at higher risk of reproductive failure. Although these genes are not expressed on the trophoblast, this might arise through linkage disequilibrium with other functionally important MHC genes. Moreover, there is inconsistency across studies in both the association between HLA-G and -E polymorphism and reproductive failures, perhaps partly owing to factors such as variation in the diagnosis of the RPL. More importantly, most existing studies on MHC polymorphism and reproductive problems focused solely on RPL, but MHC polymorphism might affect pregnancy success much earlier as HLA-C and -G expression can be detected even before implantation [89,90]. Because a vast majority of unsuccessful early pregnancies are not detected, this may, in turn, bias the results of studies that rely solely on RPL (i.e. recognizable spontaneous miscarriage).

(d) Suggestions for future studies

Above, we have discussed in detail the current state of knowledge on MHC-associated mate choice in light of results from our meta-analysis, that should inform approaches in the immediate future. We also commented on two important wider and relatively unexplored perspectives that lurk on the far horizon of this area of inquiry. In light of these, we here outline some recommendations for future work that we hope will help to ultimately clarify the extent to which MHC influences human mating. The suggestions (i–iii) highlight methodological issues, (iv–vi) focus on population- and culture-related questions, and (vii–x) stress several associated issues such as developmental and mechanistic questions.

(i)	Researchers should always perform a priori power analysis to obtain a sufficient sample size (see also [91]). Power analysis is becoming a standard procedure in other fields of behavioural research, but it is particularly needed here owing to both extreme variability in MHC genes and what appear to be, at best, small effect sizes.
(ii)	To provide more complex insights, future studies should control for genome-wide similarity. The same applies to studies on MHC-heterozygosity. Genomic studies further allow assessment of the overall level of inbreeding in the given population. This is an important issue as MHC-associated mate choice might play a role only in relatively inbred populations.
(iii)	Researchers should test for specificity of the MHC region. As was discussed above, without controlling for genome-wide level of similarity/heterozygosity, we cannot decide whether the observed effects are specific to the MHC region or whether we are dealing with more general phenomena.
(iv)	We urgently need more studies on populations of non-European descent, and particularly those with a relatively high level of inbreeding (e.g. from small-scale societies).
(v)	We need more cross-cultural comparisons assessing how shared cultural practices affect MHC-associated preferences. These include marriage practices such as various forms of endogamy.
(vi)	In any study, researchers should obtain and clearly document detailed information about interindividual differences in cultural practices of the studied population, as some practices may interfere with MHC-associated effects. These include HC use and personal hygiene practices such as fragrance use (see also [92] for a similar proposal).
(vii)	We need to distinguish between a threshold-based avoidance of very similar individuals and a fluid preference for the most dissimilar individuals.
(viii)	Currently, there is not, to our knowledge, a single study focusing on the development of MHC-associated preferences. Therefore, we do not know when in ontogeny preferences might form and how family structure affects the development of these preferences. Rodent studies show that cross-fostering tends to reverse MHC-associated preferences [93], thus similar phenomena might be expected in humans. For instance, studies with adoptive families might be particularly informative.
(ix)	We need studies testing possible mechanisms of MHC-associated preferences. These include bioassay studies testing the presence and abundance of the MHC peptide ligands. Similarly, studies testing effect of the MHC peptide ligands in the context of mate choice are of primary importance.
(x)	Finally, we should link research on MHC-associated mate choice and research on MHC-associated pregnancy loss. The two areas have to date been studied separately; however, they may jointly provide key insights into this complex area of human reproduction. Such research may also examine links between pregnancy loss and infertility with the prevalence of cultural practices (e.g. fragrance use) that may have disrupted MHC-associated mate preferences at the beginning of the relationship.