When forming personality impressions from faces, people rely on features that resemble "frozen" emotional expressions

Which Facial Features Are Central in Impression Formation? Bastian Jaeger, Alex L. Jones. Social Psychological and Personality Science, August 17, 2021. https://doi.org/10.1177/19485506211034979

Abstract: Which facial characteristics do people rely on when forming personality impressions? Previous research has uncovered an array of facial features that influence people’s impressions. Even though some (classes of) features, such as resemblances to emotional expressions or facial width-to-height ratio (fWHR), play a central role in theories of social perception, their relative importance in impression formation remains unclear. Here, we model faces along a wide range of theoretically important dimensions and use machine learning techniques to test how well 28 features predict impressions of trustworthiness and dominance in a diverse set of 597 faces. In line with overgeneralization theory, emotion resemblances were most predictive of both traits. Other features that have received a lot of attention in the literature, such as fWHR, were relatively uninformative. Our results highlight the importance of modeling faces along a wide range of dimensions to elucidate their relative importance in impression formation.

Keywords: social perception, personality impressions, overgeneralization theory, emotional expressions, facial width-to-height ratio

General Discussion

Which facial characteristics do people rely on when forming impressions of others? Some facial features, such as resemblances to emotional expressions and fWHR, occupy a central role in theories of social perception (Todorov et al., 2008; Zebrowitz, 2017). However, it is not clear whether this focus is justified, as little is known about the relative importance of different characteristics. Faces can be modeled along many dimensions, and many facial features are correlated. Yet, prior work has mostly examined one feature or a few features in isolation. These approaches cannot provide strong evidence for the claim that people rely on certain facial features in impression formation, as it remains unclear whether people relied on the facial feature in question, or on other correlated ones. In short, even though studies have identified a long list of facial features that are correlated with impressions, the question of which facial features are actually central in impression formation remains largely unaddressed. Here, we used methods from machine learning (i.e., cross-validation, regularization) to estimate and compare the extent to which a wide range of facial features predict trustworthiness and dominance impressions for a large and demographically diverse set of faces. We tested facial characteristics that have been theorized to be important in impression formation (resemblances to emotional expressions, attractiveness, babyfacedness, familiarity, and fWHR; Geniole et al., 2014; Stirrat & Perrett, 2010; Zebrowitz, 2017). We also tested a large set of other facial characteristics that have received less attention or are often held constant in social perception studies, even though they might be important in impression formation (e.g., gender, race, age, eye size, lip fullness).

When comparing different classes of facial features, we found that emotion resemblances were most predictive of both trustworthiness and dominance impressions, outperforming all other theory-driven models. When examining the importance of all 28 facial characteristics simultaneously, we found that perceptions of trustworthiness were best predicted by a face’s resemblance to a happy expression. Emotionally neutral faces were perceived as more trustworthy when facial features resembled a facial expression of happiness. Perceptions of dominance were best predicted by targets’ gender (with women being perceived as less dominant than men) and by resemblance to a facial expression of anger. Together, our results support the notion that resemblances to emotional expressions are central for explaining how people form personality impressions from facial features. Our findings are in line with overgeneralization theory (and the emotion overgeneralization hypothesis in particular; Todorov et al., 2008; Zebrowitz, 2017), which posits that personality impressions of faces are driven by an oversensitive emotion detection system: Due to their social relevance, people even perceive emotions (and associated personality traits) in emotionally neutral faces that structurally resemble emotional expressions.

Support for the importance of other facial characteristics evoked by overgeneralization theory (i.e., attractiveness, babyfacedness, and familiarity; Zebrowitz, 2012, 2017) was mixed. Facial attractiveness was the second-most informative predictor of trustworthiness impressions, whereas babyfacedness and familiarity were less informative. None of the three characteristics were among the most informative predictors of dominance impressions.

We also found that demographic factors (i.e., gender, age, and race)—which have received less attention as predictors of personality impressions—were in some instances among the most important predictors of impressions. This highlights potential problems associated with keeping features like gender and race constant when studying social perception. Certain features may guide impression formation when demographic characteristics do not vary, but they may be uninformative when more diagnostic cues such as demographic characteristics do vary.

A wealth of studies has examined the influence of fWHR on personality judgments (e.g., Geniole et al., 2014; Ormiston et al., 2017; Stirrat & Perrett, 2010). Yet, the current results suggest that fWHR is not an informative predictor of trustworthiness or dominance impressions. When comparing the predictive fit of fWHR to the four characteristics that form the basis of overgeneralization theory, fWHR emerged as the weakest predictor. When modeled alongside all other facial features that we included in our analyses, fWHR was again among the least informative predictors. Similar results were obtained in additional analyses when examining impressions of male and female targets separately and when all other variables that included some measurement of face length or width were omitted from analyses (see Supplemental Materials). Together, these findings suggest that the importance of fWHR for impression formation may have been overstated in previous studies. Previously observed associations between fWHR and personality impressions may have been due to the fact that people rely on facial features that are correlated with fWHR, but not on fWHR per se.

Interestingly, all seven classes of predictors showed better predictive accuracy for trustworthiness perceptions than for dominance perceptions. It has been suggested that emotion resemblances are particularly important for trustworthiness impressions, whereas morphological characteristics, such as fWHR, are more important for dominance impressions (Hehman et al., 2015). The current results are not in line with this notion and suggest that emotion resemblances are the most important determinant of both trustworthiness and dominance impressions. It should also be noted that even though emotion resemblances were the most important class of predictors, not all emotion resemblances were equally meaningful. Resemblance to a happy expression was the most important predictor of trustworthiness impressions, whereas resemblance to an angry expression was the most important predictor of dominance impressions.

Limitations and Future Directions

Despite the relatively good performance of some of our models, results also suggest that our list of relevant features was not exhaustive. Emotion resemblances explained 53% and 42% of the variance in trustworthiness and dominance perceptions. Even the optimized Elastic Net models explained around 68% of the variance, indicating there are other important factors contributing to personality impressions. Other facial features that might show independent contributions to personality impressions include skin texture (Jaeger et al., 2018; A. L. Jones et al., 2012) and perceived weight (Holzleitner et al., 2019). Examining the role of additional predictors will show how generalizable the present results are, as the relative importance of facial features ultimately depends on the specific set of features that is modeled. In order to conclusively establish that certain facial features are central in impression formation (and that observed associations are not due to other, unmeasured dimensions), faces need to be modeled along all potentially meaningful dimensions. From a practical perspective, achieving this goal may be unfeasible at best and impossible at worst. Still, future work should strive to test the relative importance of different features by comparing them against large sets of other features that have been shown to predict impressions.

Future studies could also investigate characteristics of the perceiver which explain a nontrivial amount of variance in impressions (Hehman et al., 2019). Moreover, while the current set of faces was relatively large and diverse in terms of gender, age, and race, we only examined U.S. individuals who were photographed in a controlled lab setting. Future studies could test whether the current findings replicate when using more naturalistic images of individuals from different nationalities (Sutherland et al., 2013).

Episodic-like memory (what, where and when specific things happened) is preserved with age in cuttlefish, molluscs that lack a hippocampus, maybe due to reproductive pressure

Episodic-like memory is preserved with age in cuttlefish. Alexandra K. Schnell, Nicola S. Clayton, Roger T. Hanlon and Christelle Jozet-Alves. August 18 2021. https://doi.org/10.1098/rspb.2021.1052

Abstract: Episodic memory, remembering past experiences based on unique what–where–when components, declines during ageing in humans, as does episodic-like memory in non-human mammals. By contrast, semantic memory, remembering learnt knowledge without recalling unique what–where–when features, remains relatively intact with advancing age. The age-related decline in episodic memory likely stems from the deteriorating function of the hippocampus in the brain. Whether episodic memory can deteriorate with age in species that lack a hippocampus is unknown. Cuttlefish are molluscs that lack a hippocampus. We test both semantic-like and episodic-like memory in sub-adults and aged-adults nearing senescence (n = 6 per cohort). In the semantic-like memory task, cuttlefish had to learn that the location of a food resource was dependent on the time of day. Performance, measured as proportion of correct trials, was comparable across age groups. In the episodic-like memory task, cuttlefish had to solve a foraging task by retrieving what–where–when information about a past event with unique spatio-temporal features. In this task, performance was comparable across age groups; however, aged-adults reached the success criterion (8/10 correct choices in consecutive trials) significantly faster than sub-adults. Contrary to other animals, episodic-like memory is preserved in aged cuttlefish, suggesting that memory deterioration is delayed in this species.

Popular version: https://www.sciencedaily.com/releases/2021/08/210817193055.htm

"The old cuttlefish were just as good as the younger ones in the memory task -- in fact, many of the older ones did better in the test phase. We think this ability might help cuttlefish in the wild to remember who they mated with, so they don't go back to the same partner," said Schnell.

Cuttlefish only breed at the end of their life. By remembering who they mated with, where, and how long ago, the researchers think this helps the cuttlefish to spread their genes widely by mating with as many partners as possible.

From 2019... In the U.S. and Côte d’Ivoire, highly educated people make decisions that are less consistent with the rational model while low-income respondents make decisions more consistent with the rational model

From 2019... Are We All Predictably Irrational? An Experimental Analysis. John A. Doces & Amy Wolaver. Political Behavior volume 43, pp1205–1226. Dec 18 2019. https://link.springer.com/article/10.1007/s11109-019-09579-0

Abstract: We examine the question of rationality, replicating two core experiments used to establish that people deviate from the rational actor model. Our analysis extends existing research to a developing country context. Based on our theoretical expectations, we test if respondents make decisions consistent with the rational actor framework. Experimental surveys were administered in Côte d’Ivoire and Ghana, two developing countries in West Africa, focusing on issues of risk aversion and framing. Findings indicate that respondents make decisions more consistent with the rational actor model than has been found in the developed world. Extending our analysis to test if the differences in responses are due to other demographic differences between the African samples and the United States, we replicated these experiments on a nationally representative analysis in the U.S., finding results primarily consistent with the seminal findings of irrationality. In the U.S. and Côte d’Ivoire, highly educated people make decisions that are less consistent with the rational model while low-income respondents make decisions more consistent with the rational model. The degree to which people are irrational thus is contextual, possibly western, and not nearly as universal as has been concluded.

Introduction

Are we all predictably irrational? Since the seminal work of Tversky and Kahneman (1974, 1981, 1986, 1991) and other behavioral economists (Akerlof and Kranton 2000; Ariely 2010; Camerer 2003; Thaler 1980), the usefulness of models assuming rationality has been questioned, if not entirely dismissed in some cases (Green and Shapiro 1994; Sen 1977). However, the vast majority of the empirical work establishing consistently irrational behaviors has been conducted on populations in the West, dubbed WEIRD for Western, educated, industrialized, rich and democratic (Henrich et al. 2010). In their review of the psychology literature, Henrich et al. (2010) indicate that 68% of subjects were from the United States, 96% from the Western Industrialized world, with 80% of the Western sample being undergraduate students. If other populations behave differently than these groups, then the implications of the new models of behavior may not be as widely applicable as we thought.

There have been some forays examining deviations from the predictions based on the rational actor model across different populations, notably studies of the impact that poverty has on decision-making. While some studies establish ways in which poverty decreases cognitive ability through the additional stresses associated with living in poverty (Mani et al. 2013; Haushofer and Fehr 2014), others argue that the influence of poverty on decisions is related more to additional constraints and a constant presence of risk in the lives of the poor (Duflo 2006; Banerjee and Duflo 2007; Carvalho et al. 2016). There is a growing body of literature that establishes that the poor are less subject to some of the cognitive biases found by Tversky and Kahneman (Shah et al. 2015, 2018). Possible explanations for the differences in decision-making by the poor are that poverty may increase attention to costs, and/or it exposes one to more risk, which causes people to give more weight to current versus future outcomes.

To determine whether these predictable irrationalities are applicable in other parts of the world, we replicate some of the most important experiments conducted by Tversky and Kahneman (1981) in Côte d’Ivoire and Ghana, and compare these samples to those from Western populations. We find that respondents from Côte d’Ivoire and Ghana make decisions that are closer to the model of rationality than Westerners. Building on this key finding, we also examine the effects of individual characteristics on decision-making to determine whether there are systematic differences within these populations. Here, we find that most sub-groups, with some exceptions, make decisions that are relatively more consistent with the predictions of the rational model than has been found in prior research. Finally, to ascertain why the differences exist between the original results from 1981 and our data, we re-consider two of the original experiments in a nationally representative sample of American adults. This sample provides support for the 1981 results of systematic irrationality, with important exceptions, and helps to contrast our findings from West Africa that the rational actor model is most applicable in the developing world.

Our empirical results, in sum, suggest there is more merit to the rational choice paradigm than perhaps has been thought, and that existing studies concluding people are predictably irrational are overstated in a number of ways. This is an important finding with implications for several areas of academic scholarship. The rational actor model has served as the cornerstone assumption about the behavior of political actors, influencing research in political science on voter choice, foreign policy making, conflict, and international political economy amongst others (de Mesquita and Smith 2011; Mansfield et al. 2000; Powell 1991; Slantchev and Tarar 2011). Recent work has extended the paradigm to explicitly non-western contexts (Hollyer et al. 2015). Nevertheless, debates about its utility in political science have been especially spirited (de Mesquita and Morrow 1999; Walt 1999), with one enduring criticism being the lack of empirical support that people behave as the model assumes, a point which even supporters acknowledge (Kahler 1998; Snidal 2002). In economics, as well, the core mainstream model assumes rationality, with applications to the law (Posner 2014) and even addiction (Becker et al. 1991). By addressing the empirical underpinnings of rational choice, we help fill an important gap in our understanding of rationality and show that the model might be most relevant for non-western populations.

Uncommon case of complete loss of hunger following an isolated left insular stroke

Uncommon case of complete loss of hunger following an isolated left insular stroke. Benjamin Hébert-Seropian, Olivier Boucher, Didier Jutras-Aswad & Dang Khoa Nguyen. The Neural Basis of Cognition, Aug 16 2021. https://doi.org/10.1080/13554794.2021.1966044

Abstract: The insula has long been among the least understood regions of the human brain, in part due to its restricted accessibility. Mounting evidence suggests that the insula is a prominent player in gustatory, interoceptive, and emotional processing, and likely integrates these different functions to contribute to the homeostatic control of food intake. Here we report the case of a young adult patient who lost the subjective experience of hunger following an ischemic stroke localized in the posterior left insula. The loss of hunger was not attributable to medication, substance use, or a clinical disorder, and lasted for a period of 15 months. In line with the role attributed to the insula in gustation and interoception, we suggest that the insula integrates information about taste, interoception, and the hedonic value of food in the service of homeostatic regulation.

KEYWORDS: Hungerappetiteinsulastrokecase report

Found some evidence that higher income is associated with less happiness and no substantive benefit to higher household income in the US after $35-40K and in Germany after €14-18K (in daily life, not as an assessment of the whole)

Kudrna, Laura, and Kostadin Kushlev. 2021. “Money Does Not Always Buy Happiness, but Are Richer People Less Happy in Their Daily Lives? It Depends on How You Analyze Income.” PsyArXiv. August 18. doi:10.31234/osf.io/4jvh5

Abstract: Do people who have more money feel happier during their daily activities? Some prior research has found no relationship between income and daily happiness when treating income as a continuous variable in OLS regressions, although results differ between studies. We re-analyzed existing data, treating household income as a categorical variable and using lowess and spline regressions to explore non-linearities. Our analyses reveal that these methodological decisions provide new insights into the relationship between income and happiness. We find some evidence that higher income is associated with less happiness and no substantive benefit to higher household income in the US after $35-40K and in Germany after €14-18K. Not all analytic approaches generate the same conclusions, which may explain discrepant results.

From 2019... Greater male cognitive variability has implications for both tails of the distribution: Danish data (n = 1.3 million) finds that twice as many boys than girls are diagnosed with intellectual disability

From 2019... Incidence Rates and Cumulative Incidences of the Full Spectrum of Diagnosed Mental Disorders in Childhood and Adolescence. Søren Dalsgaard et al. JAMA Psychiatry. 2020;77(2):155-164. Nov 20, 2019, doi:10.1001/jamapsychia

Key Points

Question  What are the age- and sex-specific incidence rates and cumulative incidences of the full spectrum of diagnosed mental disorders during childhood and adolescence?

Findings  In this nationwide cohort study of 1.3 million individuals in Denmark, the risk (cumulative incidence) of being diagnosed with a mental disorder before 18 years of age was 14.63% in girls and 15.51% in boys. Distinct age- and sex-specific patterns of occurrence were found across mental disorders in children and adolescents.

Meaning  These findings suggest that precise estimates of rates and risks of all mental disorders during childhood and adolescence are essential for future planning of services and care and for etiological research.

Abstract

Importance: Knowledge about the epidemiology of mental disorders in children and adolescents is essential for research and planning of health services. Surveys can provide prevalence rates, whereas population-based registers are instrumental to obtain precise estimates of incidence rates and risks.

Objective  To estimate age- and sex-specific incidence rates and risks of being diagnosed with any mental disorder during childhood and adolescence.

Design  This cohort study included all individuals born in Denmark from January 1, 1995, through December 31, 2016 (1.3 million), and followed up from birth until December 31, 2016, or the date of death, emigration, disappearance, or diagnosis of 1 of the mental disorders examined (14.4 million person-years of follow-up). Data were analyzed from September 14, 2018, through June 11, 2019.

Exposures: Age and sex.

Main Outcomes and Measures  Incidence rates and cumulative incidences of all mental disorders according to the ICD-10 Classification of Mental and Behavioral Disorders: Diagnostic Criteria for Research, diagnosed before 18 years of age during the study period.

Results  A total of 99 926 individuals (15.01%; 95% CI, 14.98%-15.17%), including 41 350 girls (14.63%; 95% CI, 14.48%-14.77%) and 58 576 boys (15.51%; 95% CI, 15.18%-15.84%), were diagnosed with a mental disorder before 18 years of age. Anxiety disorder was the most common diagnosis in girls (7.85%; 95% CI, 7.74%-7.97%); attention-deficit/hyperactivity disorder (ADHD) was the most common in boys (5.90%; 95% CI, 5.76%-6.03%). Girls had a higher risk than boys of schizophrenia (0.76% [95% CI, 0.72%-0.80%] vs 0.48% [95% CI, 0.39%-0.59%]), obsessive-compulsive disorder (0.96% [95% CI, 0.92%-1.00%] vs 0.63% [95% CI, 0.56%-0.72%]), and mood disorders (2.54% [95% CI, 2.47%-2.61%] vs 1.10% [95% CI, 0.84%-1.21%]). Incidence peaked earlier in boys than girls in ADHD (8 vs 17 years of age), intellectual disability (5 vs 14 years of age), and other developmental disorders (5 vs 16 years of age). The overall risk of being diagnosed with a mental disorder before 6 years of age was 2.13% (95% CI, 2.11%-2.16%) and was higher in boys (2.78% [95% CI, 2.44%-3.15%]) than in girls (1.45% [95% CI, 1.42%-1.49%]).

Conclusions and Relevance  This nationwide population-based cohort study provides a first comprehensive assessment of the incidence and risks of mental disorders in childhood and adolescence. By 18 years of age, 15.01% of children and adolescents in this study were diagnosed with a mental disorder. The incidence of several neurodevelopmental disorders peaked in late adolescence in girls, suggesting possible delayed detection. The distinct signatures of the different mental disorders with respect to sex and age may have important implications for service planning and etiological research.

Check also Greater male variability is currently universal in internationally comparable assessments; some of this heterogeneity can be attributed to some species universal mechanism or some other social/cultural phenomenon

Sex differences in variability across nations in reading, mathematics and science: a meta-analytic extension of Baye and Monseur (2016). Helen Gray, Andrew Lyth, Catherine McKenna, Susan Stothard, Peter Tymms and Lee Copping. Large-scale Assessments in EducationAn IEA-ETS Research Institute Journal 20197:2. https://www.bipartisanalliance.com/2019/03/greater-male-variability-is-currently.html

Faecal transplants from young mice can enhance cognitive function in older animals

Microbiota from young mice counteracts selective age-associated behavioral deficits. Marcus Boehme et al. Nature Aging volume 1, pages666–676. Aug 9 2021. https://www.nature.com/articles/s43587-021-00093-9

Abstract: The gut microbiota is increasingly recognized as an important regulator of host immunity and brain health. The aging process yields dramatic alterations in the microbiota, which is linked to poorer health and frailty in elderly populations. However, there is limited evidence for a mechanistic role of the gut microbiota in brain health and neuroimmunity during aging processes. Therefore, we conducted fecal microbiota transplantation from either young (3–4 months) or old (19–20 months) donor mice into aged recipient mice (19–20 months). Transplant of a microbiota from young donors reversed aging-associated differences in peripheral and brain immunity, as well as the hippocampal metabolome and transcriptome of aging recipient mice. Finally, the young donor-derived microbiota attenuated selective age-associated impairments in cognitive behavior when transplanted into an aged host. Our results reveal that the microbiome may be a suitable therapeutic target to promote healthy aging.

Popular version: Faecal transplants from young mice can enhance cognitive function in older animals. https://www.nature.com/articles/d41586-021-02184-4

Challenging the binary: Gender/sex and the bio-logics of normalcy

Challenging the binary: Gender/sex and the bio-logics of normalcy. L. Zachary DuBois, Heather Shattuck-Heidorn. American Journal of Human Biology, June 6 2021. https://doi.org/10.1002/ajhb.23623

Abstract

Background: We are witnessing renewed debates regarding definitions and boundaries of human gender/sex, where lines of genetics, gonadal hormones, and secondary sex characteristics are drawn to defend strict binary categorizations, with attendant implications for the acceptability and limits of gender identity and diversity.

Aims: Many argue for the need to recognize the entanglement of gender/sex in humans and the myriad ways that gender experience becomes biology; translating this theory into practice in human biology research is essential. Biological anthropology is well poised to contribute to these societal conversations and debates. To do this effectively, a reconsideration of our own conceptions of gender/sex, gender identity, and sexuality is necessary.

Methods: In this article, we discuss biological variation associated with gender/sex and propose ways forward to ensure we are engaging with gender/sex diversity. We base our analysis in the concept of “biological normalcy,” which allows consideration of the relationships between statistical distributions and normative views. We address the problematic reliance on binary categories, the utilization of group means to represent typical biologies, and document ways in which binary norms reinforce stigma and inequality regarding gender/sex, gender identity, and sexuality.

Discussion and Conclusions: We conclude with guidelines and methodological suggestions for how to engage gender/sex and gender identity in research. Our goal is to contribute a framework that all human biologists can use, not just those who work with gender or sexually diverse populations. We hope that in bringing this perspective to bear in human biology, that novel ideas and applications will emerge from within our own discipline.

1 | INTRODUCTION

Biological anthropologists are experts at teasing apart the complexities of biocultural interactions that inform what it is to be human, examining how broad-ranging factors such as market acculturation (Godoy et al., 2005; Liebert et al., 2013), parenting strategies (McKenna et al., 2007; Nelson, 2016), or socially constructed categories of race (Dressler & Bindon, 2000; Gravlee, 2009) relate to physiology including growth and development, immune function, and endocrinology. Yet we have not fully engaged with cutting-edge understandings of variation in gender, sex, and sexuality. This is a critical gap, especially given renewed debates regarding the boundaries of human sex, where lines of genetics, “sex hormones,” and secondary sex characteristics are drawn to defend a strict biologically based sex binary, with attendant implications for the acceptability and limits of gender identity and expression for all people. Whether regulating testosterone levels and bodies of women and girls in sports, legislating the use of gender-specific bathrooms, or enacting broadsweeping federal definitions of sex, bodily “norms” are being weaponized as a means to discriminate (Karkazis et al., 2012; Nondiscrimination in Health and Health Education or Activities, 2020). Biological anthropology is well poised to contribute to these societal conversations, but first, we need to more deeply consider our own conceptions of sex, gender, and sexuality, and how we implement such understandings in our research. In this article, we discuss biological variation associated with sex and gender and possible ways forward for conceptualizing and operationalizing these constructs within biological anthropology. We base our analysis in the concept of “biological normalcy,” which allows consideration of the relationships between “statistical distributions of biological traits and normative views about what bodies ‘should’ be like or what constitutes a ‘normal’ body” (Wiley & Cullin, 2020: p. 1; Wiley, 2021). A classic example of how bionormalcy enables critical interrogation of norms is seen in the case of dietary recommendations normalizing milk consumption culturally as “healthy” and even necessary, despite the statistical norm of lactase nonpersistence (Wiley, 2021). This can be seen as normalizing and even moralizing a biological trait present only in some individuals in some populations (Wiley & Allen, 2017; Wiley & Cullin, 2020). This example aptly demonstrates the fact that many of the statistical distributions that end up being “normalized” are based on samples drawn from predominantly white, “Western” populations (Clancy & Davis, 2019; Henrich et al., 2010), with the psychological, behavioral, and biological traits of these populations referenced as the standard from which other populations deviate (e.g., body size and growth, Thompson et al., 2014). The model of biological normalcy (Figure 1) is circular. Cultural norms and assumptions inform the development of research questions, methods of data collection, and analyses as well as interpretations of data. Statistical norms are also leveraged (albeit sometimes unconsciously) to create, reinforce, or otherwise inform those very cultural norms and assumptions. However, normalcy has not always been conceptualized in this way. The word “normal” as reflective of something to be desired in reference to an “abnormal” state arose only in the mid to late 19th century (Cryle & Stephens, 2017; Hacking, 1990). Initially, the term “normal” did not represent statistical distributions nor did it carry the morality it is imbued with today. Instead, norms provided a way to reference something “in its own right” and not necessarily through comparison to an ideal. In this way, even anomalies could be understood within a framework of “normal.” With the emergence of statistics in the late 19th century, the concept of the normal became hitched to statistical distributions and to the racist and eugenicist ideas imposed on population traits (Cryle & Stephens, 2017). And with this shift, the concept of the normal intertwines with the history of biological anthropology, as eugenic and white supremacist concepts of human traits and the categorical position of white men as both unmarked and ideal are the very foundation of much of our field (Blakey, 2020; Caspari, 2018; Marks, 2012). Racism and colonialism are equally culpable in the development of value-laden categories of sex and gender and the behavioral norms to which they are often tied. For example, conceptualizations of femininity and masculinity themselves were initially intertwined with racialized categories in an effort to hierarchically demarcate rank, reflecting a colonialist project with the “white ideal” as most differentiated between the sexes (Markowitz, 2001). As a field, biological anthropology continues to suffer from how our history influences who practices biological anthropology (e.g., Bolnick et al., 2019). As biocultural anthropologists, in this article, we aim to broaden the way that human biology engages with categorical thinking about gender and sex and to push for greater recognition of variation in these domains. We are inspired by the decades of strong work into race as a social construct with biological outcomes (Armelagos & Goodman, 1998; Dressler et al., 2005; Graves Jr, 2003; Graves Jr, 2015; Gravlee, 2009; Williams & Mohammed, 2013), and by recent work contextualizing how concepts such as violence are gendered and raced (e.g., Nelson, 2021; Smith, 2021). In our own work, we have grappled with how to better conceptualize and operationalize sex and gender, whether examining energetics and immune function in pubertal girls (ShattuckHeidorn, Reiches, & Richardson, 2020), sexual decision making among queer adolescent cis men (DuBoiset al., 2015), or immune marker and environmental conditions for (cis) men and women (Shattuck-Heidorn, Eick, et al., 2020). In some of our prior work, the category “cis” was unmarked, and at times, in our analytical strategies, we have statistically compared cis men to cis women without a clear justification as to why the sample should be divided by sex as opposed to some other trait(s). Much of our recent scholarship integrates theoretical insights from gender and feminist theory and presents challenges to simple gender/sex binaries through our research questions, study designs, and hypotheses. This is reflected for example, in work expanding understandings of stigma and embodied inequalities among trans and gender diverse people (DuBois, 2012; DuBois et al., 2017), furthering our methodological and theoretical approaches to better encompass gender/sex and sexual diversity (DuBois et al., 2021; Shattuck-Heidorn & Richardson, 2019), and interrogations into the basis for disparities in COVID-19 outcomes (Gibb et al., 2020; Rushovich et al., 2021; Shattuck-Heidorn, Reiches, & Richardson, 2020). Such interdisciplinary merging has enabled us to better conceptualize human gender/sex and enhanced our understanding of variation in embodiment and health. In this article, we address the following critical areas: (1) the problematic reliance on binary sex categories used as a priori biological categories across traits; (2) the attendant focus on group means to represent typical “male” and “female” behaviors and biology and accompanying fixation on “difference;” (3) the ways in which binary sex norms reinforce stigma and inequality regarding sex, gender, gender identity, and sexuality; (4) the need for “best practices” to effectively engage sex and gender in research; and (5) methodological suggestions to address the lack of inclusive data collection needed to enhance our understanding of gender and sex and sexual variation. Our goal is to contribute to a framework that all human biology researchers, not just those who work with gender or sexually diverse populations, can use to inform their thinking as well as decisions about best practices for whether and how to implement sex and gender analyses within their research, both theoretically and methodologically.

6 | CONCLUSIONS

As human biologists, gender/sex is central to how we understand and organize our thinking about human evolution as well as health in contemporary and historic contexts. The entwinement of gender and sex is complex, as is much of the science exploring this variation and how it develops. It is increasingly necessary for human biologists to engage novel methodologies to ensure we are capturing and engaging with gender/sex diversity. As detailed above, research in human biology and other disciplines challenges the understanding and the use of binary sex as a meaningful category explaining human biological variation across contexts. The work reviewed here is a small part of a large field of research that pushes us to continue to consider the ways in which human bodies and identities resist static categorization. Hormones vary and function in complex ecological and social environments, brains and bodies develop over time in response to varied experiences and inputs, and societal structures of gender norms, race and racism, and sexuality influence and mediate human biology. As the common-sense notion of binary categories for human gender/sex are destabilized, our discipline is well-positioned to meaningfully explore the complexity of gender/sex in terms of human variation and to understand that variation within a sociocultural context, including race, sexuality, and gender diversity. Our field has contributed substantially to an understanding of human biology in a socioecological context. We look forward to a generation of work from biological anthropologists who are incorporating intersectional analyses of gender/sex and gender identity into our understandings of human diversity.