Population affinity and variation of sexual dimorphism in three-dimensional facial forms: comparisons between Turkish and Japanese populations

Population affinity and variation of sexual dimorphism in three-dimensional facial forms: comparisons between Turkish and Japanese populations. Chihiro Tanikawa, M. Okan Akcam, Hatice Gokalp, Edlira Zere & Kenji Takada. Scientific Reports volume 11, Article number: 16634. Aug 17 2021. https://www.nature.com/articles/s41598-021-96029-9

Abstract: Examining the extent to which sex differences in three-dimensional (3D) facial soft tissue configurations are similar across diverse populations could suggest the source of the indirect evolutionary benefits of facial sexual dimorphism traits. To explore this idea, we selected two geographically distinct populations. Three-dimensional model faces were derived from 272 Turkish and Japanese men and women; their facial morphologies were evaluated using landmark and surface-based analyses. We found four common facial features related to sexual dimorphism. Both Turkish and Japanese females had a shorter lower face height, a flatter forehead, greater sagittal cheek protrusion in the infraorbital region but less prominence of the cheek in the parotid-masseteric region, and an antero-posteriorly smaller nose when compared with their male counterparts. The results indicated the possible phylogenetic contribution of the masticatory organ function and morphogenesis on sexual dimorphism of the human face in addition to previously reported biological and psychological characteristics, including sexual maturity, reproductive potential, mating success, general health, immune response, age, and personality.

Discussion

In the present study, principal components that explained 66.2% of the sample’s variance showed interaction between population affinity and sex were not significant, which indicates that both populations have statistically similar expressions of sexual dimorphism. Alternatively, our detailed analysis revealed that both population affinity characteristics of facial sexual dimorphism that were common to both the Japanese and Turkish subjects, and some characteristics that were unique to each set of subjects. The presence of both similarities and differences in facial sexual dimorphism among populations described in this study were consistent with previous controversial studies regarding population affinity9,10,11,12. The common characteristics could have arisen from a strong selective force on fundamental function to survive as a result of natural selection, and the differing sexually dimorphic characteristics could be due to environmental adaptation under a trade-off between natural and sexual selection22.

Regarding the common sexual dimorphic characteristics, both the Japanese and Turkish females had a shorter face height, especially with regard to the lower face; a flatter forehead; greater sagittal cheek protrusion in the posterior part of the infraorbital region; and less prominence of the cheek in the parotid-masseteric region. Furthermore, females in both population groups had antero-posteriorly smaller noses and greater retrusion of the columella base and subnasal region.

Males showed a greater height in the lower anterior face, especially with regard to the chin, in both population groups. It should be noted that a previous cephalometric study23 documented temporal changes in the ratios of the anterior lower face height to the total face height in the Japanese population. Females exhibited the anterior lower face height to total face height ratio almost equal to or longer than males at 6, 8, and 10 years old. Females at 6, 10, and 14 years old had lower face height ratios that were similar to those of adults (6 years old = 54.6% and adults = 54.9%). It is after 12 years of age when males begin to have increased face height ratio23. The observed increase in the lower anterior face height in males can be ascribed to sexual differences in pubertal growth potential of the mandible23, which is prolonged in males compared with females. There are several explanations regarding why men have a greater lower anterior face height, especially in the chin after pubertal growth. From the perspective of mastication, it seems likely that the acquired basic skill for most fundamental motor performance, such as mastication and locomotion, is independent of sex24. A previous study25 documented that the smoothness or skillfulness of masticatory jaw movement in terms of minimizing the jerk cost is not sex-specific. It should, however, also be noted that some parameters, such as the amount of jaw opening and movement velocity, are sensitive to sex-specific differences in jaw size and masticatory muscle properties25. Adult females show longer duration and lower peak velocity in masticatory jaw movement compared with males24; this can be ascribed to adult males generating greater muscle force and faster muscle contraction26 with greater muscle volume and size of the mandible, to which the jaw-closing muscles are attached. Sex influences on maximal molar bite force and masticatory muscle thickness17. Thus, the anatomy and function of the masticatory muscles may contribute to explaining why males generally have greater faces, especially in the lower third.

Furthermore, the allometric decomposition findings concerning sexual shape dimorphism support the phylogenetic importance of the chewing apparatus in sexual dimorphism in males. As men require more calories than women to function16, it is reasonable that their greater body size tends to correlate with a greater anterior facial height for a well-developed chewing apparatus. A previous study that examined 2D allometric and non-allometric variation in the facial shape differences between men and women showed a rather weak link with allometric variation compared with non-allometric variation in most populations, including the Turkish. As our study showed that the allometric difference was greater than non-allometric differences, this is considered to be related to the sex differences in the antero-posterior direction.

From a biological perspective, sex hormones are major factors related to sexual dimorphism. In males, higher androgen serum levels at puberty exert potent osteoanabolic effects and therefore may contribute to this skeletal sexual dimorphism. Animal experiments with anabolic steroids demonstrated a clear effect on craniofacial growth, mainly as an increase in total skull length and increase in the depth of the antegonial notch27. Interestingly, a previous study showed that mandibular and cortical human osteoblastic cells of both sexes expressed higher androgen receptor mRNA levels and significantly more androgen binding sites per cell and exhibited significantly greater mitogenic responses to the androgen dihydrotestosterone28. Those results indicate that the vertically greater mandibular height in males observed in our study could be due to skeletal site-dependent expression of the androgen receptor in the mandible. Additionally, a previous study that examined facial morphology of 1-year-old boys and girls showed the existence of early sexual dimorphism, and prenatal testosterone exposure is thought to be related to sexually dimorphic facial morphology29. Thus, it is possible that androgens in males could contribute to facial sexual dimorphism both before and after puberty.

Previous studies on anthropoids revealed only smaller muscle strains in the supraorbital region in contrast to those in the infraorbital region or the zygomatic arch during mastication30,31. Animal studies32,33 have also revealed that circumorbital structures became greater to provide rigidity against non-masticatory forces; these studies revealed that is unlikely that masticatory muscle forces contributed to the remodeling of the supraorbital torus. On the contrary, the development of the supraorbital ridge has been viewed as an ontogenetic adaptation to masticatory forces34. In primates, masticatory-stress models have been examined using in vivo experimental data. Primates have significant temporalis attachments that extend to almost the midline of the frontal bones; bending of the brow-ridges is thought to be due to the mastication force pushing upward and the masseter and temporalis muscles pulling downward35. Few of the previous computational models, using finite element analysis of primate skulls36, agree with these in vivo findings. A previous study37 found a positive correlation between the mesio-distal crown width of the mandibular first molar and the size of the supraorbital ridge in humans. Occlusal forces exerted on the molar teeth contribute to supraorbital torus formation. Because females generate weaker muscle force and slower muscle contraction than males26, and exhibit decreased maximal molar bite force and masticatory muscle thickness17, we should not rule out the possibility of contribution of masticatory muscle forces to supraorbital ridge formation in humans. Phylogenetically, the smaller supraorbital ridge observed in the female subjects in the present study may be explained by the differences in masticatory force magnitude and its relevant jaw muscle thickness between males and females17,30.

In the present study, both Japanese and Turkish males showed an antero-posteriorly greater nose when the eye distances were standardized. This result is in line with those of previous studies38,39,40. Previous studies primarily hypothesized that males have evolved to have greater nasal cavity dimensions to facilitate the oxygen intake that is needed to maintain a larger body mass37,41. The degree of sexual dimorphism in nasal shape is considered to be potentially due to the functional integration between the nasal cavity and the respiratory system42.

The extent of the cheek region is defined as “superiorly to the zygomatic arch, inferiorly to the margin of the mandible, posteriorly to the ear, and anteriorly to the corner of the mouth” and is divided into four parts as topographical regions: infra-orbital, buccal, zygomatic, and parotid–masseteric regions43).

In the present study, in the infraorbital and buccal regions, the sagittal cheek protrusion in the posterior part of the infraorbital region was greater in the female subjects on the left side. Furthermore, lesser prominence of the cheek in the parotid–masseteric region was also observed in both Japanese and Turkish female subjects.

A lesser prominence of the cheek in the parotid–masseteric region can be explained by the smaller masseter muscles in women17. Thin masseter muscles lead to a lesser prominence of the cheek in the parotid–masseteric region in women.

Effects of developmental and functional interactions on morphological variability of the head through ontogeny have been discussed in previous studies42,44. Several studies42,44 have claimed that genetic signals determine the initial geometry of craniofacial anatomy, and that geometry is altered by the local mechanical environment, such as masticatory function and respiratory function, through variations in the spatio-temporal interplay of depository and resorptive activity of bone. In contrast, there is very little concrete evidence of the relationship between functional and phylogenetic development in facial configurations. In general, it is assumed that varying environmental conditions, such as climates, geographic areas, and dietary resources, require physical characteristics, including dento-facial features, which contribute to maximizing the survival probability of individuals. Hominids are now recognized as showing higher adaptability to their surrounding environment based on related morphological changes than was previously understood.

In the past, several studies have addressed 3D morphological differences between populations. For example, between Caucasians and African-Americans, the most distinct differences were observed in the forehead, alar base, and perioricular regions using 3D facial data45; between Caucasians and Asians, differences were observed in the malar and zygomatic areas, forehead, lips, and chin46. Even in the phylogenetically related populations, there were differences seen in the nasal, malar, lips, and lower facial regions between two population groups (Budapest, Hungary, and Houston, Tex)47; differences were also observed in the nasal width, eye distances, and facial height of two European Caucasian populations of close phylogenetic and geographic proximity (UK and Netherlands)14. In short, the previous studies described the facial differences between the population groups; however, limited data has been reported regarding varied facial sexual dimorphic characteristics among populations.

In the present study, four features in the Japanese and three in the Turkish were found to be exclusive sexual dimorphic characteristics. In the Japanese subjects, females had greater eye height (i.e., brighter eyes) compared with males. A medium or high upper eyelid crease is known to represent an attractive face in East Asian females, and 50% of females exhibit a minimal or absence of a double eyelid44. Although greater eye height is also deemed an important factor for facial attractiveness in other populations, the present results indicate that eye height is a visible facial sexual dimorphism that is more discriminatory in the Japanese subjects than the Turkish subjects.

Japanese females also showed a smaller anteroposterior protrusion of the nasal dorsum at the orbital level (i.e., a flatter nose) and a superiorly positioned mouth with a vertically shorter subnasal region. Additionally, shorter horizontal mandibular width was observed in the Japanese females. These findings indicate that Japanese females had overall smaller middle and lower facial structures than males. In a previous study that examined the 3D nasal shape and genotype in 3746 individuals, nares width was correlated with temperature and absolute humidity48. This result indicates that at least sexual dimorphism in nasal shape may change because of climate adaptation.

In contrast to the Japanese females, three features were found to be characteristic of the Turkish females compared with Turkish males. There was a greater vertical distance between the eyes and eyebrows, and an increased zygomatic width compared with exocanthion–exocanthion distance. These traits reflect a stout upper facial structure. Facial ontogeny research on immature hominids with a finite element model49 showed that bone deposition was identified over the outer aspects of the orbits, lateral nasal walls, infraorbital region, zygomatico-maxillary region, parts of the mid-clivus, including the canine jugum, and interincisal protuberance, as well as portions of the nasal sill and areas lateral to the intermaxillary suture; they inferred that these changes were related to the masticatory system49.

A shallower labio-mental sulcus also characterized Turkish female compared with male faces. A recent study39 indicated that an ontogenetic decrease in chin prominence was associated with increased vertical bending resistance and vice versa. Thus, it can be inferred that a shallow labiomental sulcus was unique to the current Turkish female participants, which indicates an adaptational response of Turkish females, who have delicately constituted jaw bones and muscles, compared with Turkish males in a dietary environment that includes tougher animal proteins compared with the Japanese dietary environment.

It is well known that Africa is the ancestral homeland of modern humans50. A phylogenetic tree showed the categorization of the world population into nine sub-populations based on the polymorphisms of protein genes of 1915 populations: African; North African and West Asian; European; Amerind; Arctic Northeast Asian; Northeast Asian; Southeast Asian; Pacific Islander; and New Guinean and Australian51. The genetic distances between Japanese (Northeast Asian) and Turkish (European) were moderately far (55% of total distance) whereas European and North African were close (7%); this indicated that Japanese and Turkish (European) had different developmental route51. Genetic data also provided some indication that the spread of humans into Asia was along the coast to south and south-east Asia, from where it bifurcated to the north and south52. Thus, our comparisons of sexual dimorphism in facial forms between Turkish and Japanese populations can explain a relatively long span of genetic drift, which is the result of population variation among individual genotypes in their probabilities of survival and/or reproduction.

Several limitations associated with the present study warrant mention. First, the Turkish population was undersampled in comparison to the Japanese population. The frontal view of our 3D Turkish data was similar to that of a previous 2D study53 which used a greater number of Turkish samples (n = 264); thus it could be said that our results are possibly representative of the Turkish population. However, future studies including more Turkish subjects would us to make more general conclusions. Second, our study included only two populations, so it is impossible to draw complex conclusions regarding the geographical variability of the human face. Future studies would benefit from including an even larger number of populations. Third, in the present study, we used only the centroid size of the face to examine the allometric component. The results may vary when using the height or weight. Furthermore, in the present study, we omitted color information when analyzing the data because this information was not stable among populations. In some populations, not only sexual dimorphism in facial shape but also sex differences in skin color contribute to the overall facial dimorphism. Furthermore, it has been shown that skin color is an important trait associated with facial attractiveness in populations showing high variation in skin color, especially in Africans54,55. This means that facial dimorphism cannot be considered only by the facial shape, and there is still a place for sexual selection that may act upon non-shape-associated facial traits. Moreover, it seems that some color traits (such as iris color) are systematically associated with the sex-specific facial shape56,57. Future studies using information involving sex differences in these other attributes rather than shape should be considered. Finally, although the present study does not provide a convincing explanation about whether the sexual dimorphisms, which were determined in the present study to be unique to each population group, represent consequences of natural selection for population affinity that successfully adapted to dietary environments for many generations. Therefore, although we must be cautious about the limitations of interpreting these data, the results of the present study further enhance our understanding of human sexual dimorphism expressed in the oral and facial regions.

About 40% of students experienced depression and anxiety symptoms prior to entering/ during the transition to university: Role of Self-critical Perfectionism

Levine, Shelby L., Nassim Tabri, and Marina Milyavskaya. 2021. “Trajectories of Depression and Anxiety Symptoms over Time in the Transition to University: Their Co-occurrence and the Role of Self-critical Perfectionism.” PsyArXiv. May 27. doi:10.31234/osf.io/zxg8h

Abstract: Little is known about how mental health symptoms develop during the transition to university. Most anxiety and depression research fail to consider how symptom development differs over time across different individuals, and how symptom co-occurrence influences the severity of mental heath problems. Students (N = 658) completed online surveys on mental health prior to starting university and every 2 months until April. To better understand mental health problems during this transitional period, latent class growth curve analyses were run to determine how anxiety and depressive symptoms co-develop over time, as well, if self-critical perfectionism was a transdiagnostic risk factor for more severe symptom trajectories in this transition. About 40% of students experienced depression and anxiety symptoms prior to entering/ during the transition to university. There is substantial variation between students in terms of how they experience depression and anxiety symptoms, and research needs to take this heterogeneity into account to properly identify which students might benefit most from resources. Self-critical perfectionism was a transdiagnostic risk factor, such that students higher in this trait experienced more severe anxiety and depressive symptom trajectories during this transition. This research further implicates the importance of understanding and studying individual differences in symptom development.

Testosterone, T, was associated with more (less) advantaged socioeconomic position & better (worse) health among men (women), but previous associations of T & position may reflect influence of position on T

Testosterone and socioeconomic position: Mendelian randomization in 306,248 men and women in UK Biobank. Sean Harrison et al. Science Advances, 2021; 7 : eabf8257. July 28 2021. https://advances.sciencemag.org/content/advances/7/31/eabf8257.full.pdf

Abstract: Men with more advantaged socioeconomic position (SEP) have been observed to have higher levels of testosterone. It is unclear whether these associations arise because testosterone has a causal impact on SEP. In 306,248 participants of UK Biobank, we performed sex-stratified genome-wide association analysis to identify genetic variants associated with testosterone. Using the identified variants, we performed Mendelian randomization analysis of the influence of testosterone on socioeconomic position, including income, employment status, neighborhood-level deprivation, and educational qualifications; on health, including self-rated health and body mass index; and on risk-taking behavior. We found little evidence that testosterone affected socioeconomic position, health, or risk-taking. Our results therefore suggest that it is unlikely that testosterone meaningfully affects these outcomes in men or women. Differences between Mendelian randomization and multivariable-adjusted estimates suggest that previously reported associations with socioeconomic position and health may be due to residual confounding or reverse causation.

Rolf Degen summarizing... The brain circuit for pain is different from that for suffering, and the latter adds the bite to the former, and to other kinds of disorder

The anatomy of pain and suffering in the brain and its clinical implications. Dirk De Ridder, Divya Adhia, Sven Vanneste. Neuroscience & Biobehavioral Reviews, August 16 2021 https://doi.org/10.1016/j.neubiorev.2021.08.013

Highlights

• Acute pain is a symptom of acute or potential tissue damage, chronic pain extends beyond the period of healing of the original insult or injury, and hence lacks the acute warning function of physiological nociception.

• Pain can be anatomically and phenomenologically dissected into three separable but interacting pathways, a lateral ‘painfulness’ pathway, a medial ‘suffering’ pathway and a descending pain inhibitory pathway.

• In chronic pain the descending pain inhibitory pathway is less activated.

• Pain sensation leads to suffering via a cognitive (insula), emotional (ACC) and autonomic (ACC plus insula) processing, and is expressed as anger, fear, frustration, anxiety and depression, leading to changes in behaviour and functional disability.

• Acute pain transitions into chronic pain under influence of genetic and epigenetic factors.

• The genetic and epigenetic factors modulate neuroinflammation, which is involved in peripheral and central sensitization.

• Chronic pain, with a prevalence of 20-30% is the major cause of human suffering worldwide.

• Perceived pain disability correlates highly with suffering, little with painfulness

• Unpleasantness (and suffering) is transmitted via a phylogenetically old unmyelinated C-fibre network, linked to survival and procreation.

• Pain and suffering depend on salience and context: sadomasochistic erotic behaviour is a clear example.

• The medial pathway overlaps with the salience and stress networks, explaining that behavioural relevance or meaning determines the suffering associated with painfulness.

• Suffering explains the common neurocircuitry of many psychiatric and neurological disorders.

• Women perceive more intense acute and chronic pain and experience more unpleasantness than men.

• Women suffer more than men as evidenced by higher anxiety and depression prevalence.

• The sex-differences in pain perception are genetically encoded and dependent on immunological and hormonal modulation of pain processing.

• The (predictive) Bayesian Brain hypothesis proposes that pain (and suffering) is the consequence of an imbalance between the ascending and descending pain inhibitory pathways. This balance is theorized to be under control of the reward system.

• By categorizing the working mechanisms of each of the available treatments (pain killers, psychopharmacology, psychotherapy, neuromodulation, psychosurgery, spinal cord stimulation) to 1 or more of the 3 pathways, a rational combination can be proposed of activating the descending pain inhibitory pathways in combination with inhibition of the medial and lateral pathway, so as to rebalance the pain (and suffering) pathways.

Abstract: Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage. Chronic pain, with a prevalence of 20-30% is the major cause of human suffering worldwide, because effective, specific and safe therapies have yet to be developed. It is unevenly distributed among sexes, with women experiencing more pain and suffering. Chronic pain can be anatomically and phenomenologically dissected into three separable but interacting pathways, a lateral ‘painfulness’ pathway, a medial ‘suffering’ pathway and a descending pain inhibitory pathway. One may have pain(fullness) without suffering and suffering without pain(fullness). Pain sensation leads to suffering via a cognitive, emotional and autonomic processing, and is expressed as anger, fear, frustration, anxiety and depression. The medial pathway overlaps with the salience and stress networks, explaining that behavioural relevance or meaning determines the suffering associated with painfulness. Genetic and epigenetic influences trigger chronic neuroinflammatory changes which are involved in transitioning from acute to chronic pain. Based on the concept of the Bayesian brain, pain (and suffering) can be regarded as the consequence of an imbalance between the two ascending and the descending pain inhibitory pathways under control of the reward system. The therapeutic clinical implications of this simple pain model are obvious. After categorizing the working mechanisms of each of the available treatments (pain killers, psychopharmacology, psychotherapy, neuromodulation, psychosurgery, spinal cord stimulation) to 1 or more of the 3 pathways, a rational combination can be proposed of activating the descending pain inhibitory pathways in combination with inhibition of the medial and lateral pathway, so as to rebalance the pain (and suffering) pathways.

Keywords: painacutechroniccognitiveemotionalautonomicanterior cingulate cortex

The various disclosure & internal governance rules lead to a total compliance cost of 4.1% of the market capitalization for a median U.S. public firm; these regulatory costs only explain a small fraction of the decline in number of public firms

Regulatory Costs of Being Public: Evidence from Bunching Estimation. Michael Ewens, Kairong Xiao & Ting Xu. NBER Working Paper 29143, August 2021. DOI 10.3386/w29143

Abstract: The increased burden of disclosure and governance regulations is often cited as a key reason for the significant decline in the number of publicly-listed companies in the U.S. We explore the connection between regulatory costs and the number of listed firms by exploiting a regulatory quirk: many rules trigger when a firm’s public float exceeds a threshold. Consistent with firms seeking to avoid costly regulation, we document significant bunching around multiple regulatory thresholds introduced from 1992 to 2012. We present a revealed preference estimation strategy that uses this behavior to quantify regulatory costs. Our estimates show that various disclosure and internal governance rules lead to a total compliance cost of 4.1% of the market capitalization for a median U.S. public firm. Regulatory costs have a greater impact on private firms’ IPO decisions than on public firms’ going private decisions. However, heightened regulatory costs only explain a small fraction of the decline in the number of public firms.

Will this "up to five times" be reproducible?: Exposure to continuous or fluid theories of sexual orientation leads some heterosexuals to embrace less-exclusive heterosexual orientations

Will this "up to five times" be reproducible?: Exposure to continuous or fluid theories of sexual orientation leads some heterosexuals to embrace less-exclusive heterosexual orientations. James S. Morandini, Liam Dacosta & Ilan Dar-Nimrod. Scientific Reports volume 11, Article number: 16546 (2021). https://www.nature.com/articles/s41598-021-94479-9

Abstract: We examined whether heterosexual individuals’ self-reported sexual orientation could be influenced experimentally by manipulating their knowledge of the nature of sexual orientation. In Study 1 (180 university students, 66% female) participants read summaries describing evidence for sexual orientation existing on a continuum versus discrete categories or a control manipulation, and in Study 2 (460 participants in a nationally representative Qualtrics panel, 50% female) additionally read summaries describing sexual orientation as fluid versus stable across the life-course. After reading summaries, participants answered various questions about their sexual orientation. In Study 1, political moderates and progressives (but not conservatives) who read the continuous manipulation subsequently reported being less exclusively heterosexual, and regardless of political alignment, participants reported less certainty about their sexual orientation, relative to controls. In Study 2, after exposure to fluid or continuous manipulations heterosexual participants were up to five times more likely than controls to rate themselves as non-exclusively heterosexual. Additionally, those in the continuous condition reported less certainty about their sexual orientation and were more willing to engage in future same-sex sexual experiences, than those in the control condition. These results suggest that non-traditional theories of sexual orientation can lead heterosexuals to embrace less exclusive heterosexual orientations.

General discussion

These studies show that how heterosexual individuals perceive their sexual orientation can be influenced by exposure to different theories regarding the nature of sexual orientation. In Study 1, exposure to continuous notions of sexual orientation caused political progressive and moderate, but not conservative university students to change how they perceived their sexual orientation to a non-exclusive heterosexual orientation (in support of Hypothesis 2B)—presumably as young progressives were less resistant to accepting that they may have the capacity for same-sex sexuality. Gender did not significantly moderate the effect of the continuous manipulation on sexual orientation change contrary to Hypotheses 2A. Moreover, supporting Hypothesis 1, reading the continuous account caused all participants, regardless of political orientation, to report less certainty about their sexual orientation. Given that college students may be particularly open to sexual identity exploration it was necessary to examine these findings in a heterogeneously-aged community sample. Study 2 failed to directly replicate Study 1 in a nation-wide, more heterogeneously-aged, community sample. Nevertheless, manipulating sexual orientation impacted sexual orientation self-ratings in line with Hypothesis 1. Whereas only 8% of participants in the control condition reported being non-exclusively heterosexual post manipulation, 36% of those who read the continuous account, and 21% who read the fluid account subsequently described themselves as non-exclusively heterosexual post manipulation in this between-subject designed study. These shifts occurred irrespective of political orientation, or gender (contrary to Hypotheses 2A and 2B). Furthermore, in Study 2, the continuous account not only led heterosexuals to report less certainty about their sexual orientation, but also to express greater willingness to engage in sexual or romantic interactions with members of the same-sex. This latter finding may have been significant in Study 2 but not in Study 1 given the substantial difference in power, with the Study 2 sample size offering greater power to find a significant effect for between-subject variables, such as those.

As mentioned, unlike in Study 1, in Study 2 manipulations influenced sexual orientation self-ratings irrespective of the political orientation of participants (contrary to Hypothesis 2B). It is possible that the effects of the continuous condition were stronger in Study 2 (national sample; average age 45) than Study 1 (college sample; average age 19) because these notions of sexual orientation were more novel for the national sample who were on average middle-aged and more politically conservative (i.e., a ceiling effect may have reduced the strength of shifts in the college sample—who were perhaps already well accustomed to continuous notions of sexuality due to young age and political progressivism). Another possibility is that the differences in the design of the studies contributed to the differences in the effect of political orientation. In the first study, a within-subject assessment allowed us to assess actual within-subject changes following the manipulation. As such, it meant that participants were willing to show changes in their self-perceptions following the manipulation. As politically conservative students view same-sex attraction more negatively than their progressive counterparts35, it was arguably less taxing for the progressive (and moderates) to admit that the information in the manipulation (legitimizing gradations of heterosexuality) led them to explicitly recognise some low level of same-sex attraction in themselves. For conservatives, on the other hand, such an admission may have been more taxing and thus, less frequent, leading to the observed differences. In Study 2, on the other hand, no pre-manipulation indication of their Kinsey-type identity label was reported, thus the willingness to show that they accepted the information in the manipulation by changing their identity label immediately following the manipulation was absent, allowing the participants to avoid showing explicit endorsement of the information about continuity (or fluidity) of sexual orientation. Thus, the nature of the studies’ designs may have led conservative individuals to respond differently to the manipulations.

Across studies, neither the discrete nor stable manipulations demonstrated effects on sexual orientation self-concept—and specifically, contrary to Hypothesis 3—they failed to increase exclusivity of heterosexual feelings or decrease sexual orientation uncertainty or willingness to engage in same-sex encounters. This may be interpreted to suggest that discrete and stable notions of sexual orientation were perhaps the default assumptions in our sample. Furthermore, in Study 2, increased rates of non-exclusive heterosexual identification were observed also among those in the discrete condition (although this was a non-significant trend). On reflection, it is possible that the discrete condition provided scientific evidence that bisexuality exists—which is counter to the binary view of sexual orientation that is still predominant in society—particularly in older generations6 and those who are more politically conservative36. This may have resulted in more self-reports in the bisexual spectrum in some participants. However, the failure to observe this trend in Study 1 may indicate that this finding is spurious and requires future replication.

Contrary to our Hypotheses 2A, women appeared no more likely than men to report non-exclusive heterosexuality following exposure to informational accounts. At face value this is surprising given considerable evidence the female sexual orientation is more malleable than male sexual orientation21,22,37, and given that heterosexual men are likely to experience greater internal resistance28, and greater social backlash24,30, to adopting a non-exclusive heterosexual orientation. However, a closer look at Study 1 may explain why we did not observe gender differences in these effects. Pre-manipulation, men were much more likely than women to report an exclusively heterosexual orientation (91.7% versus 68.3%), and as discussed above, we observed a trend wherein those who were exclusively heterosexual at pre-test demonstrated stronger shifts following exposure to the continuous manipulation than those who were non-exclusively heterosexual at pre-test. Thus, a ceiling effect among the subset of women who already rated themselves as non-exclusively heterosexual at pre-test, may explain why women did not demonstrate stronger increases in non-exclusive heterosexuality than men.

Providing convergent evidence that the continuous account of sexual orientation can influence how heterosexuals view their sexual orientation—participants who read the continuous account were more uncertain of their sexual orientation than those in the control group. This may prove to be a temporary phase as individuals try to make sense of their sexual orientation in light of the new information, or alternatively, it may be a more permanent phenomenon which occurs as individuals move away from categorical and binary understandings of sexual orientation2,38. Moreover, in Study 2, reading the continuous account of sexual orientation increased participant’s willingness to engage in sexual/romantic interactions with members of the same-sex, relative to controls. Whether these behavioral intentions manifest in actual same-sex encounters will require future research. Critically, although those who read continuous and fluid accounts of sexual orientation were more likely to report a non-exclusive heterosexual orientation than controls, it is difficult to assess the generalizability of these effects to a real-world context, including whether the effects of reading these accounts persists over time (i.e., did our brief intervention permanently affect how participants view their sexual orientation?). Longitudinal studies are required to assess the stability of these effects on self-perceived sexual orientation—and thus these findings should be interpreted with caution. Such studies will benefit from integrating elements that can shed light on the mechanisms that are involved in the transitions away from the completely heterosexual descriptors. As perceptions of homogeneity are considered to be central to essentializing social categories based on elements such as sexual orientation39, and this essentialist tendency plays a role in making the members of such groups (e.g., heterosexual vs non-heterosexual individuals) seem more distinct from each others40, the role of perceptions of continuity and fluidity in reducing essentialism-derived taxometric notions offers a promising direction to explore.

Our manipulation of sexual orientation focused on influencing people’s perception of the continuity/discreteness/fluidity/stability of sexual arousal/sexual attraction, which although at the heart of what most people mean by sexual orientation41, is not the only thing which informs people’s sexual orientation. For instance, had our manipulations discussed continuity or fluidity in romantic feelings or pair bonding feelings—these may have impacted sexual orientation self-ratings in different ways. Arguably, as romantic feelings may be less gender specific than sexual arousal/attraction42, at least in males, including romantic themes (e.g., evidence that feelings of emotional intimacy/crushes toward members of the same-sex indicate non-exclusive heterosexuality) in our manipulation may have resulted in larger shifts in self-ratings. Future research may examine this possibility.

An intriguing possibility is that heterosexuals who hold a continuous or fluid view of sexual orientation are less prejudiced toward gay, lesbian, or bisexual peoples. Such a prediction is supported by evidence that indicates that perceiving sexual orientation as discrete is associated with greater anti-gay prejudice8. On the other hand, a recent study found that when heterosexual men are exposed to information that blurs the distinction between themselves and homosexual men, they enact greater homophobia, to re-establish their distinctiveness28. Future research clarifying how continuous and fluid notions of sexual orientation impact sexual prejudice is therefore of the upmost importance. Further, although the present research focused solely on heterosexual populations, future research may also examine how exposure to continuous and fluid notions of sexual orientation influences how gay men and lesbian women conceive of their sexual orientation. The potential effects of such exposure on shifts in reported sexual orientation and on levels of internalized homophobia are valuable, needed explorations.

Our findings also bring into question the meaning of “mostly” (e.g., “mostly same-sex attracted”, mostly opposite-sex attracted”) ratings on Kinsey-type measures. As commonly interpreted by researchers and lay people alike, individuals who report different positions on a Kinsey-type scale are thought to possess different sexual orientations43. But how then do we make sense of the present findings—in which participants’ self-ratings changed following our manipulations? Did we change the sexual orientation of our participants? Surely not. To make sense of the shifts observed we need to recognize that measures such as the Kinsey scale can only possibly assess “self-perceived sexual orientation”44. Although self-perceived sexual orientation is partly informed by actual sexual/romantic experiences (which gender/s we find sexually arousing, crush on, fantasize about, have sex with) these experiences are filtered through appraisals of these thoughts, feelings, and behaviours based on a range of personal beliefs and attitudes. This means that two individuals, with identical sexual experiences, could report quite different sexual orientations. The present study found that manipulating participants beliefs about sexual orientation changed how they interpreted their sexual/romantic experiences and the subsequent global assessment they made when rating their sexual orientation. As considerable effort has been undertaken to understand mental health45,46, substance use47,48, sexual health49,50,51, discrimination52, and even physiological differences43,53,54,55,56,57 between exclusive and non-exclusive heterosexual individuals, clarifying the cognitive and attitudinal variables that may predispose a heterosexual person to adopt respective labels is surely important if we are interested in the causes of differences (e.g., mental health, sexual health) between exclusive heterosexual and non-exclusive heterosexual populations.

Our findings suggest that non-exclusive heterosexual orientations might become more prevalent as continuous and fluid notions of sexuality become more culturally mainstream and provide currently-identified heterosexuals with more nuanced ways of describing themselves. We should stress that present findings do not support the contention that sexual orientation (the underlying compass that directs our sexual/romantic feelings) can be changed. Rather we show that how people understand and label their experiences can influenced by exposure to certain theories of sexual orientation, which arguably more accurately reflect their underlying feelings.