Written by Sarah MassieM.A. Student The universality of facial expressions has been widely noted. Happiness is ubiquitously signaled by the symmetrical rise of both corners of the lips. If surprised, the eyes widen, and the jaw simultaneously drops. Across cultures, humans produce the same facial expressions and infer they are indicative of the same emotional states (Ekman & Friesen, 1971). Although the list has expanded since its establishment in the early seventies, the original set of emotions includes: joy, sadness, surprise, fear, anger, and disgust. Though you may not find this phenomena earth shattering, given it has been well-known for decades, what has been less explored is why each emotion triggers each specific and unique facial expression. Humans have 42 facial muscles, meaning there are myriad combinations of expressions one could produce. So, how did each expression come to be paired with its emotion? Humans are not the only species that produce elaborate facial expressions. Even dogs make emotive faces, though there’s evidence suggesting this ability wasn’t “naturally” selected in Canis lupus familiaris (Kaminski et al., 2019). Unlike dogs, our facial expressions are not the result of domestication, but can be traced further back in our evolutionary history. Chimpanzees display a complex facial repertoire similar to our own, indicating this trait was likely selected before our ancestors diverged (5-7 million years ago). What adaptive utility did facial expressions grant our primate ancestors? Communication is an appealing candidate. However, this doesn’t explain why the trait was initially selected. For natural selection to shape adaptations, the trait in question (i.e., facial expressions) had to have enhanced the individual's fitness. Only then could the genes regulating the trait disseminate throughout the population and facilitate communication (i.e. by signaling one’s emotional state). No doubt, communication has contributed to the persistence – and refinement – of facial expressions, but this explanation fails to address the question of why facial expressions appear as they do. While contemplating the function of facial expressions, I couldn’t resist morphing my own face into theatrical caricatures of each expression. While simulating a disgust face, I noticed the expression (i.e., wrinkled nose and raised upper lip) form a crude barrier, thwarting the olfactory system’s front door: the nostrils. Despite the notion that humans have a subpar olfactory system (i.e., sense of smell), we extract a vast amount of information from the olfactory environment, which in turn guides our behavior (McGan, 2017). For example, disgusting things are often paired with disgusting scents (e.g., feces, infected sores, rotting food), and our repulsion prompts us to avoid the thing and its repugnant stench. The evolutionary benefit is readily apparent: steer clear of toxic substances and you avoid the cost of contamination. This stream of thought led me to wonder if the disgust face initially functioned to hinder sensory acquisition, an adaptive response to encountering a toxic substance. I began this post thinking I was entering uncharted scientific territory, but Google Scholar quickly disillusioned me (as it usually does). Susskind et al. (2008) tested this sensory regulation hypothesis, which suggests facial expressions first evolved as a way to increase or decrease sensory exposure. The authors reasoned it would be advantageous to enhance sensory vigilance (i.e. widening of the eyes, ears, and mouth) when frightened, and to hinder sensory acquisition when disgusted. Interestingly, the prototypical fear face is the direct inverse of the prototypical disgust face (See Figure 1). Figure 1. Fear (A) and Disgust (B) anti prototypes defined in multidimensional face space (Susskind et al., 2008) Fearful expressions were found to significantly increase the visual field and speed of eye movement compared to neutral and disgusted expressions. Supporting the notion that fearful faces help us rapidly gain more information about our surrounding environment. Next, Susskind et al. (2008) measured nasal inspiratory capacity to test if a wrinkled nose really does form a barricade to the olfactory mucosa. While fearful expressions increased air velocity and volume, disgusted expressions showed a decrease. Further, structural magnetic resonance images (MRI) revealed that fearful expressions dilated the entry to the nasal cavities, while making a disgust face closed off this pathway. This research suggests that fear and disgust expressions may have initially functioned as an adaptation to regulate the biomechanical properties of the face. Although fascinating, it’s important to note that the sensory regulation hypothesis does not account for other facial expressions such as joy, anger, and sadness. However, it’s likely these expressions will also exhibit a pattern of functionality. For example, Sell, Cosmides, and Tooby (2014) investigated the anger face, hypothesizing that this expression should display cues of formidability and strength. Theoretically, perception of strength should increase the angry individual’s bargaining power, giving them more leverage in the triggering dispute. They examined each muscle contraction independently, revealing that faces with the angry features were consistently rated as stronger than control faces. Interestingly, the features that form an angry face—lower brow ridge, prominent cheekbones, wider nose, and enlarged chin— resemble a stereotypical male skull. Although sexual dimorphism is less pronounced in modern humans, there are noticeable sex differences in skull morphology. Over the course of our evolution, males have engaged in more combat, and have denser skulls to prove it. Among other features, male skulls tend to be heavier, have more prominent brow ridges, and squarer jaws. Here’s the kicker: these traits are moderated by testosterone. When males reach puberty, testosterone makes the brow ridge protrude, augments jaw size, and increases facial width-to-height ratio. Further, males with higher levels of circulating testosterone have more masculine facial structures (Pound et al., 2008). Therefore, it seems plausible that the anger face functions to exaggerate cues of facial masculinity and formidability. This may be analogous to piloerection, the technical term for when the hair of a frightened animal rises making them appear larger to potential predators. Although some posit that emotional expressions are mere products of culture (Barett, 2011), evidence of facial expressions in other primates, newborns, and the congenitally blind make this claim hard to maintain (Burrow, 2008; Cechini et al., 2011; Parr & Waller, 2006; Valente, Theurel & Gentaz, 2018). Culture does play a role; for example, something deemed disgusting in one culture may be perfectly normal in another (e.g. entomophagy: consumption of insects). However, the link between the underlying emotion and its paired expression is consistent across cultures. If facial expressions are behavioral outputs tailored by natural selection, they should exhibit signs of functionality. While there’s no debate facial expressions function to communicate one’s emotional state, this explanation fails to address the question that inspired this post. That is, of all the combinations of muscular contractions human faces could produce, how did each emotion become tied to its unique expression? My intuition is that universal facial expressions did not emerge randomly, but instead have an underlying function (beyond communicating one’s internal state). While the function of fear, disgust, and anger faces have been explored, other facial expressions remain enigmatic. I will leave you with the reminder that an evolutionary perspective can shed light on seemingly arbitrary behaviors, facial expressions included. References Barrett L. F. (2011). Was darwin wrong about emotional expressions? Current Directions in Psychological Science, 20(6), 400-406. doi:10.1177/0963721411429125.
Burrow, A. M. (2008). The facial expression musculature in primates and its evolutionary significance. BioEssays, 30, 212–225. https://doi-org.ezproxy.csusm.edu/10.1002/bies.20719. Cecchini, M., Baroni, E., Di Vito, C., & Lai, C. (2011). Smiling in newborns during communicative wake and active sleep. Infant Behavior and Development, 34(3), 417-423. https://doi.org/10.1016/j.infbeh.2011.04.001. Ekman, P., & Friesen, W. V. (1971). Constants across cultures in the face and emotion. Journal of Personality and Social Psychology, 17(2), 124–129. https://doi-org.ezproxy.csusm.edu/10.1037/h0030377 Kaminski, J., Waller, B. M., Diogo, R., Hartstone-Rose, A., & Burrows, A. M. (2019). Evolution of facial muscle anatomy in dogs. Proceedings of the National Academy of Sciences, 116(29). 14677-14681; DOI: 10.1073/pnas.1820653116 McGan, J. P. (2017). Poor human olfaction is a 19th-century myth. Science, 356(6338):eaam726. doi:10.1126/science.aam7263 Parr, L. A., & Waller, B. M. (2006). Understanding chimpanzee facial expression: insights into the evolution of communication, Social Cognitive and Affective Neuroscience, 1(3), 221–228. https://doi-org.ezproxy.csusm.edu/10.1093/scan/nsl031 Pound, N., Penton-Voak, I. S., & Surridge, A. K. (2008). The Royal Society: Biological Studies. 276(1654): 153–159. doi: 10.1098/rspb.2008.0990 Sell, A., Cosmides, L., & Tooby, J. (2014). The human anger face evolved to enhance cues of strength. Evolution and Human Behavior, 35(5), 425-429. https://doi.org/10.1016/j.evolhumbehav.2014.05.008. Susskind, J. M., Lee, S. H., Cusi, A., Feiman, R. Grabski, W., & Anderson, A. K. (2008). Expressing Fear Enhances Sensory Acquisition. Nature Neuroscience, 11(7). 843-850. doi:10.1038/nn.2138. Valente, D., Theurel, A. & Gentaz, E. (2018). The role of visual experience in the production of emotional facial expressions by blind people: a review. Psychonomic Bulletin & Review, 25, 483–497. https://doi-org.ezproxy.csusm.edu/10.3758/s13423-017-1338-0
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