Written by BRITTANY FLORKIEWICZ and ULRICH H. REICHARD
Smiling is contagious; when people around us smile, we often immediately and spontaneously smile back and feel a sense of happiness. Why do we feel like smiling when someone smiles at us?
What makes smiling fascinating is not only how to smile but that smiles are unfailingly contagious (unless the person being smiled at suppresses the urge to smile back). Most of us have experienced that smiling is contagious, which has also been demonstrated through multiple studies; when friends and family members smile we cannot help it but reciprocate with a matching grin. According to Max Eastman, “a smile is a universal welcome” to be produced and shared. But why do we mimic smiles so easily and without hesitation? The answer is perhaps that we often don’t consciously know why we smile but at another level of being human, we feel that we ‘understand’ what has caused others to smile and perhaps even more importantly we know what it feels like to smile and smiling back unconsciously rewards us with exactly that soaring feeling smiling gives us. Therefore, smiling at each other provides us with a shared positive and emotionally satisfying experience. (Figure 1). Perhaps that is why in a recent TED Talk, Ron Gutman (a social entrepreneur from Stanford University) described smiling as a “superpower” which improves health in humans. He believed that smiling more frequently can lead to reduced stress levels and can also help improve social relationships, a notion which has been shared by other researchers.
The human brain contains hundreds of thousands of nerve cells, but one particular kind may have evolved just to allowing us replicating the emotional state we see in others, for example the happiness we assume when they smile. These are spindle-shaped neurons or simply spindle neurons, which are sometimes also referred to as von Economo neurons or ‘socializing cells’. Spindle neurons’ specialty is that they fire almost instantly when we see others ‘behave’ in ways we can relate to from our own experiences, for example, when someone yawns or smiles, and it is these neurons that ‘make us mirror’ the behavior of others, why they are sometimes also called mirror neurons. When spindle neurons fire we respond with a behavior that is nearly identical to what we see. When we see a smile we may smile back or when we see someone yawning we may feel an immediate urge to also yawn. Spindle neurons may have evolved, because they allowed us to relate to the states of others by repeating their behavior and then (assuming) we experience the same emotion the person experienced that we just copied. Thus, spindle neurons play an important part in our ability to empathize with others.
Allman and colleagues wrote in a recent paper in the Annals of the New York Academy of Sciences, that spindle neurons are activated during bouts of behavioral changes during social interactions. One example given by Allman and colleagues is crying: when infants begin to cry, mothers respond by attempting to comfort them. Identifying the change in state that leads to distress, in this case crying, is an example of a change in behavior. These changes can be subtle, and are often communicated through non-verbal means such as smiling. When spindle neurons are activated in the receiver, we are able to pick up on these signals and acknowledge them through mimicry – the copying of behavior; in this case, by smiling back.
The contagiousness of certain facial expressions has been recognized by anatomists and naturalists preceding the 18th century. In fact, one of the first naturalists who wrote about this phenomenon was Charles Darwin. In his book “The Expression of Emotions in Man and Animals” published in 1872, he suggested that the use of facial mimicry allowed an individual to empathize with others. Since his discovery, these ideas have been developed into the facial feedback and James-Lange hypotheses. These hypotheses state that if a stimulus is produced in the form of a facial expression, and if another individual mimics this facial expression then the brain of the person who mimics the expression will produce the same stimulus and endow the person who copied the behavior with the same emotional experience as the person who initially produced the stimulus. Thus, the person who mimics a facial expression experiences a feedback to his or her own feelings from the expression of another person’s facial expression and hence, emotional state. On a more practical level, the feedback mechanism allows to transfer and modulate detailed facial behavior that cannot be achieved by speech or vocalization alone. Already Darwin acknowledged that the behavior of one person could be transferred through vocal communication to another person, but he speculated that it was species who supplemented their vocalizations with facial expressions that were most successful in transmitting behavior and behavioral intent between individuals. He considered humans to be the most successful species in behavioral transmission, followed by the non-human primates and then other mammals.
Originally, spindle neurons were thought to be exclusive to great apes and humans, implying that only these species would be capable of facial feedback and fine-tuned behavioral transmission. Recent studies, however, challenge this view. Evrard and colleagues in the journal Neuron were able to show that some monkey species possess spindle neurons. Monkeys may therefore also be capable of using and experiencing facial feedback, which may also be important in the lives of monkeys to better understand the behavior and behavioral intent of conspecifics. If not only apes but also monkeys are capable of the act of ‘smiling back’ we must ask if smiling back is perhaps not a human ‘universal welcome’ at all but rather ‘universal welcome’ in all primates?
To answer such question, we must know more about the facial behavior of our closest living relatives. Unlike humans, attention to non-human primate facial expressions has been limited, even though monkeys and apes just like humans use multiple modes of communication. They use vocal, olfactory, tactile, gestural, and facial behavior to manage their everyday social lives. Vocal behavior in non-human primates has gotten the most attention thus far, perhaps because it seems easier to observe and study. Vocalizations can be recorded and analyzed with powerful computer programs. In contrast, the systematic study of facial behavior of non-human primates was only created about 8 years ago with the development of the human FACS.
FACS stands for Facial Action Coding System, which is the systematic collection of facial expressions. The human FACS is a computer program that was created by Paul Ekman to help analyze human facial behavior. The basic idea of the FACS is that individual facial muscle movements produce the various facial expressions. Thus, by exactly recording which facial muscles contracted to create a facial expression, it is possible to operationalize facial behavior and systematically record facial expressions. Although facial expressions are variable in their production and composition, FACS users can be trained to objectively identify any facial expression regardless of duration and intensity. FACS are naturally species specific and now exist for chimpanzee, orangutan, macaque, and gibbon facial behavior.
Gibbons are incredibly fascinating apes because of their diversity. Gibbons can be found all throughout Southeast Asia, from Myanmar to Indonesia (Figure 2). There are over ten species of gibbons, with each having unique colorations and patterns that differ from one another. Gibbons are also known for their beautiful pair duets which they sing in the early mornings to neighbors. Despite being referred to as a ‘lesser ape’, gibbons demonstrate complex communicative behaviors that are also observed in the great apes. A recent study, for example, has shown that gibbon vocalizations have syntax, or regular organizational structure, which can also be observed in humans through spoken sentences. Complex communicative behaviors also occurs in the facial behavior of gibbons. The creation of the gibbonFACS by Waller and colleagues has shown that similarly to humans, gibbons are capable of producing a large variety of facial expressions. Gibbons are capable of producing brilliant ‘smiles’ that can be seen in the happiest of humans. But can they produce a ‘smile’ after observing one? To learn more about the gibbon ‘smile’, we decided to collect behavioral data at the Gibbon Conservation Center using the gibbonFACS, since Facial Action Coding Systems can be used to explore whether other primates can ‘smile back’ by helping users analyze and compare individual facial behavior (Figure 3). Using the Facial Action Coding System can help differentiate various facial behaviors from one another to produce a facial repertoire, or frequently used facial expressions, for each individual.
After collecting over 200 hours of video footage containing thousands of facial expressions, I discovered that gibbons can, in fact, ‘smile back’ at each other! Each gibbon pair housed at the Gibbon Conservation Center uses a unique set of facial expressions from one another. Towards the end of my study, I could identify each pair easily based on facial behavior alone! Not only do pair members use a unique set of facial expressions, but they also produce them at similar rates. This suggests that pair members are often mimicking each other’s’ facial expressions, which is evidence for facial feedback! Like others, our study of gibbon facial behavior suggests that the act of ‘smiling back’ is not unique to our own species as was once thought. Gibbons and other non-human primates have their own ‘welcome face’ that is consistently produced, shared, and understood.
There are still many questions left unanswered about primate facial behavior. Can any other primates ‘smile back’ or is this ability restricted to higher taxa such as apes and perhaps a few monkeys who live in complex societies? And perhaps even at a more basic level, how other organisms smile and what their smiles may mean compared to our, human smiling is an open question. I think it would also be interesting to find out under which circumstances nonhuman primates are capable of ‘smiling back’? To help answer these questions, it is important to continue using systematic tools in measuring facial behavior, such as the FACS, to avoid misidentifying facial expressions. It is also important to be familiar with primate field data collection methods, which include various sampling and recording methods for behavioral data. Recording and analyzing facial behavior in non-human primates is astonishingly difficult, and requires lots of practice and patience. But this practice pays off, because when you train yourself to see the gibbon smile, then you will know what it means to ‘smile back’.
Brittany Florkiewicz is a M.A. student at Southern Illinois University Carbondale studying biological anthropology. Her research focuses on non-verbal communication in primates, with a specific interest in facial behavior.
Dr. Ulrich H. Reichard is an associate professor at Southern Illinois University Carbondale in the department of anthropology. His research focuses on the relationship between ecology, behavior, and cognition in primates from an evolutionary perspective.