The human interest in faces may have originated, as Konner (1982) suggests, from a need to unravel kinship as we looked for shared ancestry or not (friend or foe); but for millions of years, the main reason people have looked at one another’s face is to read emotion and attend to messages.
Humans produce over 20,000 different facial expressions but how many are shared? Back in 1872 Charles Darwin wrote in his ground-breaking book, ‘The Expression of the Emotions in Man and Animals’, of there being several universal facial expressions. In the 1960s, Paul Ekman sought to prove or disprove Darwin’s theory. Ekman travelled to Papua New Guinea to study the isolated Fore people. People who had never previously had any contact with outsiders, not even viewing pictures of them (still or moving). Staying with the group as he studied them, Ekman discovered what their facial expression would be in different circumstances, such as if they were preparing to fight, witnessing a child dying, or stepping on a smelly dead pig. His research suggested that Darwin was correct in his belief in the existence of universal facial expressions. Conducting many cross-cultural studies, Ekman established that six basic emotions had recognisable facial expressions: Happiness, Sadness, Anger, Surprise, Fear and Disgust. He later added contempt (Ekman, 1987).
Technically, a facial expression lasts longer than one second, otherwise it’s a micromomentary expression or micro-expression, first identified by Haggard and Isaacs (1966). Whatever the classification, expressions are usually fleeting, dynamic and spontaneous. There is evidence that people react in similar ways to seeing certain expressions. For example, Murphy & Zajonc (1993) found that people’s reactions differed depending on whether they saw a smile or a frown. The stronger the sender’s facial expressions, the more emotion is evoked in the viewer, as found Wild et al (2001), who also noted that women are more susceptible to this emotional contagion than men. Responses even occur when seeing a fleeting expression that doesn’t register consciously, as explained by Dimberg et al (1999) who found that we mimic a happy or angry face unconsciously.
Facial expressions can also evoke the actual feelings of the emotion being expressed. Findings by Strack, Martin & Stepper (1988) and McIntosh (1996), demonstrate a facial feedback theory, in that people’s moods can be influenced by their own facial expression. Given that any feeling is a manifestation of a nonconscious process (LeDoux, 2015), the feedback theory indicates the power of facial expressions.
Do universal facial expressions really exist?
There is disagreement in the scientific community on how emotions are expressed. Whilst there is controversy over the ‘established’ view of the universality of facial expressions in response to experiencing basic emotions, there is strong evidence that these basic expressions are recognisable worldwide. So, even if people don’t show the predicted facial expressions, they are quite good at labelling the emotions when presented with ‘typical’ expressions, with happiness being the easiest to recognise.
On the origin of universal facial expressions - what happens and why?
Speculation on the origin of any facial expression may provide clues to its possible universality. It’s true that through practice, culture and ritual can change automatic behaviour, as found Morsbach (1973) but, regardless of culture, humans across the world have the same facial configuration and how the face is positioned affects the senses in certain ways.
Happiness Facial Expression – non-threatening and ‘high’
From the age of five weeks old, humans smile as a way of pleasing others (keeping a parent’s attention), and we use smiles in many ways, but what of the universality of a happiness expression.
Happiness begins within - from serotonin, dopamine and oxytocin – and it manifests with a naturally emerging symmetrical Duchenne (authentic) smile. This involves the activation of the lips’ corners pulling up and causing folds or dimples to form - from the nostrils to the lips and the eyes to the cheeks - and a tightening of the lower eyelids due to a contraction of the eye muscles (orbicubris oculi), creating wrinkling around the outer eye corners as the eye area widens. This results in a loosening of the forehead’s furrow lines and a relaxing of the mouth with full lips. Universally, people read a smiling face with red cheeks/temples and a little blue around the chin, as happy.
To read a Duchenne (natural/authentic) smile, look for an activated orbicularis oculi which is seen
when the fleshy part of the eye, between brow and eyelid, moves down and the
end of the eyebrows dip slightly. Unlike the orbicularis oculi, the muscles
that engage the side of the face (the zygomatic muscles) are easily controlled
consciously.
In the words of Charles Darwin, “In joy the face expands.” When people are happy and smiling their cheeks are raised and look fuller. The face’s centre of gravity raises up as the face widens and lifts, and I think this anti-gravity action is important in the origin of the smile. When people are feeling high, lifted or in a raised mood, their bodies reflect this. The smile is a way for the face to also reflect this mood. The zygomaticus majors, which pull back the mouth and enlarge the cheeks, produce the socially bonding and infectious smile. People with naturally big cheeks often look happy or jolly.
In the words of Charles Darwin, “In joy the face expands.” When people are happy and smiling their cheeks are raised and look fuller. The face’s centre of gravity raises up as the face widens and lifts, and I think this anti-gravity action is important in the origin of the smile. When people are feeling high, lifted or in a raised mood, their bodies reflect this. The smile is a way for the face to also reflect this mood. The zygomaticus majors, which pull back the mouth and enlarge the cheeks, produce the socially bonding and infectious smile. People with naturally big cheeks often look happy or jolly.
A more established theory of origin is that the smile derives
from a submissive appeasement gesture from before we became human. Non-human
primates stretch their mouths back to reveal their teeth in a closed and
non-threatening position, evolving from a warning grimace, showing the teeth
and threatening a bite. The smile, therefore, could have evolved to signal safe
play, the absence of threat. Smiles are submissive.
Sadness Facial Expression – ‘low’ and adapting
If happiness is seen on a lifted (gravity-defying) face, then the sadness expression can be seen as the antithesis. In the words of Charles Darwin, “In grief [the face] lengthens.”
When a person feels sad the centre of gravity in their face lowers. Faces drop (“keep your chin up”). As Riskind (1984) found, “A slumped posture follows defeat and a straight posture follows success.” If facial expressions are universal, I might say that a lowered face follows defeat and a lifted face follows success. Riskind (1984) suggested that slumping is an adaptive response, allowing the person to gather their thoughts and recover internally. I would argue that a sad expression serves a similar purpose and also signals to others that a suitable response is desired. The sadness face shows that the person’s body’s metabolism has slowed due to a loss of energy and that they are feeling low. On seeing this expression people tend to avoid the sad person, making little eye contact with them and leaving them alone to their thoughts, adapting to their situation.
The recognised expression for sadness sees the mouth drop, the corner of the lips downturned, cheeks tensed to offer a slight squint with the inner corners of the eyebrows (glabella side) raised, eyes gazing downwards. The inner eyelids droop, and above, on the forehead, you may see Darwin’s ‘grief muscle’, the forehead mark that resembles a horseshoe. General forehead wrinkles tend to slope down from the centre of the face as a frown is formed.
As with the smile, the lips of sadness are typically consciously
controlled whilst the eye muscles are often only engaged when the emotion is
real. When looking for genuine sadness, ignore the lips and look how the eyes
and eyebrows are engaged. A slight or brief feeling of sadness can be seen on
the lips alone, as sadness, like happiness, grows. A pout can indicate that a
person is beginning to feel sad or sulky. The bottom lip is pushed out like a
shrug, a slight protrusion that’s often the first sign of sadness. In time the
eyes become involved if sadness develops.
In both sadness and happiness, it’s the eyebrows that provide the most readable and reliable signs. Dogs can recognise both of these ancient expressions and even exhibit a similar sadness ‘lowering’ expression when they feel sad.
Surprise Facial Expression – locate and prepare
When confronted with a sudden, unexpected event, the surprise expression allows for more sensory information in order to make the best action plan.
An easily and universally recognisable expression, despite its fleeting nature, surprise is seen in arched eyebrows and raised eyelids, a horizontally wrinkled reddening forehead and an open mouth and blue tinted chin.
Sadness Facial Expression – ‘low’ and adapting
If happiness is seen on a lifted (gravity-defying) face, then the sadness expression can be seen as the antithesis. In the words of Charles Darwin, “In grief [the face] lengthens.”
When a person feels sad the centre of gravity in their face lowers. Faces drop (“keep your chin up”). As Riskind (1984) found, “A slumped posture follows defeat and a straight posture follows success.” If facial expressions are universal, I might say that a lowered face follows defeat and a lifted face follows success. Riskind (1984) suggested that slumping is an adaptive response, allowing the person to gather their thoughts and recover internally. I would argue that a sad expression serves a similar purpose and also signals to others that a suitable response is desired. The sadness face shows that the person’s body’s metabolism has slowed due to a loss of energy and that they are feeling low. On seeing this expression people tend to avoid the sad person, making little eye contact with them and leaving them alone to their thoughts, adapting to their situation.
The recognised expression for sadness sees the mouth drop, the corner of the lips downturned, cheeks tensed to offer a slight squint with the inner corners of the eyebrows (glabella side) raised, eyes gazing downwards. The inner eyelids droop, and above, on the forehead, you may see Darwin’s ‘grief muscle’, the forehead mark that resembles a horseshoe. General forehead wrinkles tend to slope down from the centre of the face as a frown is formed.
In both sadness and happiness, it’s the eyebrows that provide the most readable and reliable signs. Dogs can recognise both of these ancient expressions and even exhibit a similar sadness ‘lowering’ expression when they feel sad.
Surprise Facial Expression – locate and prepare
When confronted with a sudden, unexpected event, the surprise expression allows for more sensory information in order to make the best action plan.
An easily and universally recognisable expression, despite its fleeting nature, surprise is seen in arched eyebrows and raised eyelids, a horizontally wrinkled reddening forehead and an open mouth and blue tinted chin.
The key feature is the eyes becoming as large/open as
possible, this allows for the cause of the surprise to be located through the
maximised peripheral vision. A 2013 study at Cornell University measured how
much light hits the retina. The study suggested that our expressions control
our sensory input. In the case of surprise, it is to allow for as much light or
brain-visual information as possible.
If the eyes are made able to make a visual sweep, the mouth has a preparatory role. A surprise expression prepares the body for potential fight or flight, the mouth being open to either breathe in more oxygen or yell for help (or warn others).
It doesn’t take long for surprise to disappear and a course of action (or no action) be enacted. Surprise can quickly become fear, amusement, relief or anger. Unlike the startle response, surprise doesn’t assume the worst and seek to protect us, it helps us make a quick unconscious decision.
Anger Facial Expression – threaten and protect
The anger facial expression can be thought of as a universally recognisable look people make when they wish to appear threatening. It would not have served humans well to have been constantly fighting but through threat fights could be avoided. In order to threaten successfully, a person needed to look like they were ready for action, and that means a face that’s protected. The anger expression is a facial configuration that tries to protect the eyes and mouth. If fighting is required it often marks a failure of this threat display. The anger display is typically used to get others to change their behaviour.
A tense, tight mouth, with gritted teeth, helps to protects the lips/teeth/tongue. The furrowed brow, moving over the eye sockets, helps to protect the eyes. And there are other physical benefits of the anger face. The eyes squint (and pupils constrict) not only for protection but to making the focus of the anger come into sharper view; and the nostrils flare (nasal wing dilation), a sign of the body oxygenating in advance of potential action (fight or flight). Nostril flaring is an intention cue that the person is about to engage in a physical act (not always in anger). A jutting and tense jaw is another sign of aggression. If a person’s face is angled ‘face on’ and their chin is thrust forward, it can be threatening. By drawing attention to the chin (which now appears larger) they are unconsciously displaying a dominance cue (testosterone levels affect chin size). In terms of colouring: the sides of an angry person’s forehead may have a blue tint whilst the centre of their forehead may redden, whilst the area around their eyes may darken with patches of blue and red.
If the eyes are made able to make a visual sweep, the mouth has a preparatory role. A surprise expression prepares the body for potential fight or flight, the mouth being open to either breathe in more oxygen or yell for help (or warn others).
It doesn’t take long for surprise to disappear and a course of action (or no action) be enacted. Surprise can quickly become fear, amusement, relief or anger. Unlike the startle response, surprise doesn’t assume the worst and seek to protect us, it helps us make a quick unconscious decision.
Anger Facial Expression – threaten and protect
The anger facial expression can be thought of as a universally recognisable look people make when they wish to appear threatening. It would not have served humans well to have been constantly fighting but through threat fights could be avoided. In order to threaten successfully, a person needed to look like they were ready for action, and that means a face that’s protected. The anger expression is a facial configuration that tries to protect the eyes and mouth. If fighting is required it often marks a failure of this threat display. The anger display is typically used to get others to change their behaviour.
A tense, tight mouth, with gritted teeth, helps to protects the lips/teeth/tongue. The furrowed brow, moving over the eye sockets, helps to protect the eyes. And there are other physical benefits of the anger face. The eyes squint (and pupils constrict) not only for protection but to making the focus of the anger come into sharper view; and the nostrils flare (nasal wing dilation), a sign of the body oxygenating in advance of potential action (fight or flight). Nostril flaring is an intention cue that the person is about to engage in a physical act (not always in anger). A jutting and tense jaw is another sign of aggression. If a person’s face is angled ‘face on’ and their chin is thrust forward, it can be threatening. By drawing attention to the chin (which now appears larger) they are unconsciously displaying a dominance cue (testosterone levels affect chin size). In terms of colouring: the sides of an angry person’s forehead may have a blue tint whilst the centre of their forehead may redden, whilst the area around their eyes may darken with patches of blue and red.
There are universal tells that something is wrong or that
the person is seeing/hearing something they don’t like. These tells are lip
compression (disappearing lips) and squinting (with pupil constriction). These
tells are often fleeting, as quick as 1/8 of a second and, of course, their
presence alone is not enough to draw conclusions. Yes, eyes may squint as a cue
of negativity but this squinting could have a different cause, such as the sun
or concentration.
Fear Facial Expression - in preparation (fight or flight)
I believe that the fear facial expression would have evolved after the surprise and anger/threat expressions. In fear, the tension in the face is as a result of the eyes and mouth trying to do two things at once. The eyes are trying to open, to see the object of the fear (perhaps a predator) but they are also trying to tighten/squint for protection (from that object of fear), resulting in a wide-eyed look, the whites of the eyes visible. The eyelids rise up high (to open the eyes) and the eyebrows lower (to protect the eyes) resulting in tense, drawn-together eyebrows. The mouth tries to open, to yell for help or warn, or inhale oxygen for fight or flight. But the mouth is also trying to close, to protect the lips/teeth/tongue from the object of fear. Hence, we see a tense mouth with horizontally stretched lips as the face tries to adopt two adaptations simultaneously.
In a state of fear, the cognitive brain is hijacked as the limbic brain readies the face in forming the best response. When experiencing fright, a person’s first response (before flight or fight) is to freeze, and the fear facial expression is a frozen one, albeit fleeting. Neither open nor closed the eyes/mouth can quickly form a new facial configuration, the eyebrows already down (if fight), the eyelids already raised (if flight). Whichever option is preferred (open/closed eyes/mouth) the active muscles in the fear face are primed to act. Of all our emotions, it is perhaps fear that has our systems 1 and 2 most at odds with one another. Our instinct is to reassure or remove the route of fear, but we also know that fears, at times, must be faced. The two paths don’t always reach same conclusions. If we constantly give in to our fears we never do or achieve anything. Epstein (1972) may have been right in saying that “The purpose of fear is that it motivates avoidance and escape,” but this isn’t always in our best interest. It’s a benefit of the freeze response that we gain a moment to decide how to react.
In addition, as the fear face itself is pulled back, so the head is slightly retracted. This pulling and retracting is instigated by the amygdala and is done before fear, as an emotion, is felt. The other instant fear facial reactions include an exposing of the teeth, the wide eyes and a reddening of the skin. Piloerection (hairs standing on their end) also occur but this is difficult to see.
The fear expression warns others and readies their survival circuits. Phelps et al (2006) found that when a person sees someone exhibiting a fear expression their own vision is improved temporarily.
Disgust Facial Expression – deny and warn
Disgust produces a nose crinkle at the same time the upper lip (or one side of it) rises. There may be a slight blue-yellow appearance around the lips, with a red-green tint around the nose and forehead. When we see, hear or smell something we deem to be disgusting, our anterior insula fires as it would if we were to taste something horrible, as if blocking poison from being swallowed.
People automatically try to block out that which is harmful.
Disgust works on the same principle as eye blocking, when people cover their
eyes, turn their face away or experience pupillary constriction at witnessing
something ugly or horrific. The origin of disgust is that the mouth closing and
nose crinkling reduces the flow of air, down to a just enough to locate an offending
smell. An exception to the closed mouth could be a protruding tongue, more
common in infants refusing food, but a behaviour which could become a habitual
response.
That the disgust face is pulled in response to an unappealing vision or noise could be to do with smell occurring first in the evolution of senses. The disgust seen on someone’s face can act as a warning to others not to taste or smell (breathe) the cause, and this is its origin or adaptive advantage.
Disgust arises easily. Unlike with anger a gradual build up is rare. One taste of poison could kill, so it’s important to display disgust instantly, regardless of the person’s mood. It also doesn’t take much to trigger disgust. As Rozin (2001) demonstrated, a single cockroach is enough to spoil a bowl of cherries.
Contempt Facial Expression – concealed superiority
The last of the universal emotions to appear in child development is contempt. It looks like a smirk (or a snarl missing the nose wrinkle). The corner of the mouth tightens on one side, producing a half smile, perhaps with one dimple, the one cheek pulling up towards its nearest eye/ear. The smile element to this expression can indicate the enjoyment of feeling superior, its one-sided nature signalling the person’s desire to keep this feeling hidden. If contempt is universal, I think it’s this act that makes it so: for if smiling is universal then trying to conceal the smile can also be. Interestingly, it’s typically the face’s left ‘dimpler’ muscle that contracts to raise up the corner of the mouth, and this is controlled by the right - emotion/instinct - side of the brain.
This superiority is a perceived one, perhaps of being a
higher social status individual than the person they are conversing with,
especially if they deem that person to be incompetent to a point of not wanting
to engage with them. This is tied in with a lack of trust, the ‘contempt’
expression being a possible cue that the person is feeling distrustful of
another. An easier to read sign of contempt is the more conscious rolling of
the eyes or looking askance.
The contempt face is also one that often tilts back as the chin raises, allowing the eyes to look down at the object of contempt. When challenged, this emotion can easily become hatred or anger. In this instance the raised chin may jut forward.
The case for the universality of facial expressions has been boosted by research into congenitally blind people, such the paper by Eibl-Eibesfeldt (1970). Whilst it may still be argued that facial expressions can be learned, despite the sender being blind from birth, the matching of their spontaneous facial expressions with the expected expressions is significant.
Konner, M (1982) The Tangled Wing: Biological Constraints on the Human Spirit. Holt.
Darwin, C (1872) The Expression of the Emotions in Man and Animals.
Ekman, P (1987) What The Face Reveals by Paul Ekman (Editor) and Erika L. Rosenberg (Editor)
Murphy, S. T., & Zajonc, R. B. (1993). Affect, Cognition, and Awareness: Affective Priming with Optimal and Suboptimal Stimulus Exposures. Journal of Personality and Social Psychology, 64, 723-739.
Wild, B. Erb, M and Bartels, M (2001) “Are emotions contagious? Evoked emotions while viewing emotionally expressive faces: Quality, quantity, time course and gender differences” Psychiatric Research 102, no. 2: 109-24.
Dimberg, U. M Thunberg & K Elmehed (1999) ‘Unconscious facial reactions to emotional facial expressions,’ Psychological Science, Research Report, Uppsala University
Strack, F., Martin, L, and Stepper S. (1988) Inhibiting and facilitating conditions of the human smile: a nonobtrusive test of the facial feedback hypothesis. J Pers Soc Psychol, 54(5):768-77.
McIntosh, D N. (1996). Facial feedback hypothesis: Evidence, Implications, and directions. Motivation and Emotion. 20(2):121–147.
LeDoux, J (2015) Anxious: The Modern Mind in the Age of Anxiety. Oneworld Publications
Morsbach, H (1973) Aspects of nonverbal communication in Japan. J Nerv Ment Dis 157: 262–277.
Epstein, S. (1972). The nature of anxiety with emphasis upon its relationship to expectancy. In C. D. Spielberger (Ed.), Anxiety: Current trends in theory and research (Vol. 2, pp. 292-338). New York: Academic Press.
Phelps, L. Ling, S and Carrasco, M. (2006). ‘Emotion Facilitates Perception and Potentiates the Perceptual Benefits of Attention’. Psychological Science 17: 292-99.
Riskind, J. H. (1984). ‘They stoop to conquer: guiding and self regulatory functions of physical posture after success and failure’. Journal of Personality and Social Psychology, 47 (3), 479-93.
Rozin, P and Royzman, E B (2001) “Negativity Bias, Negativity Dominance, and Contagion.” Personality and Social Psychology Review 5: 296-320.
Eibl-Eibesfeldt, I (1970) Ethology, the Biology of Behavior. New York: Holt, Reinhart & Winston.
Fear Facial Expression - in preparation (fight or flight)
I believe that the fear facial expression would have evolved after the surprise and anger/threat expressions. In fear, the tension in the face is as a result of the eyes and mouth trying to do two things at once. The eyes are trying to open, to see the object of the fear (perhaps a predator) but they are also trying to tighten/squint for protection (from that object of fear), resulting in a wide-eyed look, the whites of the eyes visible. The eyelids rise up high (to open the eyes) and the eyebrows lower (to protect the eyes) resulting in tense, drawn-together eyebrows. The mouth tries to open, to yell for help or warn, or inhale oxygen for fight or flight. But the mouth is also trying to close, to protect the lips/teeth/tongue from the object of fear. Hence, we see a tense mouth with horizontally stretched lips as the face tries to adopt two adaptations simultaneously.
In a state of fear, the cognitive brain is hijacked as the limbic brain readies the face in forming the best response. When experiencing fright, a person’s first response (before flight or fight) is to freeze, and the fear facial expression is a frozen one, albeit fleeting. Neither open nor closed the eyes/mouth can quickly form a new facial configuration, the eyebrows already down (if fight), the eyelids already raised (if flight). Whichever option is preferred (open/closed eyes/mouth) the active muscles in the fear face are primed to act. Of all our emotions, it is perhaps fear that has our systems 1 and 2 most at odds with one another. Our instinct is to reassure or remove the route of fear, but we also know that fears, at times, must be faced. The two paths don’t always reach same conclusions. If we constantly give in to our fears we never do or achieve anything. Epstein (1972) may have been right in saying that “The purpose of fear is that it motivates avoidance and escape,” but this isn’t always in our best interest. It’s a benefit of the freeze response that we gain a moment to decide how to react.
In addition, as the fear face itself is pulled back, so the head is slightly retracted. This pulling and retracting is instigated by the amygdala and is done before fear, as an emotion, is felt. The other instant fear facial reactions include an exposing of the teeth, the wide eyes and a reddening of the skin. Piloerection (hairs standing on their end) also occur but this is difficult to see.
The fear expression warns others and readies their survival circuits. Phelps et al (2006) found that when a person sees someone exhibiting a fear expression their own vision is improved temporarily.
Disgust Facial Expression – deny and warn
Disgust produces a nose crinkle at the same time the upper lip (or one side of it) rises. There may be a slight blue-yellow appearance around the lips, with a red-green tint around the nose and forehead. When we see, hear or smell something we deem to be disgusting, our anterior insula fires as it would if we were to taste something horrible, as if blocking poison from being swallowed.
That the disgust face is pulled in response to an unappealing vision or noise could be to do with smell occurring first in the evolution of senses. The disgust seen on someone’s face can act as a warning to others not to taste or smell (breathe) the cause, and this is its origin or adaptive advantage.
Disgust arises easily. Unlike with anger a gradual build up is rare. One taste of poison could kill, so it’s important to display disgust instantly, regardless of the person’s mood. It also doesn’t take much to trigger disgust. As Rozin (2001) demonstrated, a single cockroach is enough to spoil a bowl of cherries.
Contempt Facial Expression – concealed superiority
The last of the universal emotions to appear in child development is contempt. It looks like a smirk (or a snarl missing the nose wrinkle). The corner of the mouth tightens on one side, producing a half smile, perhaps with one dimple, the one cheek pulling up towards its nearest eye/ear. The smile element to this expression can indicate the enjoyment of feeling superior, its one-sided nature signalling the person’s desire to keep this feeling hidden. If contempt is universal, I think it’s this act that makes it so: for if smiling is universal then trying to conceal the smile can also be. Interestingly, it’s typically the face’s left ‘dimpler’ muscle that contracts to raise up the corner of the mouth, and this is controlled by the right - emotion/instinct - side of the brain.
The contempt face is also one that often tilts back as the chin raises, allowing the eyes to look down at the object of contempt. When challenged, this emotion can easily become hatred or anger. In this instance the raised chin may jut forward.
The case for the universality of facial expressions has been boosted by research into congenitally blind people, such the paper by Eibl-Eibesfeldt (1970). Whilst it may still be argued that facial expressions can be learned, despite the sender being blind from birth, the matching of their spontaneous facial expressions with the expected expressions is significant.
Konner, M (1982) The Tangled Wing: Biological Constraints on the Human Spirit. Holt.
Darwin, C (1872) The Expression of the Emotions in Man and Animals.
Ekman, P (1987) What The Face Reveals by Paul Ekman (Editor) and Erika L. Rosenberg (Editor)
Murphy, S. T., & Zajonc, R. B. (1993). Affect, Cognition, and Awareness: Affective Priming with Optimal and Suboptimal Stimulus Exposures. Journal of Personality and Social Psychology, 64, 723-739.
Wild, B. Erb, M and Bartels, M (2001) “Are emotions contagious? Evoked emotions while viewing emotionally expressive faces: Quality, quantity, time course and gender differences” Psychiatric Research 102, no. 2: 109-24.
Dimberg, U. M Thunberg & K Elmehed (1999) ‘Unconscious facial reactions to emotional facial expressions,’ Psychological Science, Research Report, Uppsala University
Strack, F., Martin, L, and Stepper S. (1988) Inhibiting and facilitating conditions of the human smile: a nonobtrusive test of the facial feedback hypothesis. J Pers Soc Psychol, 54(5):768-77.
McIntosh, D N. (1996). Facial feedback hypothesis: Evidence, Implications, and directions. Motivation and Emotion. 20(2):121–147.
LeDoux, J (2015) Anxious: The Modern Mind in the Age of Anxiety. Oneworld Publications
Morsbach, H (1973) Aspects of nonverbal communication in Japan. J Nerv Ment Dis 157: 262–277.
Epstein, S. (1972). The nature of anxiety with emphasis upon its relationship to expectancy. In C. D. Spielberger (Ed.), Anxiety: Current trends in theory and research (Vol. 2, pp. 292-338). New York: Academic Press.
Phelps, L. Ling, S and Carrasco, M. (2006). ‘Emotion Facilitates Perception and Potentiates the Perceptual Benefits of Attention’. Psychological Science 17: 292-99.
Riskind, J. H. (1984). ‘They stoop to conquer: guiding and self regulatory functions of physical posture after success and failure’. Journal of Personality and Social Psychology, 47 (3), 479-93.
Rozin, P and Royzman, E B (2001) “Negativity Bias, Negativity Dominance, and Contagion.” Personality and Social Psychology Review 5: 296-320.
Eibl-Eibesfeldt, I (1970) Ethology, the Biology of Behavior. New York: Holt, Reinhart & Winston.