Those who smile often are thought of as more likeable, competent, approachable, friendly and attractive.
Of 19 different types of smile, only six occur when we’re having a good time. The rest happen when we’re in pain, embarrassed, uncomfortable, horrified or even miserable. A smile may mean contempt, anger or incredulity, that we’re lying or that we’ve lost.

Smile ≠  happy

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Duchenne was interested in the mechanics of facial expressions, including how the muscles of the face contract to produce a smile.The Duchenne‘ smile is long and intense, though it involves the contraction of just two muscles. First the zygomatic major, which resides in the cheek, tugs at the corners of the mouth, then the orbicularis oculi, which surrounds the eye, pulls up the cheeks, leading to the characteristic ‘twinkling eyes’.

Duchenne smile

“When bonobo chimpanzees are afraid they’ll expose their teeth and draw their lips back so that their gums are exposed,” says Zanna Clay, a primatologist at the University of Birmingham.In babies, a broad grin can either mean they’re happy or distressed and studies have shown that men tend to smile more around those considered to be higher status.

Fear smile

The ‘miserable smile’ is a stoical grin-and-bear-it expression – a slight, asymmetric smile with an expression of deep sadness pasted over the top.Since Landis’ classic study, psychologists have found this tell-tale smirk on the faces those watching gory films – they were filmed by a hidden camera – and among patients suffering from depression. It's a socially acceptable way of showing that you’re sad or in pain.

Miserable smile

The dampened smile is an attempt to control an automatic, happy one and exists because some muscles, such as the ones controlling the mouth, are easier to suppress than others. “The cheeks will be raised but we pull the corners of the mouth downwards or press the lips together."Not all cultures invite a broad smile.  In Japan, where etiquette dictates that emotions are stifled in public, there’s a greater emphasis on smiling with the eyes.

The dampened smile

The ‘embarrassed smile’ is identical to the dampened smile, though the two are easily distinguished – if not by the flushed cheeks, then the uncomfortable situation which usually precedes it. Another tell-tale sign is moving the head downwards and slightly to the left.

Embarrassed smile

The ‘qualifier smile’ aims to take the edge off bad news. It begins abruptly, raising the lower lip slightly, and is occasionally accompanied by a slightly downwards and sideways tilt of the head.

Qualifier smile

The ‘contempt smile’ indicates a mixture of disgust and resentment and is disconcertingly similar to a smile of true delight, except for the corners of the lips which appear tightened.

Contempt smile

Translating roughly as ‘malicious joy’, schadenfreude is the thrill of discovering another’s misfortune.“If individuals are alone and feel unobserved, they usually express feelings of schadenfreude by so-called ‘Duchenne smiles’ and ‘Duchenne laughs’,” says Jennifer Hofmann, a psychologist at the University of Zurich.

Angry-enjoyment smile

Most people – around 71% - can voluntarily contract the inner portion of the orbicularis oculi. Judged by facial expressions alone, people are <a href="http://psycnet.apa.org/psycinfo/1974-25177-001">j</a>udged as most truthful when they are lying. As the American humourist Kin Hubbard once said: “If you haven’t seen your wife smile at a traffic cop, you haven’t seen her smile her prettiest.”

Fake smile

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As they hovered over their victims, knives at the ready, Carney Landis issued his instructions. The beheading was ready to commence.
It was 1924 and this particularly sadistic grad student had lured an assortment of fellow pupils, teachers and psychology patients - including a 13-year-old boy - into a room at the University of Minnesota.
To put his subjects at ease he had redecorated, concealing laboratory equipment, draping cloth over the windows and hanging paintings on the walls.
Landis wanted to know if certain experiences, such as pain or shock, always <a href="http://psycnet.apa.org/index.cfm?fa=buy.optionToBuy&amp;id=1926-08202-001">elicited the same facial expressions</a>. And he was prepared to inflict them in order to find out. He sat his subjects down in comfortable chairs, then painted lines on their faces so that he could better see their grimaces.
Over the course of three hours, they were repeatedly photographed while being subjected to a series of bizarre and unpleasant pranks, including placing fireworks under their seats and electrocuting their hands while they felt around in a bucket of slimy frogs. The climax came when he fetched a live white rat on a tray and asked them to cut off its head with a butcher's knife.
Landis' methods were certainly unethical, but perhaps the most uneasy revelation was what he discovered. Even during the most violent tasks, the most common reaction wasn't to cry or rage - it was to smile. He wrote: "So far as this experiment goes I have found no expression other than a smile, which was present in enough photographs to be considered as typical of any situation."
What was going on?
Watch the video above to learn the secrets behind our smiles Fast-forward to 2017 and we're head-over-heels for this simple reflex. Today reminders to 'smile' are ubiquitous, printed on fridge magnets, adverts, self-help books and occasionally hurled at us by well-meaning strangers. Those who smile often are thought of as more likeable, competent, approachable, friendly and attractive.
But the truth is far more sinister. Of 19 different types of smile, only six occur when we're having a good time. The rest happen when we're in pain, embarrassed, uncomfortable, horrified or even miserable. A smile may mean contempt, anger or incredulity, that we're lying or that we've lost.
While genuine, happy smiles exist as a reward for when we've done something helpful to our survival, the 'non-enjoyment' smiles are less about what you're feeling inside and more about what you want to signal to others. "Some evolved to signal that we're cooperative and non-threatening; others have evolved to let people know, without aggression, that we are superior to them in this present interaction," says Paula Niedenthal, a psychologist at the University of Wisconsin-Madison.
Many are polite gestures which demonstrate that we're following the rules. But they can also be an effective way of manipulating others or distracting them from our true feelings. More often than not, the universal symbol of happiness is used as a mask.
Duchenne smile The first steps to decoding this multi-purpose expression came from the 19th Century neurologist Duchenne de Boulogne. He was the son of a French pirate and had a penchant for electrocuting his patients - among other things, he was a founding father of electrotherapy. Duchenne was interested in the mechanics of facial expressions, including how the muscles of the face contract to produce a smile. The best way to study this, he decided, was to attach electrodes to a person's face and jolt their muscles into action.
<blockquote>
 In some places in the world, perceptions of genuine smiles don't seem to depend on the presence of crow's feet at all - Paula Niedenthal 
</blockquote>
The procedure was so painful, initially Duchenne was only able to experiment on the freshly severed heads of revolutionaries. Then one day, quite by chance, he met a middle-aged man with facial insensitivity in a Paris hospital - he had found his human guinea pig. 
In all Duchenne went on to discover 60 facial expressions, each involving its own dedicated group of facial muscles, which he depicted in a series of grisly photographs.
In the most famous of these, the unlucky man has his face contorted into a broad, toothless grin. He looks idiotically happy, with his cheeks pushed up and crow's feet around his eyes. 
It's since become known as the 'felt' or 'Duchenne' smile and it's associated with genuine feelings of pleasure and giddy happiness. The smile is long and intense, though it involves the contraction of just two muscles. First the zygomatic major, which resides in the cheek, tugs at the corners of the mouth, then the orbicularis oculi, which surrounds the eye, pulls up the cheeks, leading to the characteristic 'twinkling eyes'.
But there's a twist. "In some places in the world, perceptions of genuine smiles don't seem to depend on the presence of crow's feet at all," says Niedenthal.
Which brings us to a question which has been baffling scientists for over a century, from Darwin to Freud: are our expressions instinctive and universal, or do they depend on the culture we're born into?
Fear smile One clue comes from our closest cousins. In fact, though the felt smile may seem like the most natural today, some scientists think it may have evolved from an expression with a very different meaning. "When bonobo chimpanzees are afraid they'll expose their teeth and draw their lips back so that their gums are exposed," says Zanna Clay, a primatologist at the University of Birmingham.
The 'silent bared teeth display' looks so much like a smile it's often featured on birthday cards, but in chimpanzees it's a gesture of submission, used by low-status individuals to appease more dominant members of the group. Clay cites a popular <a href="https://www.youtube.com/watch?v=jealP0egJ9k">video of a chimp stealing a rock</a>. "She snuck off with it and then broke out into this big, cheeky grin. It looks like she's laughing, but she's probably nervous," says Clay. 
And though we don't tend to associate smiling with feeling fearful in humans, there are tantalising hints that the fear smile may have lingered on. In babies, a broad <a href="http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0080161">grin can either mean they're happy or distressed</a> and studies have shown that men tend to smile more around those considered to be higher status.
<blockquote>
 The 'miserable smile' is a stoical grin-and-bear-it expression 
</blockquote>
Darwin believed that facial expressions are instinctive, having originally evolved to serve practical functions. For example, raising the eyebrows in surprise increases the field of vision, which may have helped our ancestors to escape ambushes by predators. In chimpanzees, fear smiles show the teeth tightly clamped together - as if to show that they're not about to bite.
To prove his point, Darwin improvised an experiment at his home in Downe, a sleepy village just outside of London. He chose 11 of Duchenne's photographs - the two were in regular correspondence - and asked 20 of his guests to guess which emotion they represented. They unanimously agreed on happiness, fear, sadness and surprise, among others, and Darwin concluded that these expressions are universal.
Miserable smile We now know that smiling is indeed instinctive, but not just when we're happy. The 'miserable smile' is a stoical grin-and-bear-it expression - a slight, asymmetric smile with an expression of deep sadness pasted over the top.
Since Landis' classic study, psychologists have found this tell-tale smirk on the faces those watching gory films - they were filmed by a hidden camera - and among patients suffering from depression. It's a socially acceptable way of showing that you're sad or in pain.
For decades, psychologists believed that this counter-intuitive habit might be learned, but in 2009 a team from San Francisco State University uncovered tantalising evidence that it's programmed into our DNA. 
By analysing more than 4,800 photographs of athletes competing in the Athens Summer Olympic Games, they found that silver medallists who lost their final matches tended to produce these smiles - even if they had been blind from birth.
But it's a little bit more complicated than that. As it turns out, genuine, happy smiling hasn't always been as celebrated as it is today. Back in 17th Century Europe, wearing your emotions openly was considered highly improper; it was an established fact that only the poor smiled with their teeth showing. The 'smile revolution' finally kicked off over a century later in Paris, kick-started by French nobles who were having such a good time in the newly opened coffee houses that they brought the smile back into fashion.
In many parts of the world, this change of etiquette never happened. One common Russian proverb translates as 'smiling with no reason is a sign of stupidity', while a <a href="http://www.actirisinternational.be/documents/NORVEGE%20%E2%80%93%20NORWAY%20-%20NOORWEGEN/Living%20and%20working%20in%20Norway%20(EN).pdf">government leaflet on working in Norway</a> warns that you've been in the country too long if you assume smiling strangers are drunk, insane or American.
<blockquote>
 In Japan, where etiquette dictates that emotions are stifled in public, there's a greater emphasis on smiling with the eyes 
</blockquote>
The dampened smile is an attempt to control an automatic, happy one and exists because some muscles, such as the ones controlling the mouth, are easier to suppress than others. "The cheeks will be raised but we pull the corners of the mouth downwards or press the lips together, like "I shouldn't be smiling",' says Zara Ambadar, a cognitive psychologist at the University of Pittsburgh.
This is thought to explain why in Japan, where etiquette dictates that emotions are stifled in public, there's a greater emphasis on <a href="http://www.sciencedirect.com/science/article/pii/S0022103106000321">smiling with the eyes</a>. Intriguingly, this discrepancy even stretches to the way smiles are typed: vertically, <a href="http://onlinelibrary.wiley.com/doi/10.1111/jcom.12086/full">with a flat mouth and squinting eyes</a>, as opposed to dotted eyes with a curved mouth. This ^_^ instead of this : ).
The urge to smile may be universal, but when it's acceptable to - and how it is interpreted - depends on cultural rules. As usual, Darwin hit the nail on the head, remarking that though facial expressions have been hardwired by evolution, 'once acquired, such movements may be voluntarily and consciously employed as a means of communication.'
Embarrassed smile The 'embarrassed smile' is identical, though the two are easily distinguished - if not by the flushed cheeks, then the uncomfortable situation which usually precedes it. Another tell-tale sign is moving the head downwards and slightly to the left.
Qualifier smile From the check-out assistant who watched you queue for 10 minutes, only to tell you sweetly that 'returns are only available on the fourth floor', to the receptionist who explains that the next available appointment is in a year's time, the 'qualifier smile' aims to take the edge off bad news. 
It begins abruptly, raising the lower lip slightly, and is occasionally accompanied by a slightly downwards and sideways tilt of the head. It's perhaps the most irritating of all the smiles, since it often traps the recipient into smiling back.
It's the spitting image of three others, though they have quite different uses; the compliance smile, often awkwardly deployed by the victims of the qualifier to show that they aren't going to make a fuss, the coordination response smile, which shows agreement, and the listener response smile, which tends to accompany 'mm-hmm' noises and a reassuring nod that you're still paying attention.
Contempt smile Another tricky expression to swallow is the rictus of utter contempt. The 'contempt smile' indicates a mixture of disgust and resentment and is disconcertingly similar to a smile of true delight, except for the corners of the lips which appear tightened.
In East Asian culture, which is less centred around the needs of the individual, negative emotions are often concealed with a smile to maintain social harmony. "Where I'm from in Indonesia, anger is usually not considered socially acceptable. Instead people tend to smile a lot when they're angry," says Ambadar.
Angry-enjoyment smile Translating roughly as 'malicious joy', schadenfreude is the thrill of discovering another's misfortune.
For obvious reasons, this deliciously mischievous emotion is best concealed from others. But that's not always easy. "If individuals are alone and feel unobserved, they usually express feelings of schadenfreude by so-called 'Duchenne smiles' and 'Duchenne laughs'," says Jennifer Hofmann, a psychologist at the University of Zurich.
When we know someone's watching, the best we can do is plaster an expression of anger over the top, resulting in the fixed, creepy grin which has become a staple of horror movie villains. 
This blended expression is just one of several smiles with a similar formula, such as enjoyable-contempt, enjoyable-fear and enjoyable-sadness.
Fake smile Thanks to Duchenne, it's widely held that you can easily spot a fake smile by simply looking to the eyes - he believed that the eye muscle only contracts when we really mean it. But we now know that most people - around 71% - can voluntarily contract the inner portion of the orbicularis oculi.
<blockquote>
 A 2013 study found that simply faking one in a shop mirror makes you more likely to buy what you're trying on 
</blockquote>
"There is nothing intrinsically genuine about Duchenne smiles and evidence shows that they are easily faked," says Alan Fridlund, a psychologist at the University of California, Santa Barbara. Most of us have a lot of practice. Since smiles tend to accompany greetings, we're used to politely lying about our true feelings - saying we're fine, even when we're not - with these expressions fixed on our faces.
And from currying favour in the courtroom to bonding with your boyfriend's nightmare parents and even landing yourself a better job, there are plenty of good reasons to. Smiles are so universally appealing, a 2013 study found that simply faking one in a shop mirror makes you more likely to buy what you're trying on.
In fact, when judged by their facial expressions alone, people are <a href="http://psycnet.apa.org/psycinfo/1974-25177-001">judged as most truthful when they are lying</a>. As the American humourist Kin Hubbard once said: "If you haven't seen your wife smile at a traffic cop, you haven't seen her smile her prettiest."
How can you tell if a smile is authentic?
When we encounter a face in everyday life, our brains instantly compare its geometry to thousands of others that we have encountered, to see which expression it fits. Next we think about the context - is a smile expected? Finally, automatically mimicking their face allows us to test how it makes us feel.
Niedenthal warns against placing too much emphasis on context. ''It is important not to assume that a smile that you see in a situation that wouldn't make you smile is false. It may be genuine for this person in this culture or situation!'
But there are other tell-tale signs. When they're used deliberately, smiles may be too abrupt or too lingering, or occur too soon or too long before the phrase they should accompany. There's more to a convincing smile than squinty eyes and a flash of teeth.
Flirtatious smile No list would be complete without a reference to the most famous smile of all - that depicted in the Mona Lisa. For all its mystery, categorising this vanishing smile is easy. Psychologists have known for decades that Leonardo da Vinci's masterpiece captures an act of flirtation; his sitter smiles radiantly while gazing into the distance, then risks a sideways glance and an 'embarrassed' smile before quickly looking away again.
So the next time someone tells you to 'smile', remember - it's up to you which one you choose. 
If you liked this story, <a href="http://pages.emails.bbc.com/subscribe/?ocid=fut.bbc.email.we.email-signup">sign up for the weekly bbc.com features newsletter</a>, called "If You Only Read 6 Things This Week". A handpicked selection of stories from BBC Future, Earth, Culture, Capital, and Travel, delivered to your inbox every Friday. 
</div>

As they hovered over their victims, knives at the ready, Carney Landis issued his instructions. The beheading was ready to commence. It was 1924 and this particularly sadistic grad student had lured an assortment of fellow pupils, teachers and psychology patients - including a 13-year-old boy - into a room at the University of Minnesota.

Bite‑sized knowledge
to upgrade
your career

There are 19 types of smile but only six are for happiness

Smile ≠ happy

Duchenne smile

Fear smile

Miserable smile

The dampened smile

Embarrassed smile

Qualifier smile

Contempt smile

Angry-enjoyment smile

Fake smile

10 Main Types of Smiles and What They Really Mean

The Superpowers of Your Smile

Smiling Victorians: why it's a myth that our ancestors didn't smile for pictures

Bite‑sized knowledgeto upgradeyour career

There are 19 types of smile but only six are for happiness

Smile ≠ happy

Duchenne smile

Fear smile

Miserable smile

The dampened smile

Embarrassed smile

Qualifier smile

Contempt smile

Angry-enjoyment smile

Fake smile

10 Main Types of Smiles and What They Really Mean

The Superpowers of Your Smile

Smiling Victorians: why it's a myth that our ancestors didn't smile for pictures

Bite‑sized knowledge
to upgrade
your career