Happy, sad and angry do not even begin to cover the range of emotions we express in our faces, a study has found.
Using new computer software, scientists mapped no less than 21 emotional states, including apparently contradictory examples such as "happily disgusted" and "sadly angry".
The research more than triples the number of known emotional facial expressions.
Dr Aleix Martinez, from Ohio State University in the US, said: "We've gone beyond facial expressions for simple emotions like happy or sad. We found a strong consistency in how people move their facial muscles to express 21 categories of emotions.
"That is simply stunning. That tells us that these 21 emotions are expressed in the same way by nearly everyone, at least in our culture."
In future, the computer model could aid the diagnosis and treatment of conditions such as autism and post-traumatic stress disorder (PTSD), said the researchers.
Since the time of Aristotle, scholars have tried to understand how and why our faces betray our feelings.
Today, cognitive scientists try to link facial expressions to emotions in order to track the genetic and chemical pathways that govern emotion in the brain.
Until now they have focused on six basic emotions: happy, sad, fearful, angry, surprised and disgusted.
But restricting emotions to just six categories is like painting only using primary colours, said Martinez. The new study, published in the journal Proceedings of the National Academy of Sciences, has tripled the colour palette available to researchers.
"In cognitive science, we have this basic assumption that the brain is a computer," Martinez added. "So we want to find the algorithm implemented in our brain that allows us to recognise emotion in facial expressions.
"In the past, when we were trying to decode that algorithm using only those six basic emotion categories, we were having tremendous difficulty. Hopefully with the addition of more categories, we'll now have a better way of decoding and analysing the algorithm in the brain."
The scientists photographed 230 mostly student volunteers – 100 male, 130 female – making faces in response to verbal cues designed to trigger emotional states.
The words "you just got some great unexpected news", for instance, produced an expression that was happily surprised, while "you smell a bad odour" prompted a disgusted face.
A search was then made for similarities or differences between the 5,000 resulting images and an expression database widely used in body language analysis called the facial action coding system (Facs).
This yielded 21 emotions which included subtle combinations of the basic six.
Tagging prominent landmarks for facial muscles, such as the corners of the mouth or the outer edge of the eyebrow, enabled the scientists to match emotions to movement.
Putting on a happy face was done the same way by almost everyone, with 99% of study participants drawing up the cheeks and smiling. Surprise involved widening the eyes and opening the mouth 92% of the time.
Happily surprised was marked by wide-open eyes, raised cheeks, and a mouth that was both open and stretched into a smile.
Another hybrid emotion, happily disgusted, created an expression that combined the scrunched up eyes and nose of disgusted with the smile of happy.
It was the emotion felt when something "gross" happens that is also incredibly funny, Martinez explained.
He described how the research might help people with PTSD who were likely to be unusually attuned to anger and fear.
"Can we speculate that they will be tuned to all the compound emotions that involve anger or fear, and perhaps be super-tuned to something like 'angrily fearful'?" said Martinez. "What are the pathways, the chemicals in the brain that activate those emotions?
"We can make more hypotheses now, and test them. Then eventually we can begin to understand these disorders much better, and develop therapies or medicine to alleviate them."
Here is the full list of emotional states identified by the scientists from facial expressions: Happy, sad, fearful, angry, surprised, disgusted, happily surprised, happily disgusted, sadly fearful, sadly angry, sadly surprised, sadly disgusted, fearfully angry, fearfully surprised, fearfully disgusted, angrily surprised, angrily disgusted, disgustedly surprised, appalled, hatred, awed.
