Scientists have mapped a total 21 different emotions we express in our faces - from happy and sad to 'happily disgusted' and 'sadly angry'.
Using new computer software, the research conducted at Ohio State University in the US more than triples the number of known emotional facial expressions.
In future, the computer model could aid the diagnosis and treatment of mental 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 Dr Martinez.
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".
Here is the full list of emotional states identified by the scientists from facial expressions:
- Happily surprised
- Sadly fearful
- Sadly angry
- Sadly surprised
- Sadly disgusted
- Fearfully angry
- Fearfully surprised
- Fearfully disgusted
- Angrily surprised
- Angrily disgusted
- Disgustedly surprised