A commonly-held belief, first proposed by Dr Paul Ekman, posits there are six basic emotions which are universally recognised and easily interpreted through specific facial expressions, regardless of language or culture. (Agencies)
These are: happiness, sadness, fear, anger, surprise and disgust.
Scientists at the University of Glasgow, UK have reduced the list to four, after deciding that fear and surprise could be combined because they both cause wide eyes, while anger and disgust make us wrinkle our noses in a similar way.
The researchers studied the range of different muscles within the face - or Action Units - involved in signaling different emotions, as well as the time-frame over which each muscle was activated.
This is the first such study to objectively examine the 'temporal dynamics' of facial expressions, made possible by using a unique Generative Face Grammar platform.
The team claim that while the facial expression signals of happiness and sadness are clearly distinct across time, fear and surprise share a common signal - the wide open eyes - early in the signalling dynamics.
Similarly, anger and disgust share the wrinkled nose. It is these early signals that could represent more basic danger signals.
Later in the signalling dynamics, facial expressions transmit signals that distinguish all six 'classic' facial expressions of emotion.
"Our results are consistent with evolutionary predictions, where signals are designed by both biological and social evolutionary pressures to optimise their function," lead researcher Dr Rachael Jack said.
The Generative Face Grammar uses cameras to capture a three-dimensional image of faces of individuals specially trained to be able to activate all 42 individual facial muscles independently.
"Our research questions the notion that human emotion communication comprises six basic, psychologically irreducible categories. Instead we suggest there are four basic expressions of emotion," Jack said.
"We show that 'basic' facial expression signals are perceptually segmented across time and follow an evolving hierarchy of signals over time - from the biologically-rooted basic signals to more complex socially-specific signals.
"Over time, and as humans migrated across the globe, socio-ecological diversity probably further specialized once-common facial expressions, altering the number, variety and form of signals across cultures," said Jack.
The study was published in the journal Current Biology.
A commonly-held belief, first proposed by Dr Paul Ekman, posits there are six basic emotions which are universally recognised and easily interpreted through specific facial expressions, regardless of language or culture.