A study by scientists at Glasgow University challenges the commonly held belief that expressions of happiness, sadness, fear, anger, surprise and disgust are identified by specific facial movements, recognised across the world.
The research, published in the journal Current Biology, suggests that there are only four basic emotions.
Their conclusion was reached by studying the range of different muscles within the face - or "action units" as researchers refer to them.
This is the first such study to objectively examine facial expressions' "temporal dynamics", made possible with a unique generative face grammar platform developed by Professor Philippe Schyns, Dr Oliver Garrod and Dr Hui Yu.
The team from the Institute of Neuroscience and Psychology at Glasgow University claim that while the facial expression signals of happiness and sadness are clearly distinct, fear and surprise share a common signal - the wide open eyes.
Similarly, anger and disgust share the wrinkled nose.
Lead researcher Dr Rachael Jack said: "Our results are consistent with evolutionary predictions, where signals are designed by both biological and social evolutionary pressures to optimise their function.
"First, early danger signals confer the best advantages to others by enabling the fastest escape. Secondly, physiological advantages for the expresser - the wrinkled nose prevents inspiration of potentially harmful particles, whereas widened eyes increases intake of visual information useful for escape - are enhanced when the face movements are made early.
"What our research shows is that not all facial muscles appear simultaneously during facial expressions, but rather develop over time supporting a hierarchical biologically-basic to socially-specific information over time."