According to researchers from the University of Glasgow, the universally-held notion that there are six basic emotions that can be easily identified using facial expressions regardless of language or culture is inaccurate.
By studying the different muscles of the face -- called Action Units -- researchers were able to identify which muscles were activated during different emotions.
Their findings, which have been published in Current Biology, revealed that that there were only four basic human facial expressions used to convey emotion. Happiness and sadness were distinctive expressions, while fear and surprise shared a common signal, as did anger and disgust.
Researchers found that while expressing fear and surprise the common basic signal was wide open eyes. And in the case of anger and disgust it was a wrinkled nose. It is these early signals that were used to represent more basic danger signals, and later in the signaling dynamic different signals were added to express all six emotions.
“Our results are consistent with evolutionary predictions, where signals are designed by both biological and social evolutionary pressures to optimise their function," said lead researcher Dr. Rachael Jack.
During early signaling, these expressions were used to warn others of danger. They also served some physiological purposes -- wide eyes increased intake of visual information and wrinkled nose prevented inhalation of potentially harmful particles.
“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,” Jack said.
The researchers used the Generative Face Grammar that uses cameras to capture a three-dimensional images of the faces of individuals specially trained to be able to activate all 42 facial muscles individually.
“Over time, and as humans migrated across the globe, socioecological diversity probably further specialised once-common facial expressions, altering the number, variety and form of signals across cultures,” said Jack.