Humans have four basic expressions, rather than six, new study finds

The six basic emotions - happiness, sadness, fear, anger, surprise and disgust - may have just gotten a little more basic.

First proposed by Paul Ekman, the commonly-held belief that there are six basic emotions was recently challenged by new research published in Current Biology. The study's authors, scientists from the University of Glasgow, suggest that there are only four basic emotions.

The range of different muscles within the face while signaling emotion, as well as the time frame over which each muscle was activated, was studied using a unique Generative Face Grammar platform developed at the university.

The team's objective examination of "temporal dynamics" was the first of its kind and led to the combination of anger and disgust, as well as fear and surprise, as similar basic emotions.

While happiness and sadness were found to be clearly distinct over time, fear and surprise share a common early signal - the wide open eyes. Similarly, anger and disgust begin with a wrinkled nose.

These four early signals may have formed from early biological evolution, while the six "classic" expressions diverged from later social pressures.

Lead researcher Rachael Jack said: "Our results are consistent with evolutionary predictions, where signals are designed by both biological and social evolutionary pressures to optimize 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," she said. 

The Generative Face Grammar crafted for the study uses cameras to capture a three-dimensional model of individuals trained specifically to be able to activate all 42 muscles of the human face independently. Research participants then viewed recreations of the individuals faces, shown at random, and identified which emotion was being expressed.

It was through this methodology that they found signals for fear and surprise, as well as anger and disgust, were confused at the early stage of transmission.

"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," explained Jack. "Over time, and as humans migrated across the globe, socioecological diversity probably further specialized once-common facial expressions, altering the number, variety and form of signals across cultures."

Looking forward, the researchers intend to focus on facial expressions of different cultures. They have already ascertained that East Asian populations interpret some of the six classical emotions differently than Westerners, placing more emphasis on eye signals than mouth movements.