A commonly-held belief posits there are six basic emotions universally recognized through specific facial expressions. New research by scientists at the University of Glasgow challenged this view, suggesting that there are only four basic emotions.

In Dr Paul Ekman's theory, the six emotions are: happiness, sadness, fear, anger, surprise and disgust. However, the new study, published in Current Biology, claimed 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.

Their conclusion was reached by studying the range of different muscles within the face involved in signalling different emotions, as well as the time-frame over which each muscle was activated.

"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, "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."

The researchers pointed out that, "basic' facial expression signals are perceptually segmented across time and follow an evolving hierarchy of signals over time. 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.