Researchers at University of Glasgow suggest that a human face can express just four basic expressions rather than six.
The researchers analyzed the range of different muscles, or action units, in the face using a computer program called Generative Face Grammar. Prior to the study, it was thought that a face expresses six different feelings: happiness, sadness, fear, anger, surprise and disgust.
However the researchers said that fear and surprise and anger and disgust are part of the same category and share a common signal, wide open eyes in case of former and wrinkled nose in case of latter. The study has been published in the journal Current Biology.
"What our research shows is 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. Our results are consistent with evolutionary predictions where signals are designed by both biological and social evolutionary pressures to optimize their function", lead researcher Dr Rachael Jack said.