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Happy faces make less of an impression on the brain than fearful ones, new research has shown.
Tests showed the brain becomes aware of a scared face faster than it does one that is smiling. The reaction is thought to be an instinct that evolved as a "threat radar" millions of years ago.
Psychologist Dr David Zald, from Vanderbilt University in Nashville in the US, who co-led the study, said: "There are reasons to believe that the brain has evolved mechanisms to detect things in the environment that signal threat. One of those signals is a look of fear.
We believe the brain can detect certain cues even before we are aware of them, so that we can direct our attention to potentially threatening situations in our environment."
