Why it is hard to keep a straight face

It really is contagious, say psychologists

If you meet someone who looks angry or happy, it is often hard to remain expressionless yourself - and now scientists believe they know why.

Researchers in Sweden believe your unconscious mind exerts direct control of your facial muscles.

However much you struggle to keep a blank face, your brain may be letting you down.

It is well known that emotions can find their way into your facial expression, body language, and gestures, but researchers at Uppsala University are trying to work out to what extent we are in control of these movements.

Dr Ulf Dimberg told volunteers to react to a series of pictures of expressionless, happy or angry faces.

In a way, the person automatically enacts the expression they see

Professor Ruth Campbell, University College London

They were told to make frowning, smiling or expressionless faces in return.

Often the face they were told to attempt was the opposite of what might be expected - meeting a smile with a frown, or a frown with a smile.

Movements in their facial muscles were measured using equipment that picked up electrical signals from the fibres.

However, the results showed that volunteers simply did not have total control over their facial muscles.

No control

While it was easy to frown back at a picture of an angry man, it was much more difficult to pull a smile.

Even though study subjects were trying consciously to curb their natural reactions, the twitching in their muscles told a different story.

Dr Dimberg describes this as "emotional contagion".

Direct action

A British expert in communication science, Professor Ruth Campbell, from University College London, said the work might support the theory there was a brain "short-cut" called a "mirror neuron".

This appears to start in the part of the brain responsible for the recognition of faces and their expressions, and bypasses the section associated with conscious processing, generating an instant "mimicking" action.

She said: "In a way, the person automatically enacts the expression they see."

Prof Campbell said it was still unknown exactly why humans tended to mirror the expressions, body language and hand movements of others, although it might be a social device which helped them fit in within larger groups.

She said it was also possible the reaction was a left-over from a primitive method of learning involving imitation.