French neurologist Guillaume Duchenne used electrical stimulation to trigger facial expressions.

A more contemporary expert on facial emotions is psychology professor Dr Paul Ekman. He has developed a Facial Action Coding System (FACS) to determine which of the 43 different muscles in the face are used for different emotions. His work has many practical applications: the FBI has often asked for his help in reading the faces of suspected criminals and he has been consulted by animators from Pixar who are trying to create life-like characters.

But Ekman has also found out more about the Duchenne smile. It seems that in addition to the different facial muscles involved, a different area of the brain is activated: the left prefrontal cortex where positive emotions come from. And the connection between a smile and this part of the brain can work backwards as well: by putting on a true smile, you can activate this pleasure center and make yourself happy.

It may sound too good to be true, and actually it is. A spontaneous smile activates more areas than a self-produced one - and so the two are not completely equivalent. There is no simple route to happiness - just a few clues to detect those who are faking it...

For more info:

Want to see if you can detect a fake smile? Try this online experiment:
BBC - Spot the Fake Smile
http://www.bbc.co.uk/science/humanbody/mind/surveys/smiles/

The New York Sun - A Smile Detector
http://www.nysun.com/article/25251