When we’re comfortable with someone, we’ll tend to smile in a more genuine and spontaneous way. Smiling without showing our teeth is more of a forced smile.
This is backed up by a 2009 Dutch study, in which men were filmed performing posed smiles and spontaneous smiles, with the latter provoked by watching a funny film. When the researchers compared the two types of smile, they found that genuine smiles were both wider and toothier than posed ones.
Dr Christian Jarrett is a cognitive neuroscientist, science writer and author. He is the Deputy Editor of Psyche, the sister magazine to Aeon that illuminates the human condition through psychology, philosophy and the arts. Jarrett also created the British Psychological Society's Research Digest blog and was the first ever staff journalist on the Society's magazine, The Psychologist. He is author of Great Myths of The Brain and Be Who You Want: Unlocking the Science of Personality Change.