People want to match their behaviour with their attitude. Usually we think behaviour follows attitude, but if behaviour is constrained in some way, people tend to warp their attitude to fit the behaviour. That's one of the arguments for dress codes in the workplace, it puts you in the role, and also explains why you feel happier when you smile, etc.
Also, we sound subtly different when smiling, it's a bio-physical thing too, not just a manifestation of psychological state. I believe most people can actually detect if the person on the phone is smiling or not.
There's probably specific industrial/organisational psych research on the topic, but more broadly: http://en.wikipedia.org/wiki/Cognitive_dissonance
People want to match their behaviour with their attitude. Usually we think behaviour follows attitude, but if behaviour is constrained in some way, people tend to warp their attitude to fit the behaviour. That's one of the arguments for dress codes in the workplace, it puts you in the role, and also explains why you feel happier when you smile, etc.
Also, we sound subtly different when smiling, it's a bio-physical thing too, not just a manifestation of psychological state. I believe most people can actually detect if the person on the phone is smiling or not.