Heartfelt Smile (also known as the Duchenne Smile): This is known as the straight from the heart, genuine and sincere smile. The true and impulsive feelings of pure joy, gladness, happiness, amusement and pleasure are spontaneously expressed through this smile. The facial muscles will raise the corners of the mouth, while both the cheeks will be lifted up and at the same time retracting the skin around the eye socket inwards. The stronger the emotion gets, the more distinct this muscle action will become. This heartfelt smile normally only last between two-thirds of a second to four seconds and it is very seldom held longer than that. Font: http://www.egodevelopment.com/choose-your-smile-10-smile-types/