Smile Intensity in Photographs Predicts Longevity - Ernest L. Abel, Michael L. Kruger, 2010
References Ekman, P. (2007). The directed facial action task: Emotional responses without appraisal. In Coan, J.A., Allen, J.J.B. (Eds.), Handbook of emotional elicitation and assessment (pp. 47-53). New York: Oxford University Press. Google Scholar Ekman, P., Friesen, W.V. (1978). The Facial Action Coding System: A technique for the measurement of facial movement.