sadness shown in picture Photograph courtesy of Bridge PointOpens in new window

A smile is a facial expression formed by contracting muscles around the mouth and eyes. It often involves upturned lips and sometimes an open mouth.

In humans, a smile may signify feelings of happinessOpens in new window, pleasure, or amusement, but it can also be a sign of anxietyOpens in new window, embarrassmentOpens in new window, or fearOpens in new window. Smiles may serve to communicate greeting, appeasement, or apology.

In animals (including humans and other primates), a snarlOpens in new window (which resembles a smile) may communicate fear, submission, or threatening intentions. For example, a dog baring the teeth might signify a warning. A chimpanzee “smile,” or a grimace with bared teeth, generally signifies fear (Waller, Vick , Bard, & Smith Pasqualini, 2007).

A smile can be produced deliberately, by contracting the facial muscles, or spontaneously, in response to emotional or social stimuli. In 1862, French neurologist Guillaume-Benjamin-Amand Duchenne (also known as Duchenne de BoulogneOpens in new window, 1806–1875) studied the muscles involved in different types of smiles.

Specifically, Duchenne differentiated between involuntary smiles (usually expressing enjoyment) and intentional, deliberate smiles. Duchenne observed that natural, involuntary smiles contracted both the zygomaticus major muscleOpens in new window (raising the corners of the mouth) and the orbicularis oculi muscleOpens in new window (raising the cheeks and forming crow’s feet around the eyes).

Charles Darwin referred to Duchenne’s work in his 1872 book The Expression of the Emotions in Man and AnimalsOpens in new window. Later, American psychologist Paul Ekman suggested that the spontaneous smile described by Duchenne (and Darwin) be referred to as a Duchenne smileOpens in new window (Ekman, Davidson, & Friesen 1990).

A non-Duchenne (or polite) smile only involves the zygomaticus major muscleOpens in new window. Ekman has researched facial expressions extensively using the Facial Action Coding SystemOpens in new window (FACS; Ekman, Friesen, & Hager, 2002).

Ekman notes that Duchenne smiling is more often associated with actual positive feelings such as enjoyment, amusement, happiness, excitement, or interest.

Non-Duchenne smiles are more often associated with negative emotions, may function as social markers (e.g., to communicate appeasement), or may be used to mask feelings or to deceive (Papa & Bonanno, 2008).

Duchenne smiles tend to correspond with activation of left, anterior (front) brain regions, while non-Duchenne smiles more activate right anterior brain regions. Other research has shown that positive emotions tend to activate left brain regions, while negative feelings more often activate the right side (Ekman et al., 1990). Duchenne smiles have been associated with higher levels of social integration and better health and well-being (Papa & Bonanno, 2008).

Human infants have been observed to smile from birth, with a large increase after 10 days of age and a further increase after about two months of age. Younger infants tend to have unilateral (one-sided) smiles, while bilateral smiling (engaging both sides of the mouth) occurs in older infants (Kawakemi et al., 2006).

Typically developing infants usually engage in anticipatory smilingOpens in new window—smiling at an object then continuing to smile while gazing at a nearby person—between 8 and 12 months.

Anticipatory smiling is a precursor to developing joint attention (coordinating visual attention between an object and a social partner), which is a crucial milestone in intentional communication (Venezia, Messinger, Thorp, & Mundy, 2004).

There is a widespread belief that lower social status is associated with more smiling (Hall, Horgan, & Carter, 2002). While studies show that girls and women tend to smile more, the degree to which boys and men smile less varies by culture, role, and social expectations (LaFrance, Hecht, & Levy Paluck, 2003).

GelotologyOpens in new window—the study of laughter—has led to several types of therapy that make use of humor, laughter, and smiling. These include humor therapy, clown therapy, laughter therapy, laughter meditation, and laughter yoga. Promising health benefits (physical and psychological) have been demonstrated for some of these therapies (MacDonald, 2004).

See Also:
  1. Darwin, C. (1998). The expression of the emotions in man and animals (3rd ed.). New York: Oxford University Press. (Original work published 1872).
  2. Duchenne, G. B. (1862). Le Mecanisme de la Physionomie Humaine (Mechanism of human physiognomy). Paris: J. –B. Bailliere et Fils.
  3. Ekman, P., Davidson, R.J., & Friesen, W.V. (1990). The Duchenne smile: Emotional expression and brain psychology: II. Journal of Personality and Social Psychology, 58, 342 – 353.
  4. Hall, J. A., Horgan, T.G. & Carter, J.D. (2002). Assigned and felt status in relations to observer-coded and participant-reported smiling. Journal of Nonverbal Behavior, 26 63 – 81.
  5. Kawakami, K., Takai-Kawakami, K., Tomonaga, M., Suzuki, J., Kusaka, T., & Okai, T. (2006). Origins of smile and laughter: A preliminary study. Early Human Development, 82, 61 – 66.
  6. LaFrance, M., Hecht, M.A., & Levy Paluck, E. (2003). The contingent smile: A meta-analysis of sex differences in smiling. Psychological Bulletin, 129, 305 – 334.
  7. MacDonald, C.M. (2004). A chuckle a day keeps the doctor away: Therapeutic humor & laughter. Journal of Psychological Nursing & Mental Health Services, 42, 18-25.
  8. Papa, A., & Bonanno, G.A. (2008) Smiling in the face of adversity: The interpersonal and intrapersonal functions of smiling. Emotion, 8, 1 – 12.
  9. Venezia, M., Messinger, D. S., Thorp, D., & Mundy, P. (2004). The development of anticipatory smiling. Infancy, 6, 397 – 406.