National Smile Day

Children smile roughly 400 times per day, while the average adult manages closer to 20. National Smile Day lands on May 31 in part to bridge that gap, drawing attention to what decades of neuroscience research have confirmed: the simple act of smiling triggers measurable changes in brain chemistry, lowering cortisol and releasing dopamine, serotonin, and endorphins.

The observance grew out of a dental practice in suburban Chicago, but its scope extends well beyond oral health. From Guillaume Duchenne's 1862 experiments mapping the muscles of genuine joy to modern studies linking smile intensity with longevity, the science behind a smile has proven far more complex, and far more consequential, than most people realize.

The human smile has biological roots stretching back roughly 30 million years. In primates, the expression began as a "fear grin," a display of clenched teeth used to signal submission or harmlessness to dominant group members. Over millennia, that defensive reflex evolved into a social greeting, and in humans it became one of the most universally recognized facial expressions across cultures.

The first scientist to treat the smile as a subject of rigorous anatomy was Guillaume Duchenne de Boulogne. Working in Paris in the 1850s and 1860s, Duchenne applied electrical currents to facial muscles to isolate their individual contributions to expression. His 1862 book, Mecanisme de la Physionomie Humaine, identified two muscles critical to a genuine smile: the zygomatic major, which lifts the corners of the mouth, and the orbicularis oculi, which crinkles the skin around the eyes.

From Anatomy to Emotion

Duchenne's distinction between a full-face smile and a mouth-only grin proved foundational. A decade later, Charles Darwin cited Duchenne's work in The Expression of the Emotions in Man and Animals and proposed that facial movements do not merely reflect emotions but can actively shape them. This idea, later formalized as the facial feedback hypothesis, gained experimental support over the following century.

In 2002, Robert Soussignan published a controlled study showing that participants who produced Duchenne smiles reported more intense positive feelings when exposed to pleasant images. The mechanism involves the release of dopamine, serotonin, and endorphins when the brain detects the activation of smile-related muscles, even if the smile is initially deliberate rather than spontaneous.

A Dental Practice Origins Story

Against this scientific backdrop, Dr. Tim Stirneman and Jim Wojdyla of Compassionate Dental Care in Lake in the Hills, Illinois, launched National Smile Day in 2018. Stirneman, a practicing dentist, and Wojdyla, the practice's director of business development, registered the May 31 observance through the National Day Calendar to spotlight the connection between oral health and overall well-being.

The following year, the practice marked the day by hosting a Dentistry from the Heart event, offering free cleanings, fillings, and extractions to community members without dental insurance. The observance has since been adopted by dental practices and wellness organizations across the United States as an annual prompt to promote both the physical and psychological benefits of a healthy smile.