The involuntary closing, crinkling, or squinting of the eyes that occurs when a person smiles strongly is a common facial phenomenon. This action is not a mistake, but rather a direct result of the mechanics of facial muscles working in concert. It provides a nonverbal signal about the authenticity of the emotion being felt. Understanding this process involves looking closely at the specific muscles at work and the neurological signals that govern them.
The Anatomy of the Squint
The primary muscle responsible for pulling the corners of the mouth up into a smile is the Zygomatic Major. This muscle extends from the cheekbone down to the corner of the mouth, acting as the main elevator for the lips. When this muscle contracts intensely, it pushes the surrounding soft tissues upward, including the cheeks.
The upward movement of the cheeks exerts pressure on the lower eyelid area, triggering the contraction of the Orbicularis Oculi. This circular muscle surrounds the entire eye socket. Specifically, the pars orbitalis portion of this muscle contracts involuntarily when the cheek pushes against it. This contraction pulls the lower eyelid up, resulting in the eye closing or squinting action and creating the distinctive lines often called “crow’s feet.” The squint is a secondary effect, mechanically linked to the primary action of a wide smile.
The Science of the Genuine Smile
The co-activation of these two muscle groups defines a genuine, felt smile, which scientists call the Duchenne smile. This term is named after 19th-century anatomist Guillaume Duchenne, who mapped facial expressions. He discovered that a true smile of enjoyment involves both the voluntary action of the Zygomatic Major (mouth) and the involuntary action of the Orbicularis Oculi (eyes).
A smile that only involves the mouth, without the accompanying eye crinkle, is known as a non-Duchenne smile, often serving as a polite or social gesture. True, joyful emotion activates areas of the brain that send signals down two distinct pathways. One pathway controls conscious muscle movement, like the mouth, and the other controls involuntary muscles, such as the eye-crinkling portion of the Orbicularis Oculi. Because the eye muscle movement is difficult to consciously fake, its contraction serves as a reliable marker of authentic positive emotion.
Control and Cosmetic Considerations
Since eye crinkling is a reflex driven by genuine emotion, it is difficult to completely override the response through conscious effort. While some people learn to control the intensity of their mouth-based smiles to limit eye involvement, the involuntary nerve signal remains hard to suppress. An intense activation of the Zygomatic Major muscle will almost always induce the secondary contraction of the eye muscle.
For individuals concerned about the appearance of the lines that form around the eyes during a strong smile, cosmetic procedures are available. Treatments like botulinum toxin (Botox) can be injected directly into the Orbicularis Oculi muscle to temporarily weaken its contractile function. This reduces the dynamic wrinkles, known as crow’s feet. However, relaxing this muscle too much can inhibit the natural expression, sometimes leading to a smile that appears less authentic because it lacks the involuntary eye component.