Your eyes squint when you smile because of the activation of the orbicularis oculi muscle, specifically the pars orbitalis section, which is a key component of a genuine, or Duchenne, smile.
The Duchenne Smile and the "Squinch"
A true, heartfelt smile, often called a Duchenne smile, involves not only the muscles around your mouth (zygomatic major) but also the muscles around your eyes. The key muscle responsible for the squint is the orbicularis oculi. This muscle encircles the eye and when activated, causes the cheeks to raise and the skin around the eyes to wrinkle, creating the characteristic squint or "smize" (smiling with your eyes).
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Orbicularis Oculi: This muscle has two main parts: the pars palpebralis (responsible for blinking) and the pars orbitalis (responsible for the squint). The pars orbitalis is more difficult to consciously control.
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Involuntary Action: The contraction of the orbicularis oculi, particularly the pars orbitalis, is largely involuntary. This means it's harder to fake a genuine smile that reaches the eyes compared to a polite or posed smile that primarily involves the mouth.
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Sign of Authenticity: Because the orbicularis oculi is harder to control consciously, a squinting of the eyes during a smile is often interpreted as a sign of sincerity and authentic happiness.
Fake vs. Genuine Smiles
You can often distinguish between a fake and a genuine smile by looking at the eyes. A forced or polite smile typically only involves the mouth muscles, while the eyes remain relatively unchanged. The absence of the squint or "smize" suggests the smile may not be entirely authentic.
In summary:
Your eyes squint when you smile because a genuine smile activates the orbicularis oculi muscle around your eyes. This action is largely involuntary and is a key indicator of a true, heartfelt expression of happiness. This action is often referred to as "squinching".
