The prevalence of double eyelids reveals fascinating variations in a defining facial feature. The presence or absence of an upper eyelid crease is a naturally occurring difference determined by anatomical structure and genetic inheritance. This biological variation is a key differentiator across global populations, requiring an understanding of underlying morphology, inheritance patterns, and demographic distributions.
Defining Eyelid Morphology
A double eyelid, often called a supratarsal crease, has a visible fold above the lash line, creating the appearance of two sections on the upper lid. This crease forms because fibers from the levator aponeurosis, the tendon of the muscle that lifts the upper eyelid, extend forward and attach directly to the skin.
In contrast, a monolid (or single eyelid) lacks this visible crease, presenting a smooth, uninterrupted surface from the lash line to the eyebrow. Anatomically, the levator aponeurosis fibers do not connect to the skin. Instead, the orbital septum, a thin membrane, extends lower and contains a preaponeurotic fat pad that covers the area where the crease would otherwise form. This difference in muscle and connective tissue attachment determines whether the skin folds upon opening the eye or remains smooth. The monolid is also often characterized by an epicanthal fold, a skin fold at the inner corner of the eye.
The Genetics of Eyelid Creases
The double eyelid feature is generally considered a dominant genetic trait. This means that if a person inherits the gene for a double eyelid from even one parent, they are highly likely to exhibit the trait.
The monolid, or the absence of the crease, is typically considered a recessive trait. However, eyelid inheritance is not a simple, single-gene Mendelian pattern, but rather a more complex polygenic trait. Multiple genes influence specific anatomical structures, such as the thickness of the orbital fat pad and the precise insertion point of the levator aponeurosis. This complexity explains why variations in crease height and definition exist even among people with a double eyelid.
Global Prevalence and Demographic Distribution
Globally, the double eyelid is the majority trait across most of the world’s population, including people of African, Caucasian, South American, and South Asian descent. In these large demographic groups, the presence of an upper eyelid crease is near-universal.
The monolid feature is most highly concentrated in populations of East Asian descent, where its prevalence is significant, though it is not universal. Studies suggest that approximately 50% of people of East Asian heritage have a monolid, meaning the other half naturally possess a double eyelid.
Within East Asia, there are further distinctions in prevalence. Estimates suggest that in populations like those in Korea, the monolid may be the configuration for 70% or more of the population. In other areas, such as Southern China or Southeast Asia, the double eyelid is far more prevalent, sometimes exceeding 90% of the population.
In Malaysian populations, for instance, nearly all Malays have a double eyelid, while the prevalence among Chinese living in the same region is closer to 70%. These statistics highlight that the double eyelid is highly common globally, but its frequency is balanced in East Asia, where the monolid is a frequent and entirely normal variation.
Non-Genetic Influences on Eyelid Appearance
While genetics establishes the fundamental structure of the eyelid, non-genetic factors can alter its appearance over a person’s lifetime. The most significant non-genetic influence is the natural aging process. Over time, the skin loses collagen and elastin, leading to a loss of firm structure and elasticity.
This age-related change can cause the eyelid skin to stretch and sag, a condition known as dermatochalasis, which can create new creases or folds where none existed before. Conversely, aging can also cause a genetically determined double eyelid crease to weaken or disappear entirely due to the descent of orbital fat.
Temporary factors can also affect eyelid appearance, such as fluid retention from high salt intake, allergies, or lack of sleep. These can cause puffiness that temporarily obscures or alters the appearance of an existing crease.