Eyelid morphology, the structure of the upper eyelid, is a prominent example of natural human variation observed across global populations. This feature is primarily categorized by the presence or absence of a visible crease. The term “monolid” describes one of these variations, often sparking questions about its prevalence and rarity worldwide. Examining the anatomy and underlying genetic factors reveals that this eyelid type is a normal, genetically determined trait, highly concentrated in specific geographic regions.
Defining the Monolid and Eyelid Anatomy
The monolid is scientifically defined by the smooth, uninterrupted surface of the upper eyelid, which lacks a defined fold or crease. This contrasts with the “double eyelid,” which features a distinct supratarsal crease that runs parallel to the lash line. The difference between these two structures lies in the connection of the levator palpebrae superioris muscle.
In a double eyelid, fibers from the levator aponeurosis, the tendon-like structure of the muscle that lifts the eyelid, extend forward to insert into the skin of the upper eyelid. This insertion pulls the skin inward when the eye opens, forming the visible supratarsal crease. Conversely, the monolid structure is characterized by the absence of this direct skin attachment.
The levator muscle fibers insert much lower, or not at all, into the eyelid skin of a monolid. This anatomical difference often results in a thicker upper eyelid due to a lower-set orbital septum and a greater amount of subcutaneous fat extending closer to the lash line. The smooth, seamless appearance from the lash line to the eyebrow is the signature characteristic of this eyelid morphology.
Global Prevalence: Answering the Question of Rarity
The question of whether monolids are rare depends entirely on the population being considered. Globally, the monolid is not a rare feature, though it is highly concentrated in East Asian populations. It is the dominant eyelid type among many ethnic groups in East Asia, including the majority of people in Korea and Mongolia.
In Korea and Japan, studies estimate that the natural monolid configuration is present in 60% to 80% of adults. The prevalence is closer to a 50:50 ratio in the Chinese population, with Northern Chinese populations tending to have a higher rate of monolids than Southern Chinese. In contrast, the double eyelid is dominant among Southeast Asian populations, such as those in Thailand, the Philippines, and Indonesia, where prevalence can be as high as 90% or more.
Considering the global population, the creased or double eyelid is the more common morphology across European, African, and most South American and South Asian populations. However, the monolid is also observed in non-East Asian groups, including indigenous peoples of the Americas and certain communities in Africa and the Arctic region. Therefore, while the feature is a minority characteristic worldwide, its concentration in the most populous region of the world makes it a common feature, not a rare one.
The Genetics Behind Eyelid Variation
Eyelid morphology is determined primarily by inherited genetic factors, presenting as a classic example of human genetic diversity. The inheritance pattern is not a simple dominant or recessive trait but is understood to be polygenic, meaning it is influenced by the combined action of multiple genes. This polygenic nature explains the range of eyelid types, from a sharp crease to a complete monolid.
Recent genomic research has begun to pinpoint specific genetic markers associated with the presence of the eyelid crease. Genome-wide association studies (GWAS) have identified single nucleotide polymorphisms (SNPs), such as rs12570134 and rs1415425, which show a significant association with the double eyelid phenotype.
The expression of these genes affects the amount of fat and connective tissue in the upper eyelid, ultimately determining whether a crease is formed. This complex genetic interplay results in the high degree of variation seen in eyelid structure, even within the same geographic or ethnic groups.
Clarifying Terminology: Monolid versus Epicanthic Fold
A common source of confusion is the interchangeable use of the terms monolid and epicanthic fold, but they describe two distinct anatomical features. A monolid refers to the absence of the supratarsal crease across the entire upper eyelid. The epicanthic fold, or epicanthus, is a specific vertical fold of skin that extends from the upper eyelid and covers the inner corner of the eye, known as the medial canthus. While the epicanthic fold is frequently present in individuals with monolids, the two features are not mutually exclusive. It is possible to have an epicanthic fold alongside a visible eyelid crease, though this crease is often partially obscured. The epicanthic fold can also be observed in non-Asian populations, particularly in infants before the bridge of the nose fully develops, and as a feature associated with certain genetic syndromes.