Is It Rare to Have One Monolid and One Double Eyelid?

Having one monolid and one double eyelid is not rare. The condition, sometimes called asymmetric eyelid creases, is common enough that plastic surgeons and ophthalmologists see it routinely, and researchers studying eyelid anatomy have documented it across populations of all ethnic backgrounds. Most people have some degree of asymmetry between their left and right upper eyelids, and a mismatch where one side has a visible crease while the other does not sits on the more noticeable end of a spectrum that nearly everyone falls on.

What Creates the Fold in the First Place

A double eyelid forms because of tiny fibrous connections between the muscle that lifts the eyelid (the levator muscle) and the skin of the upper lid. When these fibers attach to the skin at or above the edge of the tarsal plate (the stiff cartilage-like structure that gives the eyelid its shape), they pull the skin inward each time the eye opens, creating a visible crease. Anatomical dissection studies have confirmed the existence of these connections using both light and electron microscopy, supporting a theory first proposed decades ago by the surgeon Sayoc.

In a monolid, those fibrous connections either do not form, attach lower, or are buried under a thicker layer of skin and soft tissue so that no crease appears on the surface. An ultrasound study comparing single and double eyelids in Asian patients found that the levator muscle itself, the underlying Mueller muscle, and the tarsus were essentially the same thickness in both lid types, with no statistically significant differences between them.¹ The structural difference is not about the muscles being bigger or smaller on one side. It comes down to whether those fine fibrous attachments reach the skin surface in the right spot to produce a fold.

This is exactly why one eye can have a crease and the other may not. Fibrous attachment patterns do not have to develop identically on both sides of the face. Small differences in soft tissue distribution, fat pad thickness, or the precise insertion point of those fibers can produce a visible fold on one lid and no fold on the other, even when the underlying muscle anatomy is symmetrical.

How Common Eyelid Asymmetry Really Is

Virtually no human face is perfectly symmetric, and the upper eyelids are one of the areas where left-right differences show up most clearly. A study using three-dimensional photogrammetry in Caucasian populations measured the absolute difference in upper eyelid crease height between the two eyes and found average side-to-side differences of about 0.65 mm in women and 0.90 mm in men.² Those numbers represent the average, meaning plenty of individuals had even larger differences. When the crease height on one side dips below a certain threshold, it effectively disappears, producing the one-monolid-one-double-eyelid pattern.

In East Asian populations, where monolids are more prevalent, the odds of having asymmetric creases are even more visible. A survey-based study classifying 485 eyes from 247 participants identified ten distinct upper eyelid shapes, ranging from single eyelids through various double eyelid configurations (fan-shaped, parallel, crescent, hidden fold, and others), and the distribution of these shapes differed significantly between men and women.³ The fact that researchers classified eyes individually rather than in pairs tells you something: left and right eyelids on the same face frequently fall into different categories. Having one monolid and one double eyelid is simply one of the more dramatic versions of a mismatch that occurs on a sliding scale.

Why One Side Can Differ from the Other

Several factors explain why eyelid creases develop unevenly. The most straightforward is developmental variation. Just as one foot can be slightly larger than the other, the soft tissue architecture around each eye develops semi-independently. Differences in orbital fat distribution, skin thickness, and how the levator fibers insert into the lid tissues during growth can all tilt one eye toward a crease and the other toward a smooth lid.

Aging adds another layer. Over time, skin elasticity decreases and the soft tissue that once kept a crease hidden can thin out, letting a fold emerge on one side before the other. Some people who had bilateral monolids in their twenties gradually develop a crease on one eye in their thirties or forties and the other much later, or not at all. Weight changes can have a similar effect: gaining fat around the orbits may obscure a shallow crease, while losing weight can expose one.

Sleep habits are an underappreciated contributor. People who consistently sleep on one side apply sustained pressure to that eyelid for hours each night. Over months and years, this can stretch the skin and alter the fold on the compressed side. Allergies and chronic eye rubbing cause localized inflammation and skin stretching that may affect one eye more than the other, especially if you tend to rub one eye preferentially.

Hormonal shifts, particularly around puberty and pregnancy, can change fluid retention and fat deposition around the eyes in ways that are not always symmetrical. None of these causes are alarming on their own. They are normal variations in how the body develops and ages.

Can Eyelid Tape or Glue Create Asymmetry

Eyelid tape and glue products are widely used to temporarily create a double eyelid fold, and they deserve a closer look in this discussion because they can both mask and cause asymmetry. A large survey of people who used double eyelid tape found that roughly 70% reported permanent changes to their eyelid shape after sustained use, and about the same proportion experienced discomfort during use. Complications including asymmetry, numbness, and hypertrophic scarring were found exclusively among tape users and did not appear in the non-user group.⁴

A separate clinical study of 191 patients who had used double-fold tape or glue for an average of about two years documented a range of side effects. The most common symptom was itching, followed by decreased skin elasticity, dryness, and changes in skin color. The most common clinical sign was skin laxity, with eruption, thickening of the skin, pigmentation changes, and localized inflammation also documented.⁵

The practical implication is that if you are using tape or glue on one eye to match the other, you may eventually create a lasting crease on that side, but you also risk ending up with stretched, irritated skin that looks different from the untreated eye in other ways. Asymmetry caused by tape use can be harder to correct surgically because the skin quality on the treated side has changed.

How Noticeable Is One Monolid and One Double Eyelid

People who have this asymmetry often worry it is more obvious than it actually is. Research on how well observers detect facial asymmetry offers some reassurance, along with an important threshold. In a study that presented standardized photographs with controlled amounts of eyelid asymmetry to a panel of observers, differences of 1 mm were correctly identified as asymmetric by only about 14% of participants. Once the difference reached 2 mm, detection jumped sharply, with observers correctly identifying the asymmetry more than 90% of the time.⁶

In real life, people rarely examine your face under the controlled lighting and static framing of a clinical photograph. Movement, conversation, expression, and the natural tendency to look at both eyes together rather than comparing them side by side all reduce how much asymmetry casual observers pick up on. That said, the monolid-versus-double-eyelid difference is larger than a subtle crease height mismatch. When one eye has a visible fold and the other has none, the difference is often well above 2 mm and can change the apparent size and shape of the eye enough that people do notice, particularly in photographs and video.

When Asymmetry Might Signal Something Medical

Most of the time, having one monolid and one double eyelid is a cosmetic variation with no health significance. But a crease that appears suddenly on one side, especially later in life, can occasionally point to a medical issue worth checking.

Ptosis, a drooping of the upper eyelid caused by weakness or stretching of the levator muscle, can change the appearance of the crease on one side. As the muscle weakens, the lid may sit lower, and the crease can either deepen, shift, or disappear. If one eyelid starts drooping enough to partially block your vision or if you notice a new asymmetry that develops over weeks rather than years, it is worth seeing an ophthalmologist. Ptosis can stem from aging, nerve problems, or conditions affecting the muscles.

Dermatochalasis, which is excess skin and tissue on the upper eyelid, can also produce asymmetric creases and sometimes progresses to the point of obstructing peripheral vision. A review of the literature on eyelid-related conditions and their effects on vision found that disorders like ptosis and dermatochalasis can produce measurable changes in corneal shape, and that surgical correction of these conditions sometimes alters or resolves those changes depending on the patient’s age and the technique used.⁷ In other words, eyelid asymmetry that crosses over from cosmetic into functional territory can actually affect how well you see.

A useful rule of thumb: if the asymmetry has been present since childhood or developed gradually over many years, it is almost certainly a normal anatomical variation. If it appeared over a short period, is getting worse, or is accompanied by drooping, double vision, or eye fatigue, get it evaluated.

Surgical Options for Those Who Want Symmetry

Double eyelid surgery, known as Asian blepharoplasty, is one of the most commonly performed cosmetic procedures in East Asia and is frequently sought by people who want to create a crease on a monolid side to match the other eye. The procedure involves creating a connection between the levator muscle and the skin, essentially mimicking the natural fibrous attachments that produce the fold.⁸

Techniques vary. One approach uses small incisions, sometimes as short as 3 to 4 mm, through which the surgeon isolates and trims excess muscle tissue before suturing the skin to create a fold. This mini-incision method was applied to 110 patients in one reported series and aims to reduce scarring while still producing a lasting crease.⁹ Other techniques involve longer incisions that allow more direct access for removing fat and reshaping the lid, which may be preferred when there is significant asymmetry in the amount of soft tissue between the two sides.

Revision surgery is also an option for people who have had a previous procedure that did not hold or that produced an uneven result. A study examining a specific revision technique in Asian patients found generally good outcomes, though mild recurrence of problems occurred in a small percentage of eyes. Among 37 eyes that had preoperative ptosis, mild recurrence was observed in about 5%, and one eye regressed fully. Among eyes with retraction, about 9% showed mild recurrence. None of the patients in that series developed serious complications like inability to close the eye or granuloma formation.¹⁰

An important caveat for anyone considering surgery specifically to correct asymmetry: achieving perfect symmetry is difficult. Because the underlying anatomy of the two eyelids differs, the same surgical technique can produce slightly different results on each side. Surgeons experienced with this procedure will typically discuss the limits of symmetry correction before operating.

Satisfaction and the Psychology of Eyelid Appearance

For people who do choose surgery, the psychological outcomes tend to be positive. A validation study using the FACE-Q questionnaire in 200 young Asian patients after double eyelid blepharoplasty found high scores across several quality-of-life measures, including social confidence and psychological well-being, with patients reporting strong satisfaction with both the decision to have the procedure and its outcome.¹¹ These findings come from a specific population of young people who actively sought the procedure, so they do not generalize to everyone with asymmetric eyelids, but they suggest that for people who are bothered enough to pursue surgery, the intervention tends to deliver on its promise.

That said, many people with one monolid and one double eyelid live comfortably with the asymmetry and have no interest in changing it. The cultural conversation around eyelid creases has shifted in recent years, with more public figures and beauty influencers embracing monolids and asymmetric features. Whether you find the asymmetry charming, neutral, or bothersome enough to address is an entirely personal judgment, and all three positions are common.

The Evolutionary Backdrop of Eyelid Variation

Eyelid morphology varies considerably across human populations, and researchers have proposed evolutionary explanations for why monolids and epicanthal folds became prevalent in certain groups. One hypothesis frames the epicanthal fold as a feature that developed during the broader evolutionary process of eyelid adaptation, rather than as a response to any single environmental pressure like cold or UV exposure.¹² The details remain debated, but the key point for anyone wondering about their own eyelids is that monolids, double eyelids, and every configuration in between are normal outcomes of human anatomical diversity. Having one of each is not a defect or a medical oddity. It is one of the many ways that the small, independent developmental decisions happening on each side of your face can produce a result that is not perfectly mirrored.