Why are Asian's eyes slanted?

An epicanthic fold, epicanthal fold, or epicanthus is a skin fold of the upper eyelid, from the nasal bone to the inferior side of the eyebrow, covering the inner corner (medial canthus) of the eye. It is a normal trait for a large percentage of humans. This lower fold of the upper eyelid gives the eyes of some East Asians an appearance which seems relatively narrower and almond-like in comparison to most persons of Western (Caucasian) and Sub-Saharan African descent, whose eyes seem wider due to a higher upper eyelid fold.

The term "epicanthic fold" refers to a visually categorized feature of the eyelid; however, there are different underlying explanations of the causes. For instance, one theory is based on the fact that the fold is almost always associated with absent to near-absent brow ridges and vice versa, thus causing some to posit that the absent brow ridge is responsible for the occurrence of the fold.

Other names for this trait include plica palpebronasalis, palpebronasal fold, and Mongolian eye fold.

Distribution

The epicanthic fold occurs frequently among people of North Asian, Central Asian, and East Asian descent, such as the Mongols, Han Chinese, Tibetans, Koreans, Japanese, Kazakhs, Hazaras, and Kyrgyz. It is also commonly though less frequently found among Southeast Asians and certain other Asian ethnic groups like Vietnamese, Filipinos, Burmese, Cambodians, Thais, Bhutanese, northern Nepalis, Ladakhis, people (mostly tribal) from Eastern Bangladesh and North Eastern India (for example, the Chakma, Marma, the Sylhetis, the Mizo and the Garo), among others. Mixed race people of partially Asian descent, such as Eurasians and Afro-Asians may inherit epicanthic folds.

Epicanthic folds are also common among Native Americans and Inuit. They can also be found inherited among some Oceanic peoples including Tongans, Samoans, Micronesians and Hawaiians.

A minority of people within some African ethnic groups also have epicanthic folds; these include the Khoisans (Capoids) in Africa and certain groups from southern Sudan such as the Dinka and the Nuer.

Epicanthic folds are also found in a minority of Europeans having no obvious Asian background, especially in eastern Europe and across northern Europe in areas such as Scandinavia and Poland, as well as within Ireland and Britain. One of the existing accounts suggests that Europeans who possess the fold also have less prominent brow ridges.

Genetic factors

All humans initially develop epicanthic folds in the womb, with some children losing them at birth.[citation needed] Children who have one parent with a pronounced epicanthic fold, of non-medical causation, and one without the fold can have varying degrees of epicanthic fold, indicating that the trait is in some degree heritable.

Conditions underlying expression

Evolutionary

One hypothesis as to why epicanthic folds came about involves the climates in which populations expressing them arose. Sunlight reflects more intensely off light colored surfaces, such as those prevalent in snowy regions or savannahs and deserts. The theory is that an epicanthic fold in such an environment would protect the eyes from extra UV radiation.[citation needed] The trait may also be useful against strong winds and cold weather such as the ones on the Central Asian steppes.[citation needed] For instance the country of Mongolia is hot in the summer and extremely cold in the winter, with January averages dropping as low as -30°C (-22°F). The country is also subject to occasional harsh climatic conditions known as zud. The capital Ulaanbaatar has the lowest average temperature of any national capital in the world. Mongolia is high, cold, and windy. It has an extreme continental climate with long, cold winters and short summers, during which most of its annual precipitation falls.

With regard to the vast dry grasslands of the savannah regions, the yellow grasses also reflect a higher intensity of sunlight[citation needed][clarification needed]. Additionally, analogous conditions are present among desert groups such as the Dinka or Nuer of Sudan who live in environments in which sunlight reflects off the light colored sands of the desert.

Developmental

Epicanthic folds may be seen in young children of any ethnicity before the bridge of the nose begins to elevate. They may persist where birth is pre-term, and sometimes also where the mother is alcoholic. Epicanthic folds can cause a child's eyes to appear crossed, a condition known as pseudostrabismus.

Medical

Epicanthic-like folds appearing on individuals from ethnic groups that do not traditionally display the characteristic, especially when combined with other symptoms, can be a sign of a number of disorders. These include the chromosome abnormalities associated with Down syndrome, Cri du chat syndrome, Williams syndrome, and Triple-X syndrome, as well as fetal alcohol syndrome, Ehlers-Danlos syndrome, and pre-term birth. This coincidental visual similarity previously led to individuals with such disorders being described as mongoloid (referring to the Mongol people); this usage is no longer in favor.

Surgical alteration

In some Asian ethnicities, the presence of an epicanthic fold is associated with a less prominent upper eyelid crease, commonly termed "single eyelids" as opposed to "double eyelids". The two features are distinct; a person may have both epicanthic fold and upper eyelid crease, one or neither.

The procedure of reducing or removing epicanthic folds is epicanthoplasty. It is now an extremely rare procedure. Asian blepharoplasty, however, is a popular form of cosmetic surgery in East Asia. The use of "eyelid glue" or "eyelid tape" (sometimes referred to as "eye glue" or "eye tape") is used to create the appearance of the "double eyelid" by finding a "natural fold" in the eyelid and tucking it in and holding it with an adhesive.