An epicanthic fold, epicanthal fold, or epicanthus is a skin fold of the upper eyelid (from the nose to the inner side of the eyebrow) covering the inner corner (medial canthus) of the eye. The trait arises because the eyelid muscles are weaker or lower compared with people who do not have this epicanthic fold, resulting in a lower fold in the eyelid, when the eyes are open. The fold gives the eyes of East Asians a characteristic shape which is narrower and almond-like in comparison to most Westerners, whose eyes appear rounder.

The term "epicanthic fold" refers to a visually categorized feature of the eyelid; however, there are different underlying causes. 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. The trait may also be useful against strong winds and cold weather such as the ones on the Central Asian steppes. 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.

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