Mongolian fold:

The most well-known and probably most widespread eye fold is the Mongolian fold. Sometimes erroneously confused with the epicanthus, although both folds often appear in combination. The Mongolian fold is characterised by the fold of the upper lid running along the whole length of the lid, so that it unites with nasal skin at the corner of the eye in a crescent-shaped arch. Sometimes the fold is so strong that the plica semilunaris is fully covered. Similarly, the eyelashes are partially covered. This gives the impression of slanting eyes and leads to a narrowing of the lid opening. E.g. Japanese have an average lid height of 8.8 mm while Europeans have 10 mm. In some cases, the lower lid shows a fold as well, this leads to the double Mongolian fold. In many individuals the Mongolian fold weakens in old age. It usually shows its strongest expression between the ages of 15 and 25 years and during childhood. The Mongolian fold does not only appear in Mongoloids. It may appear in Native Americans and even Africans (e.g. among Nilotids, Bantuids, Khoisanids, and Mangebtu), rarely Europeans, but in all those groups usually only in a weak form, where the fold is less persistent during adult age. Individuals of those ethnic groups who possess the Mongolian fold usually do not show other features of the Mongoloid eye. In the Mongoloid eye, the levator extends further down than in Europids and reaches the lid. In addition, the whole muscle is far thicker than in non-Mongoloids, and the whole lid contains more fat, which pushes the lash line deeper. Also, in Mongoloids the medial palpebral ligament is much longer and the whole orbit is higher. The Mongoloid fold is most likely an adaption to cold climate (during the last ice age), where fatty layers that pad the lids protect the eyeball. The narrow slit also protects the eye against snow glare.