Epicanthic fold

Epicanthic folds are skin folds of the upper eyelid covering the inner corner of the eye. It is most common in Asians, where the term "slanted eyes" is used to describe them. Their eyes are not really slanted, they just appear that way.

One evolutionary hypothesis for why epicanthic folds came into existence involves the climate in Asia, especially that of central Asia. Most Asian people originated from Mongolia, where there are lots of snow and sunlight. Because the sunlight hitting the snow would eventually make people blind, humans of this region developed an extra fold to protect the eyes from extra UV radiation.

All humans develop epicanthic folds in the womb but most lose them by birth. If they show on someone whose lineage does not display this trait, it may be a sign of Down syndrome or Williams syndrome, among other disorders. This can be caused by inbreeding, endogamy, fetal alcohol syndrome, etc.