[6] Usually, as multiple spots or one large patch, it covers one or more of the lumbosacral area (lower back), the buttocks, sides, and shoulders.

[6] It results from the entrapment of melanocytes in the lower half to two-thirds of the dermis during their migration from the neural crest to the epidermis during embryonic development.

[7][8][9] People who are not aware of the background of the slate grey nevus may mistake them for bruises, possibly resulting in mistaken concerns about abuse.

Gessain spent time with the Huehuetla Tepehua people in Hidalgo, Mexico, and wrote in 1947 about the spot's "location, shape, colour, histology, chemistry, genetic transmission, and racial distribution".

He also says that "the observation made for the first time by Saabye about Inuit children has been completely confirmed by Captain Holbøll", who sent him a fetus pickled in alcohol.

That year, the Danish anthropologist Soren Hansen drew the connection between the observations of Bälz in Japan and Saabye in Greenland.

Fridtjof Nansen, the Norwegian polar explorer, said that the spot was widespread in the mixed Danish-Inuit population of West Greenland.

A missionary in Bethel, Alaska, a traditional gathering place of Yup'ik people, reported that the spots were common on children.

Rudolf Trebitsch, an Austrian linguist and ethnologist, spent the summer of 1906 on the West Coast of Greenland, and listed all the examples he came across.

[14] The Journal of Cutaneous Diseases Including Syphilis, Volume 23 contained several accounts of the slate grey nevus on children in the Americas: Holm ("Ethnological Sketch.

In the Mayas of Central America, Starr's (Data on the Ethnography of Western Mexico, Part H., 1902) facts are corroborated by Herman (Aparecimiento de la Mancha Mongolica.

The birthmark is prevalent among East, South, Southeast, North and Central Asian peoples, Indigenous Oceanians (chiefly Micronesians and Polynesians), certain populations in Africa,[15] Amerindians,[16] non-European Latin Americans and Caribbeans of mixed-race descent.

[17] Approximately 90% of Polynesians and Micronesians are born with slate grey nevus, as are about 46% of children in Latin America,[20] where they are associated with non-European descent.

Nowadays it is completely accepted that the big majority of Mexico's and Latin America's mixed-race populations have the Mongolian spot[26] and that its presence works as an indicator of the actual degree of mestizaje present in a given population,[27] having its lower frequency in Uruguay with 36%,[27] followed by Argentina with an incidence 44%,[28] Mexico with 50%-52%,[29] 68% in Hispanic-Americans[30] and 88% in highland Peruvians.

Among common folk it is said to be caused by the Buddhist goddess of childbirth Songzi Guanyin (送子观音, pinyin: Sòng Zǐ Guān Yīn; lit.

A small portion of people wrongfully believe it is caused when the doctor is slapping the baby's backside to make it cry.

In Ecuador, the native Indians of Colta are insultingly referred to in Spanish by a number of terms which allude to the slate grey nevus.