A study from Sweden has reported that Somali immigrants in Stockholm experience three to four times more cases of autism and other related disorders than non-Somalis in the same area. A similar situation exists in Minnesota, among its Somali immigrants. Autism is unknown in Somalia.
It is speculated that the problem may be that the Somali immigrants get less sun, and therefore less vitamin D, in these northern regions than they would in their own country. People with darker skin do not synthesize vitamin D as quickly as those who have lighter skin.
"And there is other evidence for a vitamin D link: Last November, Cornell University researchers published a study in Archives of Pediatrics & Adolescent Medicine showing that children in rainy (and therefore more overcast) counties of Oregon, Washington and California were two times more likely to be diagnosed with autism than their counterparts in drier parts of the state."
http://www.scientificamerican.com/article.cfm?id=vitamin-d-and-autism