There are officially 68 ethnic groups in Laos. About 68% of all Laotians are Lao-Loum, or lowland Lao, a people related to the people of Thailand; thought to have migrated to Laos from southwestern China in the 8th century, the Lao-Loum are concentrated in the lowlands along the Mekong. On the hillsides live the Lao-theung, or slope dwellers, a diverse group dominated by the Lao-tai (with various subgroups, including the Black Tai), who are ethnically related to the Lao-Loum. They account for 22% of the population. At higher altitudes are the Lao-soung, or mountain dwellers, a diverse group of ethnic minorities of mainly Malayo-Polynesian or proto-Malay backgrounds. They constitute 9% of the population. Important among the Lao-soung, and more prosperous than most Lao because of the opium poppies they grow, are the Hmong (Meo), a people of Tibeto-Burman origin who supported the American presence until 1975 and, because of their continuing insurgency, became the targets of harassment by government and Vietnamese troops. Other important upland tribes, all with customs and religions considerably different from those of the lowland Lao, are the Ho, Kha, Kho, and Yao (Mien). Ethnic Vietnamese and Chinese account for 1% of the population.