The Yao are one of the 56 ethnic groups officially recognized by the People's Republic of China, where they reside in the mountainous terrain of the southwest and south. Yao origins in southern China can be traced back some 2,000 years. From the 15th to 19th century, the Yao migrated into Thailand, Vietnam and the highlands of Laos. The migration was agitated by the opium trade and as the result of revolts in Southern China during this period. Marriage among the Yao is traditionally arranged by go-betweens who represent the boy's family to the girl's parents at the age of 12. If the union is acceptable, a bride-price is negotiated. The wedding takes place in two installments, first at the bride's house, followed by a procession to the groom's house where a second ceremony occurs.