Yemeni ethnic groups are predominantly Arabs, followed by Afro-Arabs, South Asians and Europeans. When the former states of North and South Yemen were established, most resident minority groups departed. Yemen is a largely tribal society. In the northern, mountainous parts of the country, there are 400 Zaidi tribes. There are also hereditary caste groups in urban areas such as Al-Akhdam. There are also Yemenis of Persian origin. According to Muqaddasi, Persians formed the majority of Aden's population in the 10th century.