Historians estimate that between the advent of Islam in 650CE and the abolition of slavery in the Arabian Peninsula in the mid-20th century, 10 to 18 million sub-Saharan Black Africans were enslaved by Arab slave traders and transported to the Arabian Peninsula and neighboring countries. This number far exceeded the number of slaves who were taken to the Americas. Several factors affected the visibility of descendants of this diaspora in 21st-century Arab societies: The traders shipped more female slaves than males, as there was a demand for them to serve as concubines in harems in the Arabian Peninsula and neighboring countries. Male slaves were castrated in order to serve as harem guards. The death toll of Black African slaves from forced labor was high. The mixed-race children of female slaves and Arab owners were assimilated into the Arab owners' families under the patrilineal kinship system. As a result, few distinctive Afro-Arab black communities have survived in the Arabian Peninsula and neighboring countries.