The only human society that has not made marriage in one form or another central to the way it is organised is the Na people, of which there are about 30,000 living in the Yunnan Province of southwest China. They live in sibling-based households, and sexual relations occur in the form of nan-sese, meaning to visit furtively: the men walk to the houses of the women at night, but return in the morning. They are, however, the sole exception in recorded history.