France, Jan. 4 -- In a small town near Mahajanga, chants of the Sakalava, one of Madagascar's ethnic groups, drift through the open windows of a house where around 60 guests, mostly young people, are gathered in a living room filled with incense and music.

During the ceremony, some participants fall into a trance, their bodies believed to be temporarily inhabited by ancestral spirits.

At one point, five people are possessed at the same time. Some bodies tremble beneath cloths, while others suddenly grow still. A woman inhabited by a male spirit removes her clothing before slowly returning to her normal state.

Watching closely is Josiane Lazare, 30, seated on a mat in the room.

She continues to welcome guests and serve drinks until wel...