 Families who use this system trace descent through both parents simultaneously and recognize multiple ancestors, but unlike with cognatic descent it is not used to form descent groups.While bilateral descent is increasingly the norm in Western culture, traditionally it is only found among relatively few groups in West Africa, India, Australia, Indonesia, Melanesia, Malaysia, the Philippines, and Polynesia.Anthropologists believe that a tribal structure based on bilateral descent helps members live in extreme environments because it allows individuals to rely on two sets of families dispersed over a wide area.