The Kula Exchange system determines many social hierarchies and authoritative positions among the trade islands. For example the chiefs living on the Trobriand Islands in the 1970s owned the most shells and also the most valuable shells. Therefore they were responsible for most of the organization and direction of the trade systems in their region. (The Trobrianders of Papua New Guinea (1988) by Annette B. Weiner) Also, the ten most influential men among all the islands control about 90 per cent of the Kula items. With this amount of control and competition, relationships between traders become fragile and ruined by manipulation and deceit. (Malinowski, B. (1920). "Kula; the Circulating Exchange of Valuables in the Archipelagoes of Eastern New Guinea". Man (Man, Vol. 20) 20: 97–105).