Measuring Multi-Membership in Economic Integration and Its Trade Impact: A Comparative Study of ECOWAS and SADC

One of the intriguing aspects of African regional trade agreements (RTAs) is the extent of multi-membership, where many African countries are members of more than one RTA. Using a gravity model for 25 countries and the years 1980-2006, we measure the extent of multi-membership and compare its impact in two major African regional blocs, Economic Community of West Africa States (ECOWAS) and Southern Africa Development Community (SADC). We find that the impact of multi-membership critically depends on the characteristics of the multi-membership of regional integration initiatives. We find a positive impact if an additional membership complements the integration process of the original regional integration initiative: overlapping memberships had a much stronger and significant positive effect on bilateral trade within ECOWAS compare with an insignificant impact within the SADC