stone sweet and sandholtz also put forward a supranationalist argument but rather than focusing on the interests of economic or legal actors they suggest that it is the interests of member states themselves which have shaped integration Whilst transnational activity may drive integration it cannot explain its timing or form Instead this depends on the extent and manner to which member states are incentivized to harmonize their policy positions which then depends on the costs Whilst it is costly to maintain separate trade policies for example it is not particularly costly to retain separate security and defence policies