The imminence of the British referendum lays the European integration project at a crossroads. One tabled policy proposal is to offer different membership options – shallow integration (economic only) and deep integration (economic and political). This column presents new evidence comparing these two options. Focusing on Norway, a country that is economically but not politically associated with the EU, deep integration is estimated to bring a 6% productivity gain in the first five years, compared with shallow integration. These findings bring new economic arguments to debates about EU integration and membership.