Which states join international institutions? Existing theories of the multilateral trade regime, the GATT/WTO, emphasize gains from cooperation on substantive policies regulated by the institution. We argue that political ties rather than issue-area functional gains determine who joins, and we show how geopolitical alignment shapes the demand and supply sides of membership. Discretionary accession rules allow members to selectively recruit some countries in pursuit of foreign policy goals, and common interests attract applicants who are not yet free traders.We use a duration model to statistically analyze accession time to application and length of accession negotiations for the period 1948–2014. Our findings challenge the view that states first liberalize trade to join the GATT/WTO. Instead, democracy and foreign policy similarity encourage states to join. The importance of political ties for membership in the trade regime suggests that theories of international institutions must look beyond narrowly defined institutional scope.