Boundary change and external differentation in EU enlargement

This paper reports a quantitative analysis of changes in EU external boundary closure to measure and explain the enlargement of the European Union. It goes beyond the member/non-member dichotomy to study the patterns and determinants of gradual changes in boundary openness and closure with non-member states since 1980 and across more than 40 functional boundaries. This allows for a nuanced and differentiated view of enlargement as horizontal European integration. The paper will test several political, economic, and geographic factors of boundary formation.