This research paper investigates how the internal decision-making structures of interest organisations mediate how reactive these groups are at the European level. There are two causal mechanisms which explain the interplay between leadership autonomy and interest group reactivity in the European Union. The paper argues that through the inclusion of members in their intra-organisational decision-making (and thereby low leadership autonomy), interest groups internalise conflicts and thereby become less actionable in the European political sphere (internalisation mechanism). Reversely, by not including members in their internal decision-making (and having high leadership autonomy), interest groups are more able and flexible to react at the European level (externalisation mechanism). However, this actionability comes at the cost of members ‘lobbying solo’, i.e. members circumventing the group structures and becoming active on their own. Building on a least-similar cases design, the paper uses twenty-eight expert interviews, group documents and data from the European Transparency Register from twelve German, British and Portuguese groups in order to verify its analytical claims. The findings of the paper go to show that member inclusion within group structures comes at the cost of being less reactive at the European level. At the same time, high leadership autonomy comes at the cost of higher competition with these same members at the European level.
