The EU is currently struggling to implement coherent coexistence regulations on genetically modified (GM) and non-GM crops in all member states. While it stresses that any approach needs to be "proportionate to the aim of achieving coexistence", very few studies have actually attempted to assess whether the proposed spatial ex ante coexistence regulations (SEACERs) satisfy this proportionality condition. In this article, we propose a spatial framework based on an existing landscape and introduce the concept of shadow factor as a measure for the opportunity costs induced by SEACERs. Our empirical findings led us to advance the proposition that flexible SEACERs based on pollen barriers are more likely to respect the proportionality condition than rigid SEACERs based on isolation distances. Particularly in early adoption stages, imposing rigid SEACERs may substantially slow down GM crop adoption. Our findings argue for incorporating a certain degree of flexibility into SEACERs by advising pollen barrier agreements between farmers rather than imposing rigid isolation distances on GM farmers. The empirical questions of proportionality and flexibility have been largely ignored in the literature on coexistence and provide timely information for EU policy makers. (C) 2009 Elsevier Ltd. All rights reserved.