This assumption has been challenged by the rather persistent observation of cooperation in situations where e.g., defecting would be individually preferable. This has advanced search for alternative ways of understanding choice. Proposals dominantly implies including internal motivations into the utility function.

The strengths and weaknesses of this type of explanation are evaluated, and an alternative model based on institutional theory is put forward. It is based on the idea of plural rationality and the hypothesis that institutions have the capacity to signal when e.g., individual or cooperative rationality is expected.

The model is confronted with data from existing experiments within economics and social psychology. Some profound implications for environmental policy are emphasized.

About Arild Vatn
Arild Vatn is professor of environmental sciences at the Department of International Environ-ment and Development Studies at the Norwegian University of Life Sciences. His main expertise is in institutional and ecological economics.

He is the current president of the European Society of Ecological Economics. He is also leader of the board for the research program ‘Environment 2015´ under the Norwegian Research Council.

Finally, he is responsible for the summer school series ‘Emerging Theories and Methods for Sustain¬ability Research (Themes)´ 2006-2009 funded by Marie Curie.

He has about 150 publications. His book Institutions and the Environment from 2005 summarizes his position in institutional theory.