Abstract: Compliance with a social norm is a matter of self-enforceability and endogenous motivation to conform, which are relevant not just to the study of social norm, but also to a large array of social institutions. Endogenous mechanisms become effective once the game description is enriched with pre-play communication ad unanimous pre-play agreements over a norm (which remains nevertheless not binding in any sense). Behavioral models understand conformity as the maximization of some “enlarged” utility function, addressed to make room for an individual “desire” to comply with a norm reciprocally adhered to by other participants - which in turn is seen as depending on the expectation that the norm will be in fact reciprocally adhered. In this paper we present an experimental study of the “conformity-with-the-ideal preference theory” (Sacconi and Grimalda 2005), based of a simple experimental three person game called “exclusion game”. In case the players participate in a “constitutional stage” (under a veil of ignorance ) in which they decide unanimously the rule of division, the experimental data show a dramatic chance of the participants’ behavior pattern. Most of them conform to the fair rule of division they have agree before. This behavior is largely consistent with the predictions of our theory, but also suggest a weak form of what John Rawls (1971) called the “sense of justice”.

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