The richness of today’s electronic communications mirrors physical world: activities such as shopping, business and scientific collaboration are conducted online. Current interactions have become a form of social exchange where participants must deal with complexity, uncertainty and risk. We propose a policy specification approach that combines the social sciences and trust theory to facilitate ad-hoc interactions of self-interested parties in open environments. Our socio-cognitive approach allows us to reason about uncertainty and risk involved in a transaction, and automatically calculate the minimum trust threshold needed to mitigate the vulnerabilities. The trust threshold comprises the core of security policies that govern the interactions. The threshold calculation is based on balancing objective and subjective trust components, which together guarantee that a transaction will result in an acceptable outcome. We propose to apply the Prospect Theory [20] to specify policies tha...