A difficulty in the testing of strategic experts

We consider a contracting problem between a principal who wants to be informed about relevant stochastic processes and an expert who claims to know which process will generate the data. The data generating process is known to belong to a given class.We show that if the expert discounts the future and the set of allowed processes is convex then there is no screening contract that separates informed and uninformed experts. Our main proviso of convexity is immediately satisfied by any class of processes that can be characterized in a De Finetti-style result. This proviso is also satisfied when the expert is required to produce a prior over the relevant parameter space. Thus, the main difficulty in screening informed and uninformed experts has not yet been fully resolved.