Third party assessments in trust problems with conflict of interest: An experiment on the effects of promises

Is it possible to elicit reliable assessment from an assessor having a conflict of interest (e.g. a professor that writes a recommendation letter for a formal PhD student)? We propose an experimental test and show that compared to a not-incentivized assessment, a promise to give a truthful assessment reduces misreporting to the same extent as an incentivized assessment (i.e. when the assessor gains higher payoff if the assessment is correct).