  A key element in the education of youths is their sensitization to historical
and artistic heritage. We analyze a field experiment conducted in Florence
(Italy) to assess how appropriate incentives assigned to high-school classes
may induce teens to visit museums in their free time. Non-compliance and
spillover effects make the impact evaluation of this clustered encouragement
design challenging. We propose to blend principal stratification and causal
mediation, by defining sub-populations of units according to their compliance
behavior and using the information on their friendship networks as mediator. We
formally define principal natural direct and indirect effects and principal
controlled direct and spillover effects, and use them to disentangle spillovers
from other causal channels. We adopt a Bayesian approach for inference.
