Abstract
Behavioural interventions often create unintended effects on the other aspects of tax compliance that were not targeted during the design stage, or known as spillover effects. This study utilises data collected from two randomised controlled experiments that have been done in Indonesia to investigate the spillover effects that may arise from the interventions to annual income tax return compliance. In the previous experiments, we examined the impact of the interventions involving individual taxpayers. The sample consist of more than 51,000 taxpayer, where half of the sample were allocated to Control (status quo-received no treatment) and the other half to Treatment Group (received either physical letters or digital messages as the intervention). To investigate whether there are spillover effects from those two previous experiments, an instrumental variable regressions (IV) were used. The endogenous variables are instrumented by the initial random allocations to represent assuming that the taxpayers who although allocated to treatment group but in reality did not received the intervention would not be impacted by the treatments. This study could be a reference for policymakers when designing interventions. When designed should use the results as a reference when designing compliance-related interventions so it could cover a broader spectrum of tax compliance.