Treatment Effect Analysis for Pairs with Endogenous Treatment Takeup

We study causal inference in a setting in which units consisting of pairs of individuals (such as married couples) are assigned randomly to one of four categories: a treatment targeted at pair member A, a potentially different treatment targeted at pair member B, joint treatment, or no treatment. The setup includes the important special case in which the pair members are the same individual targeted by two different treatments A and B. Allowing for endogenous non-compliance, including coordinated treatment takeup, as well as interference across treatments, we derive the causal interpretation of various instrumental variable estimands using weaker monotonicity conditions than in the literature. In general, coordinated treatment takeup makes it difficult to separate treatment interaction from treatment effect heterogeneity. We provide auxiliary conditions and various bounding strategies that may help zero in on causally interesting parameters. As an empirical illustration, we apply our results to a program randomly offering two different treatments, namely tutoring and financial incentives, to first year college students, in order to assess the treatments' effects on academic performance.