INTRODUCTION
Though family relationships crucially determine health and wellbeing, their role in shaping the uptake of individual health interventions is not well‐understood. Using family network data from a large population‐based cohort, we quantified the degree to which the efficacy of a randomized individual‐level health intervention—a financial incentive for HIV testing—is shaped by its pattern of distribution among family members.Improving testing programmes can increase access to anti‐retroviral therapy (ART) which effectively eliminates HIV transmission at the individual level [Through a post‐hoc analysis of Home‐Based Intervention to Test and Start (HITS) study data, we quantified how the effectiveness of a financial incentive for HIV testing changes depending on whether it is offered to an individual or offered to an individual along with family members. HITS, which was conducted in South Africa, investigated the effects on HIV testing and linkage to care of a ZAR 50 (USD 3) incentive and a male‐targeted HIV‐specific decision support programme. We previously reported that among men, the uptake of HIV testing increased from 17.1% in the standard of care to 27.5% in the financial incentives arm (risk ratio = 1.55, 95% CI: 1.31−1.82) [We build on this finding by testing the hypothesis that, for a given individual, the effectiveness of the financial incentive is augmented by offering incentives to family members prior to, or at the same time as, the individual. For many South Africans, resources are shared among extended family across different households [