PROJECT SUMMARY Over 40% of children in Brazil live in poverty that is often severe and carries with it a significant risk for mental illness. Existing interventions can help alleviate mental health problems in children, yet the capacity to implement these interventions is limited across the globe, and particularly in low-resourced settings. Because they are highly scalable, social interventions such as cash transfer programs offer a promising approach that may limit a critical social driver of mental illness, adverse childhood experiences (ACEs), thereby reducing an important social burden arising from poverty – child mental illness. We propose a 24-month randomized clinical trial involving 300 households to examine causal effects of cash transfers on child neurodevelopment and mental health. Our study builds off of the largest cash transfer program in the world, Bolsa Familia, that has been in place in Brazil for over 2 decades. Consistent with the goals of RFA-MH-21-160, our study includes a mechanistic framework, testing whether cash transfers reduce new onset ACEs, protecting child brain and behavioral development by limiting inflammation and hypothalamic- pituitary-adrenal (HPA) activity (cortisol). With pre- and post-intervention assessments that include brain MRI scans, behavioral assays, and symptom measures, we will index the neurodevelopmental effects of cash transfers across multiple units of analysis. Although cash transfer programs are becoming increasingly common, their effects on child mental health remain poorly characterized particularly in low/middle income countries where poverty is widespread. Because our proposed study builds off of Bolsa Familia, a well-established program with a successful, national infrastructure for transferring cash, our findings could rapidly move toward implementation. Moreover, by providing mechanistic insights, our study will take a key step needed for refining Bolsa Familia and other cash transfer programs, facilitating studies aimed at targeting the specific populations that are most likely to benefit, optimizing the dose/amount of the transfers, and determining the ideal timing/duration for intervention.