The evaluation of payment for performance (P4P) programmes has focused mainly on understanding contributions to health service coverage, without unpacking causal mechanisms. The overall aim of this paper is to test the causal pathways through which P4P schemes may (or may not) influence maternal care outcomes.

The research used data from an evaluation of a P4P programme in Tanzania. Data were collected from a sample of 3000 women who delivered in the 12 months prior to interview and 200 health workers at 150 health facilities from seven intervention and four comparison districts in Tanzania in January 2012 and in February 2013.

The findings show that:

• P4P reduced the probability of women paying for delivery care (−4.5 percentage points) which mediates the total effect of P4P on institutional deliveries (by 48%) and on deliveries in a public health facility (by 78%).
• P4P reduced the stock-out rate for some essential drugs, specifically oxytocin (−36 percentage points), which mediated the total effect of P4P on institutional deliveries (by 22%) and deliveries in a public health facility (by 30%).
• P4P increased kindness at delivery (5 percentage points), which mediated the effect of P4P on institutional deliveries (by 48%) and on deliveries in a public health facility (by 49%).
• P4P increased the likelihood of supervision visits taking place within the last 90 days (18 percentage points), which mediated 15% of the total P4P effect on the uptake of two antimalarial doses during antenatal care (IPT2).

The study illustrates that there is great potential to apply the method of causal mediation analysis to help unpack the causal mechanisms of complex health systems interventions such as P4P, shedding light on how they impact the health system to achieve population health goals. The authors encourage further research of this kind to strengthen the evidence base about how health system interventions works.