Effect of a lottery intervention on gender-based violence among female sex workers in Dar es Salaam, Tanzania: results from a randomised trial

Introduction: Financial incentives are a promising approach for HIV prevention. Some studies have also shown positive spillover effects on gender-based violence (GBV); however, little is known about impacts of financial incentives on GBV among female sex workers (FSW). We investigated the impact of lottery-based incentives on GBV among FSW in Dar es Salaam, Tanzania. Methods: tests. We conducted an intent-to-treat analysis using additive risk models (binomial distribution, identity link) to compare physical and sexual GBV, intimate partner violence (IPV) and non-partner violence between study arms at endline, with estimates expressed as prevalence differences (PDs) and 95% CIs. Causal, population-level impacts were estimated using g-computation. Results: All forms of violence declined over the study period in the endline sample (n=1117; 49.4% lost to follow-up). There were no significant differences between arms at endline in the primary analysis: GBV - 34.1% control versus 30.7% lottery, PD: -0.03, 95% CI -0.09 to 0.02; IPV - 18.9% control versus 15.9% lottery, PD: -0.03, 95% CI -0.07 to 0.01; non-partner violence - 21.1% control versus 20.4% lottery, PD: -0.01, 95% CI -0.06 to 0.04, nor in the g-computation analysis. Conclusion: This study did not find any significant impact of low-probability, high-reward lottery incentives on any form of GBV, including IPV and non-partner violence, suggesting that this intervention does not influence risk of violence for FSW in the context of HIV prevention. Nonetheless, given the high attrition in the study sample, effects of the intervention cannot be ruled out, and additional investigation is warranted among a larger sample to further assess the potential benefits and harms of this approach for FSW. Trial registration number: AEA RCT registry: AEARCTR-0002677.