rather than a lottery ticket with a chance to win.<sup>21</sup> By comparing outcomes across the two trials, we found that the prevalence of any of the two STIs followed the same pattern in the control group over the study periods: the prevalence of any of the two STIs was reduced from 13.8% to 3.6% and 13.5% to 3.8% in the Tanzania and Lesotho trial, respectively. de Walque et al. (2012) further document a significant (27%) reduction in STI prevalence in the Tanzania trial in the group eligible to the high ($20) conditional cash transfer – a transfer over the full trial period twice as large as the expected conditional transfer in the pooled intervention group in the Lesotho trial.<sup>22,23</sup> In the group eligible to the low ($10) conditional cash transfer – a transfer over the full trial period of $30, so essentially the same as the expected conditional transfer in the pooled intervention group in the Lesotho trial – and in the pooled intervention group with a 50% higher expected conditional transfer relative the pooled intervention group in the Lesotho trial – no measurable effects were found in the Tanzania study. As a comparison, and as reported in table 5, STI prevalence was reduced by 84% and 81% in the pooled intervention group and low lottery arm, respectively, in the Lesotho trial, with the largest reduction occuring in the group of risk-loving individuals (see table 9).<sup>24</sup> The two trials differed also in other dimensions (see footnote 21), and context does matter. The comparison should therefore at best be viewed as suggestive. Taken together, however, the results suggest that the introduction of stochastic rewards do increase the effectiveness of financial incentives as a HIV prevention strategy.

The use of lotteries could plausibly affect the costs and potential scalability of financial

> 21The design of the Lesotho trial was inspired by the RESPECT study. The two trials differed in more dimensions than payment amount and the use of lotteries. In the RESPECT study the financial incentives were tied to test results for chlamydia, gonorrhoea, and trichomonas, while in the Lesotho trial the incentives were tied to test results for syphilis and trichomonas. Study participants in the Tanzania trial were also screened for chlamydia, mycoplasma genitalium, HIV, herpes (HSV-2), and syphilis. All participants in the RESPECT study were invited to participate in a monthly group counseling session, while counseling was provided as part of testing every fourth month in the Lesotho trial. The HIV prevalence rate at baseline was much lower in the Tanzania trial compared to the Lesotho trial (3.5% as compared to 16.7%) and the Tanzania trial was not powered to detect impact on HIV incidence. The Tanzania trial evaluated impact after 12 months rather than 24 months as was done in the Lesotho trial.

22As noted in footnote 21, STI prevalence in the RESPECT study was measured as the combined prevalence of any of four STIs. The reduction in the prevalence of syphilis and/or trichomoniasis only, however, broadly mirrors the reduction in the four STIs combined, with an adjusted 28% reduction in the high conditional cash transfer arm relative to the control arm, and no significant reductions in the low conditional cash transfer and combined intervention arms.

> 23PPP adjusted GDP per capita in 2012 was $1,685 in Tanzania and $2,433 in Lesotho, so expressed in GDP per capita terms, the conditional payment in the high intervention group was 116% and 73% higher in the Tanzania trial (0.036) than in the combined lottery arm (0.016) and high prize lottery arm (.206) in the Lesotho trial.

24STI prevalence among risk-loving individuals was 0.8% in the pooled intervention group and 5.1% in the control group at the end of the trial as compared to 0% in the pooled intervention group and 2.2% the control group for the sub-sample of risk-averse individuals (see table 9 for details).

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