Incentivising safer sexual behaviour: Using lotteries to prevent HIV

Martina Björkman Nyqvist, Lucia Corno, Damien de Walque, Jakob Svensson

07 January 2017



Despite more than 20 years of awareness campaigns, HIV/AIDS is still a major health problem in many countries of the world and especially in Africa, where nearly 37 million people are living with HIV and an estimated 1.4 million new HIV infections occurred in 2015 alone (UNAIDS 2016). Unsafe sexual behaviour is the main mode of HIV transmission. Unsurprisingly, then, traditional behavioural change programmes based on information and education campaigns have been the main tools used to fight the epidemic. Unfortunately, these types of campaigns have proven to be less effective than anticipated at stemming the tide of the HIV/AIDS epidemic (Bertrand et al. 2006, Napierala Mavedzenge et al. 2010).

A natural question that arises from this evidence is “Why do people continue to engage in risky sexual behaviour if they know how to protect themselves and are informed about the dramatic consequences of HIV/AIDS?” A potential answer is that most of the infections occur among the least risk-averse individuals – i.e. individuals that are willing to take risks when it comes to sexual behaviour – and current information campaigns have failed to specifically target them.

Lotteries as incentives for HIV prevention

In a recent paper, we designed an HIV prevention intervention to appeal specifically to the people who are risk lovers and therefore are, presumably, the most at risk of HIV infection (Björkman Nyqvist et al. 2016). Specifically, we use lotteries to design a financial incentive programme aimed at reducing HIV infections in Lesotho – the lottery offered relatively low expected payments but high prizes conditional on negative sexually transmitted infection (STI) test results.

Introducing a gamble into an otherwise standard financial incentive programme has at least two potential advantages. First, with lotteries, the programme becomes relatively more attractive to individuals that are willing to take monetary risks. If the willingness to take monetary risks is correlated with other risky behaviour, such as smoking, taking drugs, or engaging in risky sex, then lottery incentives may better target those at higher risk of getting infected by HIV.

Second, there is growing evidence from psychology and behavioural economics that people tend to overestimate small percentages, and therefore prefer a small chance at a large reward to a guaranteed small reward (Kahneman and Tversky 1979, Kahneman 2011, Barberis 2013). If so, the perceived return from participating in a gamble (lottery) is higher than the return from an incentive programme that pays the expected return with certainty, or likewise lotteries may provide stronger incentives for behavioural change compared to a traditional conditional cash transfer (CCT) holding the budget constant.

The study was a parallel group randomised trial. It had three separate arms: a control arm (N= 1208) and two intervention arms (859 in a low-value lottery arm and 962 in a high-value lottery arm). Participants randomised into the low-value lottery arm were eligible to win lottery prizes worth 500 malotis/South African rands, or approximately $50, every four months. In the high-value lottery arm, individuals were eligible to win lottery prizes of twice that amount.

Individuals in the intervention arms were awarded a lottery ticket if they tested negative for two curable STIs (syphilis and trichomoniasis) in the week before the lottery draw. Village level lotteries were organised every four months and four lottery winners (one male and one female per lottery arm) per village were drawn.

Individuals in the intervention arms testing positive for any of the two STIs did not receive a lottery ticket. They could, however, continue as study participants and thus become eligible in subsequent rounds if they took the free treatment offered and subsequently remained STI negative. Participants randomised into the control arm were not eligible for lottery tickets, but all other study procedures were identical between the control and intervention arms. Anyone testing positive for an STI (regardless of arm) was offered counselling and free STI treatment and individuals testing positive for HIV were referred to public health clinics offering AIDS treatment for appropriate follow-up.

Overall, the lottery incentives had a significant impact in reducing HIV incidence, the rate of new HIV infections among participants who were HIV negative at baseline. Over the two-year trial period, the HIV incidence rate was reduced by 2.5 percentage points, or 21.4%. This lead to a HIV prevalence rate at the end of the trial that was 3.4 percentage points lower in the pooled intervention group compared to the control group. To the best of our knowledge, this is the first HIV prevention intervention focusing on sexual behaviour changes (as opposed to medical interventions) to have been demonstrated to lead to a significant reduction in HIV incidence, the ultimate objective of any HIV prevention intervention.

Do risk-lovers like lotteries?

We further explored whether individuals with preferences for risk, based on the perceived value of a risky gamble, are more likely than risk-averse individuals to respond to a prevention scheme with a high but uncertain return conditional on behavioural change. Participants’ preference for risk was measured using a hypothetical risk-aversion question in the baseline questionnaire – 62% of the participants report they would prefer a fixed amount of money below the expected value of a lottery instead of taking part in the lottery (i.e. they are risk-averse), while the remaining 38% are risk-loving. At baseline, risk-averse and risk-loving individuals had similar demographic and socioeconomic characteristics, but risk-loving participants were less likely to report that they practice safe sex and more likely to be HIV or STI positive.

Did risk-loving individuals respond differently than risk-averse individuals to the lottery program? Our results suggest they did. HIV incidence was 12.3 percentage points higher for risk-loving compared to risk-averse individuals in the control group. HIV incidence among risk-lovers was, however, 12.2 percentage points lower in the intervention relative the control group – an effect size of 58% in this sub-group. The treatment effect for risk-averse participants was insignificant and the point estimate close to zero, so we cannot rule out that the observed decrease in HIV incidence in the pooled intervention compared to the control group was driven solely by the changed behaviour of risk-loving individuals.

The practical advantages of lottery incentives

Our findings provide suggestive evidence that lotteries can be used to target groups at higher risk of HIV infection. In addition to targeting the individuals most at risk, the use of lotteries also has practical advantages which are important if we consider scaling-up such programmes. First, the administrative costs of a lottery programme are likely to be lower than those of a traditional CCT programme as only winners need to be paid. Second, we could think of a lottery system in which only a fraction of the study participants are tested, reducing the cost of testing, an important fraction of the total cost. While the research protocol for the Lesotho study required that all project participants were offered testing, the incentive for behavioural change will, under certain conditions, remain the same if only lottery winners are tested or if STI screening also is subject to a lottery. Finally, and importantly, while we study the impact of lottery incentives for a specific outcome here, our results open up avenues to explore whether lotteries can successfully be used in many other types of programs and domains to enhance the demand for less risky behaviour.


Barberis, N C (2013) "Thirty years of prospect theory in economics: A review and assessment", Journal of Economic Perspectives, 27(1): 173–196.

Bertrand J R, K O’Reilly, J Denison, R Anhang, M Sweat (2006) “Systematic review of the effectiveness of mass communication programs to change HIV/AIDS-related behaviours in developing countries”, Health Education and Research, 21: 567–597.

Björkman Nyqvist, M, L Corno, D de Walque and J Svensson (2016) “Incentivizing safer sexual behaviour: Evidence from a randomized controlled trial on HIV prevention”, CEPR Discussion Paper No. 11542.

Kahneman, D (2011) Thinking, fast and slow, New York: Farrar, Straus, and Giroux.

Kahneman, D and A Tversky (1979) "Prospect theory: An analysis of decision under risk", Econometrica, 47(2): 63–91.

Napierala Mavedzenge, S, A Doyle, D Ross (2010) “HIV prevention in young people in sub-Saharan Africa: A systematic view”, February (accessed November 9, 2010).

UNAIDS (2016) "Fact Sheet 2016",, Geneva, Switzerland.



Topics:  Health economics

Tags:  HIV, AIDS, health, infection, Africa, South Africa, lottery, incentive, risk preferences, risk loving, intervention, experiment

Assistant Professor, Stockholm School of Economics

Assistant Professor of Economics, Catholic University and Executive Director, Laboratory for Effective Anti-poverty Policies (on-leave from Queen Mary, University of London)

Senior Economist in the Development Research Group, World Bank

Professor of Development Economics, Institute for International Economic Studies at Stockholm University and CEPR Research Affiliate