In total 1395 papers were found of which 76 papers were duplicates. Based on title and abstract, 1308 publications were excluded. Eleven full-text papers were selected and scored according to the in- and exclusion criteria individually by two reviewers. Finally, three papers, describing randomized controlled trials (RCT) were included (Fig. 1). These studies are summarized in Table 1.
Study populations, designs and settings
All three included studies describe a RCT. Harland et al. evaluated the effectiveness of several combinations of methods to promote physical activity using brief (one) or extended (six) motivational interviews and a financial incentive for PA promotion (30 vouchers each for one episode of aerobic activities at a local leisure center or swimming pool). This study was performed in the United Kingdom in the primary care setting and involved the local leisure center. In total, 523 adults between 40 and 64 years old were recruited from one urban general practice in a socioeconomically disadvantaged region of Newcastle.
The study of Duggins et al. was designed to address the question, of whether eliminating financial barriers to physically activity leads to weight loss. This study was performed in the USA in the primary care setting in combination with the local Young Men’s Cristian Association (YMCA). In total, 83 children between 5 and 17 years old were recruited in two family medicine clinics and a specialized pediatrics clinic. Patients were eligible if they had a BMI at or above the 85th percentile for age and sex, and the socioeconomic status of the participants varied widely. In the study, participating families were randomized in an intervention group and a control group. Both groups received nutrition advice through four nutrition classes, and to promote physical activity the intervention group received a financial incentive (family membership of the local YMCA). The materials were available in English and Spanish in order to also include Spanish-speaking families.
The study of Islam evaluates a financial incentive in a physical activity program for 22 women of at least 18 years old, who have used cocaine regularly in their lives. The study was performed at Rubcion, a non-profit organization for substance abuse in the USA. Women were eligible if they were approved for 60 days of residential treatment at Rubicon and received medical clearance from the physician to participate. Both groups had an exercise schedule of three weekly sessions for a period of six weeks. In addition, the intervention group had an incentive scheme. If they met their targets in their exercise schedule, participants were allowed to draw tokens from a prize gym bag.
All three studies have combined a financial incentive with some other technique, such as motivational interviewing, education or exercise sessions. However, these additional techniques were provided to the individuals in both the intervention group and the control group. As studies were only included in this review when the financial incentive was the only difference between study groups, any effect observed can be assigned to the financial incentive. The incentives in the included studies diverge in their characteristics, such as the value they represent, the requirements to receive the incentive and the moment of handing out the incentive.
Both the studies of Harland et al. and Duggins et al. chose an incentive that is linked to physical activity. The study of Islam chose an incentive in the form of simply a compliment or presents of different values, such as toiletries, jewelry or a digital camera. The higher the value of the incentive, the lower the chance they could grab that prize from the prize gym bag. The study of Islam set requirements in such a way that the participants were only allowed to grab a prize from the prize gym bag if they met their target of 30 min of observed treadmill walking. Some additional prizes could be earned if their adherence to the program was high. In contrast with the study of Islam, the studies of Harland et al. and Duggins et al. did not have requirements that the participants had to meet before they received the incentive.
The studies of Harland et al. and Duggins et al. did not report that the content of the financial incentive was matched with the preferences of the target group. The study of Islam surveyed the participants beforehand and during the intervention to identify which prizes were preferred and whether they were still incentivizing during the intervention. They did not report that they surveyed the preferences for other characteristics, such as the moment of handing out and the requirements for receiving the incentive.
Harland et al. evaluated the effectiveness of several combinations of methods to promote physical activity. Data were collected at baseline, at 12 weeks, and after one year. After 12 weeks of intervention, significantly more participants in the intervention group had improved physical activity scores compared to the control group (38 % vs. 16 %, p = 0.001). A significant interaction was found between the two intervention conditions (interviews and vouchers) with the greatest effect in the group offered both vouchers and extended interviewing. In general, this pattern was also found when focusing on only vigorous and moderate physical activity. Comparing the matching groups with regard to the number of motivational interviews, no statistically significant effects were found for providing vouchers as a financial incentive as opposed to not providing this incentive. Moreover, effects found at 12 weeks were not maintained one year after the intervention, regardless of the intensity of the intervention. However, the use of vouchers was higher (44 % versus 27 %) among the group that received the intensive intervention (vouchers + six interviews) than in the group that received the brief intervention (vouchers + one interview).
In the study of Duggins no differences in Body Mass Index (BMI) or weight change were seen between the intervention and control group after the one-year intervention period. In the intervention group, the relationship between the number of visits to the YMCA and the loss of either BMI or weight was positive, but very small and not statistically significant.
After the six week intervention period, the study of Islam reported no significant changes over time in both groups for attitude and perception on benefits of participating in exercise, physical activity levels, compliance, BMI, and Waist Hip Ratio (WHR).