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Written commitments, such as behavioural contracts, may increase weight loss and improve diet in the short-term in adults with obesity or who are overweight when combined with lifestyle change supports

Coupe N, Peters S, Rhodes S, et al. The effect of commitment-making on weight loss and behaviour change in adults with obesity/overweight; a systematic review BMC Public Health. 2019;19:1-16.

Review question

Is adding the use of soft commitment devices (such as written pledges or behavioural contracts) to lifestyle change supports effective for weight loss in adults with obesity or who are overweight? 


Obesity is a rising global issue responsible for an increased risk of heart disease, stroke, type 2 diabetes, and some cancers. The risks of developing these disease can be decreased with weight loss through changes to diet and exercise. Despite the benefits, keeping up with these changes over long periods of time can be challenging. A soft commitment device is designed to help people commit to and follow through with a behaviour change—such as a change around diet or exercise—and/or reach a specific goal such as weight loss. It generally takes the form of a written pledge, behavioural contract, or verbal agreement that is witnessed by another person and does not come with tangible incentives such as money. Soft commitment devices are simple and affordable, but their effect on behaviour change and weight loss needs to be established.

How the review was done

This is a systematic review of ten randomized controlled trials and non-randomized controlled trials published between 1980 and 2017, including a total of 1,320 participants. Three randomized controlled trials were included in a meta-analysis.

Key features of the studies:

  • Participants were adults living with overweight or obesity.
  • Participants used written soft commitment devices in combination with supports to make healthy lifestyle changes, with the goal of weight loss. Soft commitment devices included: membership contracts, contracts to record health habits, contracts to achieve a specific health goal, online pledges, and commitments made by displaying one’s name and health goal in a public place. All commitment devices were witnessed directly or indirectly by another person and addressed one or more of the following: diet, exercise, or weight loss. Supports included structured exercise and diet programs, generally with an educational or counselling component. The studies ranged from four weeks to 12 months.
  • Researchers mainly measured one or more of the following: changes in weight, diet and exercise.
  • Generally, the results were compared to people in control groups who were either: not receiving any other interventions, using supports to make healthy lifestyle changes but not using any commitment device, or using supports to make healthy lifestyle changes and a different type of soft commitment device or the same type of commitment device but with different components.

What the researchers found

The review found that written soft commitment devices—generally behavioral contracts—may improve short- and long-term weight loss, as well as short-term diet changes in adults living with overweight or obesity when combined with lifestyle change supports. Some evidence also shows that having a public component to a contract—such as it being witnessed by a peer or health provider—may help. However, due to the small number of studies the results were based on, more long-term high-quality research is needed to establish the findings, as well as the most effective components of soft commitment devices.

Weight loss

  • Participants who used soft commitment devices lost an average of 1.52kg more weight than participants in control groups in the short term (up to six months) and an average of 1.7kg more than participants in control groups in the long-term (up to 12 months).

Diet and exercise lifestyle changes

  • Participants who used soft commitment devices improved diet in the short-term by consuming 205 less calories than participants in the control group, increasing intake from more healthy food groups, and decreasing intake from unhealthy food groups. There were mixed results about long-term diet changes and no improvements in exercise in the short or long term.


The use of written soft commitment devices in combination with supports to make healthy lifestyle changes may increase short and long-term weight loss and improve diet in the short-term in adults living with overweight and obesity. As of yet, their use does not appear to be beneficial for long-term diet changes, or for short or long-term exercise changes, but more research is needed.


Control group
A group that receives either no treatment or a standard treatment.
Advanced statistical methods contrasting and combining results from different studies.
Non-randomized controlled trial
Studies where people are assigned to one of the treatments, but not purely by chance (for example by the date they enter the study, or other methods).
Randomized controlled trials
Studies where people are assigned to one of the treatments purely by chance.
Systematic review
A comprehensive evaluation of the available research evidence on a particular topic.

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DISCLAIMER These summaries are provided for informational purposes only. They are not a substitute for advice from your own health care professional. The summaries may be reproduced for not-for-profit educational purposes only. Any other uses must be approved by the McMaster Optimal Aging Portal (

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