Can commitment contracts boost participation in public health programmes

•A personal commitment contract significantly raised attendance and completion rates in a community health programme.•Increased attendance was associated with higher weight loss, but there was no evidence that the commitment contract directly affected weight outcomes.•Heterogeneous treatment effects were detected, with evidence suggestive of positive and significant effects for people with more short-termist health attitudes.•Exploratory analysis suggests the commitment contract helped people who lacked other commitment elements to the weight management programme, but it did not add to those wider commitment elements to further increase participation. This suggests threshold effects in the way commitment devices work.