Social bridges to healthy habits: Expert feedback and peer observations in photo-based healthy eating apps

In the framework of Persuasive Systems Design, diet-related apps often connect users with peers or healthcare experts as the indirect and direct routes of persuasion to retain users and promote healthy behaviors. This connectivity reaches broad and diverse populations, yet its causal effects on users’ eating behaviors in digital environments, particularly those centered on photo-based interactions, remain unclear. Drawing on social norms theory, media richness theory, and social comparison theory, we examine the effects of two socially delivered features—photo-informed expert feedback and photo-based peer observation—through a four-month randomized field experiment with 337 participants, orthogonally manipulating their presence in a photo-based healthy eating app. We find that photo-informed expert feedback increases app usage and slows disengagement over time, consistent with the influence of injunctive norms and the richer information conveyed through meal photos. In contrast, photo-based peer observation reduces engagement and has no significant aggregate effect on food choices, consistent with predictions based on the similarity hypothesis of social comparison and defensive avoidance in photo-based environments. We further find that this disengagement mainly occurred among relatively unhealthy eaters. This study advances theoretical understanding of how socially delivered direct and indirect routes of persuasion operate in photo-based, computer-mediated environments. It further provides causal, feature-level evidence from a longitudinal randomized field experiment and offers practical guidance for designing mHealth apps that improve user retention and promote healthy eating behaviors.