Participants in the iApp group start using the app immediately (Time 0; T0) for 16 consecutive days (until Time 1; T1).

Participants in the dApp group start using the app at Time 1 (T1; 16 days after the iApp group) and use the app for the following 16 days (Time 2; T2).

Brief, game-like, training exercise for challenging body image disorder symptoms and associated psychological features

Assessed with the Questionario sul Dismorfmismo Corporo (Questionnaire about Body Dysmorphic Disorder; QDC; Cerea et al., 2017), a self-report questionnaire assessing body dissatisfaction and body dysmorphic disorder symptoms made up of 40 items. Items are rated on a 7-point Likert scale (from 1 = strongly disagree to 7 = strongly agree; score range: 40-280), with higher scores indicating extreme body dissatisfaction and body dysmorphic disorder symptoms. Individuals who score above the cut-off of the questionnaire (QDC total score = 130) should be referred for further assessment because they might present extreme body dissatisfaction/body dysmorphic disorder symptoms or may be at risk of developing body dysmorphic disorder

Eating Disorder Inventory-3 (EDI-3; Garner, 2004; Italian version by Giannini et al., 2008): a self-report questionnaire assessing eating disorders core features made up of 91 items on a 6-point Likert scale (from always to never), with higher scores indicating higher eating disorder symptoms. It contains three primary subscales: Drive for Thinness (score range = 0-28; higher scores indicating higher drive for thinness), Bulimia (score range = 0-32; higher scores indicating higher bulimic symptoms), and Body Dissatisfaction (score range = 0-40; higher scores indicating higher body dissatisfaction). These subscales compose the Eating Disorder Risk Composite (EDRC) score. The other nine additional scales are Low Self-Esteem, Personal Alienation, Interpersonal Insecurity, Interpersonal Alienation, Interoceptive Deficits, Emotional Dysregulation, Perfectionism, Asceticism, and Maturity Fears.

Assessed with the Rosenberg Self Esteem Scale (RSES; Rosenberg, 1965; Italian version by Prezza, Trombaccia, and Armento, 1997), a self-report questionnaire assessing self-esteem. The Rosenberg Self Esteem Scale is made up of 10 items rated on a 4-point Likert scale (from 1 = "strongly disagree" to 4 = "strongly agree" ), with higher scores indicating higher self-esteem