Eating disorder patients are usually ambivalent about changing their eating behaviour. They frequently avoid treatment and non-compliance and drop-out are common, even if the potentially severe consequences of their disorder are explained to them. ‘Readiness or motivation to change’ refers to the willingness of patients to introduce changes that lead to improvements in their disorder, and is an important component of the treatment process. The transtheoretical model of behaviour change (Prochaska & DiClemente, 1982) suggests that change is an intentional process that involves a continuous movement towards actions directed at stopping symptoms related to a disorder. The stages of change model seek to understand the patient's progress towards achieving a real desire for change and recovery. The model has six stages of motivation with different degrees of involvement in the process of behavioural change: pre-contemplation (patients are unaware of the problem and have no intention to change), contemplation (patients acknowledge that they have a problem and are thinking about change but not yet ready to make a commitment), preparation (patients are considering specific actions in order to introduce changes), action (patients are actively working to improve their abnormal behaviours) and maintenance (patients work to prevent going back to their previous behaviours). These concepts have been applied to different behaviours and disorders such as physical activity (Berry, Naylor, & Wharf-Higgins, 2004), smoking (DiClemente, Prochaska, Fairhurst, Velicer, Velasquez, & Rossi, 1991), alcohol abuse (Rollnick, Heather, Gold & Hall, 1992) or obesity (Prochaska, Norcross, Fowler, Follick, & Abrams, 1992). Several studies have stressed the importance of motivation to change in adults with eating disorders with regard to ensuring clinical improvement and maintenance (Bewell & Carter, 2008; Geller, Cockell & Drab, 2001; Hasler, Delsignore, Milos, Buddeberg, & Schneyeder, 2004; Rodriguez-Cano & Beato-Fernández, 2005; Vitousek, Watson, & Wilson, 1998; Wolk & Devlin, 2001). In adolescents with anorexia nervosa, motivation to change has also been related to better treatment response (Ametller, Castro, Serrano, Martínez, & Toro, 2005; Castro-Fornieles et al., 2007). Some instruments evaluate motivation to change in eating disorders as a whole: for example, the Readiness Motivation Interview (Geller & Drab, 1999) which has also been validated for adolescent patients (Geller et al., 2008). Other instruments are addressed specifically to anorexia nervosa patients, for instance, the Anorexia Nervosa Stages of Change Questionnaire (ANSOCQ) (Rieger, Touys, Schotte, Beaumont, Russell, Clarke, 2000), or to bulimia nervosa, e.g. the Bulimia Nervosa Stages of Change Questionnaire (BNSOCQ) (Martínez et al., 2007).
Higher motivation to change in adult eating disorder patients has been related with age (Casanovas, Fernandez-Aranda, Granero, Krug, Jiménez-Murcia, Bulik, 2007), but the association with duration of disorder is not clear (Casanovas et al., 2007; Perkins, Schmidt, Eisler, Treasure, Berelowitz, Dodge, 2007). In adolescent patients with anorexia nervosa (Serrano, Castro, Ametller, Martinez, & Toro, 2004) no significant correlation with age, duration of illness or body mass index was found. In adolescent patients with bulimia nervosa (Martínez et al. 2007), age and depressive symptomatology were related to motivation to change, but duration of disorder and body mass index were not. Research has been conducted on the process of change specifically in patients with bulimia nervosa (Ward, Troop, Todd, & Treasure, 1996) and several authors have found that motivation to change predicted the results of an intervention program in adult patients with this disorder (Wolk & Devlin, 2001; Treasure, Katzman, Schmidt, Troop, Todd, & de Silva, 1999). Halmi et al. (2002) found that patients with bulimia nervosa who relapsed after treatment had lower motivation to change. Nevertheless, there are no longitudinal studies analysing the influence of motivation to change in treatment outcome in adolescent patients with bulimia nervosa, in spite of the fact that patients at these ages usually have low insight into the consequences of their disorder (Fisher, Schneider, Burns, Symons, & Mandel, 2001). In view of the increase in incidence of the disorder at younger ages (Currin, Schmidt, Treasure, & Jick, 2005), more research specifically addressed to adolescent bulimia nervosa patients is necessary.