Abstract: Critical scholars have indicated that the assumptions underlying most sport-for-development (SFD) initiatives tend to align with a ‘deficit model’ of youth: young people from disadvantaged areas are uniformly deficient and in need of development, which can be achieved through sport (Coakley, 2011; Coalter, 2013). In this article, we investigated these assumptions within six urban SFD initiatives that work with young people in socially vulnerable situations in a ‘first’ world nation, Belgium. We conducted a survey at two moments in time amongst 14- to 25-year-old participants in order to test two assumptions: i) ‘participants are deficient and in need of development’; and ii) ‘participation in SFD initiatives leads to positive personal development’. We operationalised ‘development’ as the commonly used outcomes of perceived self-efficacy and self-esteem. These are ‘household words’ both inside and outside SFD research, practice, and policy and carry the assumption that boosting them will by itself foster positive outcomes. The findings refute the supposition that young people from disadvantaged urban areas are uniformly in need of more perceived self-efficacy and self-esteem and show that there is no simple and predictable change in participants’ ‘development’. We suggest that, in designing and researching programs, SFD stakeholders start from an open-ended bottom-up approach which is tailored to the actual life situations of young people and their individual differences and consider more interpersonal and critical conceptualisations of ‘development’.