Voluntary medical male circumcision (VMMC) to protect against sexual transmission of HIV is a key part of HIV prevention interventions in 15 priority countries in Southern and Eastern Africa. Ensuring that VMMC programs reach adolescent males is important in countries with large young populations. We designed a methodology to explore the joint decision-making dynamics among caregivers and adolescents aged 10-19, and the drivers and barriers for circumcision, in order to identify levers which can drive uptake of VMMC. Our approach was grounded in behavioral science to address some of the limitations of survey-based research (e.g., the "say-do gap," social desirability bias, respondent fatigue). Our methods included 1) interviews with adolescent boys and their caregivers to understand how adolescents interact with their families, other key stakeholders, and the healthcare system; 2) journey mapping to understand how boys and caregivers move through the stages of progress toward the decision for VMMC, and the influence of context, family, and community members; and 3) Ethnolab, a decision-making game that tests behavioral hypotheses in hypothetical situations mimicking the real-life context of decision-making about VMMC, enabling an understanding of boys' and caregiver's motivators, barriers, and mental models via observation as well as questioning. Factors influencing the decision for VMMC included anticipated pain of the surgical procedure, mistrust about safety, the boy's uncertainty about his caregiver's consent, and caregiver's uncertainty about the adolescent's assent, and caregiver's concern about their adolescent boy's maturity level and ability to deal with VMMC, among others. Conversely, in-group seeking, the belief that being circumcised is appreciated by women, and improved hygiene were among the positive factors motivating decisions for VMMC. Demand generation should involve the whole family unit, encouraging discussion and trust within and among households, and recognizing and addressing the ways decision dynamics change as the boy ages through adolescence.