Open-access resources like MOOCs have expanded opportunities for adult learners to pursue self-directed learning. However, sustaining engagement remains a significant challenge, marked by high attrition rates. Adult learners often juggle work, family responsibilities, and learning, while navigating how to connect their learning to real-world applications. This thesis explores how behavioral change interventions, grounded in Bandura's social cognitive framework, address challenges by targeting environmental, behavioral, and personal factors. Through three studies, it examines how intervention design elements, such as cultural contexts, message frequency, and task complexity, influence adult learners' behavioral engagement and real-world application of skills. Study 1 explores the impact of social norm messages on behavioral engagement across cultures. Study 2 investigates the effects of social norm messages combined with design elements like message frequency and task complexity. Study 3 identifies effective planning tactics and suggests the role of flexibility in fostering long-term engagement. Findings highlight the importance of culturally tailored interventions, frequent and actionable prompts, and adaptable planning tactics to support adult learners in self-directed learning environments. This research provides theoretical insights and practical guidelines for designing behavioral change interventions to promote sustained behavioral engagement and support inclusive participation among adult learners in online self-directed learning environments.