Thinking skills in transdisciplinary curriculum integration: A service-learning project in a Spanish secondary school

• An analysis is presented regarding the development of creative thinking, problem-solving, decision-making, collaborative thinking, and systemic thinking within a transdisciplinary Service-Learning project focused on sustainable development. • A concurrent triangulation design was implemented from a mixed methods QUAL+QUAN perspective. • The interconnected nature of these thinking skills throughout the project's development is highlighted. • The transdisciplinary Service-Learning project underscores the potential of integrating transdisciplinary and Service-Learning frameworks to promote meaningful, integrated, and holistic learning experiences in secondary education. Curricular integration based on a transdisciplinary approach poses a significant challenge in secondary education. This study analyses the development of creative thinking, problem-solving and decision-making, collaborative thinking and systemic thinking within a transdisciplinary service-learning project focused on sustainable development. The participants included 192 students and 17 teachers from 12 academic departments in a Spanish secondary school. A concurrent triangulation design was implemented using QUAL+QUAN data for a mixed-methods research approach with the results showing a connection between the explored thinking skills, and how problem-solving and decision-making were characterized by the development of analytical and applicative cognitive processes. Creative thinking in artistic areas was marked by originality and divergent thinking, which contrasted with the convergent and analytical nature of the thinking used in the scientific and mathematical fields. Critical thinking was cultivated through cognitive processes of analysis and evaluation across all areas, both transversally and through continuous feedback during the various activities. Collaborative thinking emerged at an integral level, influenced by the methodological approach. The transdisciplinary approach significantly contributed to the development of systemic thinking, which enabled a comprehensive understanding of complex social and ecological issues. This approach fostered: (1) a world-centered perspective in the various disciplines; (2) the application of knowledge in real-world situations through a practical and experiential focus; (3) creative expression through various forms of communication; (4) the awareness of social and environmental issues, all leading to the proposal of innovative solutions.