<p>This study explored differences in high school students’ decision-making modes, reasoning modes and reasoning levels when engaging with global socio-scientific issues (GSSI) versus local socio-scientific issues (LSSI). Depending on cultural, social, and regional characteristics, socio-scientific issues (SSI) can be categorized as GSSI and LSSI. Previous research suggests that individuals’ informal reasoning may be influenced by the locality and personal relevance of an SSI. However, this idea has not been thoroughly examined through direct comparisons of decision-making modes, reasoning modes, and reasoning levels across different types of SSI. The participants in this study were 44 twelfth-grade students (25 male, 19 female) who completed two open-ended written questionnaires: one focused on a GSSI topic (global warming) and the other on an LSSI topic (water pollution in Weishan Lake, Jining City). Each questionnaire required students to express their opinions, provide reasoning and formulate rebuttals. Through qualitative coding and quantitative analysis, the results revealed significant differences in students’ decision-making modes between GSSI and LSSI. In reasoning modes, students relied more on personal experience and value considerations when addressing LSSI, whereas their reasoning for GSSI was imbalanced, with limited reference to scientific considerations. In terms of reasoning levels, students generated significantly more rebuttals in the LSSI context than in the GSSI context. These findings underscore the need for differentiated instructional approaches when teaching GSSI vs. LSSI. Science teachers should support students in developing stronger informal reasoning related to GSSI, given that students naturally tend to engage more readily with LSSI.</p>