Informal reasoning is the basic reasoning frequently used by people to solve complex daily life problems. Unlike scientific reasoning, informal reasoning includes cognitive and affective processes in which the types of reasoning can be intuitive, emotive and rational. This cross sectional study aims at analysing development pattern of students' informal reasoning at elementary school, junior high school, and senior high school. Moreover, the study also identifies differences between boys and

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... ls reasoning. The participants were 20 elementary school students, 30 junior high school students, and 30 high school students who attended schools managed by the same foundation. The data were collected using five items test on issues found in everyday life. Students' responses were grouped into intuitive, emotive, or rational reasoning. The study finds that students' informal reasoning tends to develop in accordance with the school grade. Related to gender, the study finds that girls tend to use rational reasoning while boys tend to use intuitive reasoning.