In order to prepare learners to engage in discussion and be able to make informed decisions on socio-scientific issues (SSIs), SSI-research has become an important field in biology- and science education. Research has revealed that justifications from various resources are involved in students’ informal reasoning and argumentation on SSIs. The importance of multi-disciplinary involvement of reasoning is shown in connection to the quality of argumentation as well as the number of reasons presented in the argumentation. In this study, to investigate the resources of reasons in students’ argumentation on SSIs in relation to study backgrounds, a model termed SEE-SEP covering three aspects (of knowledge, value and personal experience) and six subject areas (of Sociology/culture, Economy, Environment/ecology, Science, Ethics/morality and Policy), was adopted to analyze students’ reasons in different SSIs. A total of 208 upper secondary students (105 social-science majors and 103 science majors) from Sweden were invited to justify and expound their arguments on four SSIs including global warming, genetically modified organism (GMO), nuclear power and consumption. The results showed that (1) the group of social-science majors generated more numbers of reasons than the science majors; (2) the aspect of value was found to be used most in students’ argumentation without differences between students’ study backgrounds; (3) reasons from the subject area of science were presented most in the topics of nuclear power and GMO, with no difference between students’ study backgrounds; (4) personal experience was referred less often in students’ argumentation by both groups of students than the aspects of value and knowledge, especially in the topics of global warming, GMO and nuclear power, in which, the subject area of economy was also discussed less; (5) the social science major students used more resources from the subject area of environment/ecology than the science majors, regardless of SSI. In addition, we found that the social-science majors could provide more solutions of the SSIs and also use reasons from different subject areas in discussing the consumption issue, in contrast to science majors. The implications to SSI-research and teaching are discussed.