School subjects' synergy and teacher knowledge: Do biology and chemistry teachers benefit equally from their second subject?

German preservice teachers always study two teaching subjects instead of only one. This offers an almost paradigmatic approach for the investigation of effects of specific subject combinations on professional competence. So far, few studies focused on potential beneficial effects or risks on preservice teachers' motivational orientations, beliefs, and professional knowledge. In our exploratory study, we chose a cross-sectional quasi-experimental design to compare preservice teachers studying a combination of biology and chemistry to those who study only one of these two subjects and any other, as it is expected that the differences between less similar subjects (such as chemistry and art) are greater than those between two science subjects. A total of N = 570 preservice teachers with a minimum of one of their subjects being biology or chemistry were recruited in 12 universities in Germany. Nonparametric analyses using Westenberg-Mood's median test show that preservice teachers studying both biology and chemistry score significantly different on some central aspects of professional competence compared to those who study only one of the two subjects and any other. However, advantages regarding motivational orientations and professional knowledge can only be found for the domain of biology, whereas in the domain of chemistry it does not seem to matter which second subject is studied beside chemistry. Furthermore, students taking both sciences show a significantly lower educational motivation and corresponding self-concept, suggesting that dual science students' teaching and learning beliefs are less education-focused than those of students combining one science with a social or language subject. Our findings regarding synergy effects of the dual study of different natural sciences on aspects of professional knowledge can be relevant beyond the specific German context, that is, they can help to supplement or restructure a university curriculum.