They keep on learning: Teachers’ pedagogical/psychological knowledge increases over 15 years in the teaching profession

• Teachers’ pedagogical/psychological knowledge (PPK) increases significantly during the first 15 years of their professional career. • The induction phase proves especially effective for PPK acquisition. • Teachers with initially low PPK benefit the most during induction, indicating that initial gaps can be effectively closed. • Reflection on teaching experiences is especially effective for promoting PPK acquisition. Pedagogical/psychological knowledge (PPK) is an important aspect of teachers’ professional competence, linked to instructional quality and students’ learning. While research results indicate that PPK increases during formal teacher education, less is known about its long-term progression. This study explores the long-term development of PPK and its predictors among secondary school mathematics teachers. Using a validated competence test, PPK was assessed at the beginning (T1, n = 746) and at the end (T2, n = 567) of the induction phase, and after 15 years in the profession (T3, n = 116). Latent growth curve modeling showed significant growth in PPK from T1 to T3, with 55 % of the mean changes occurring within that period during the induction phase (between T1 to T2). Substantial interindividual differences in the trajectories were observed. Reflection on teaching experiences proved to be a predictor of positive development of PPK over time. The study provides important insights into the necessity and possible starting points for systemically supporting the development of PPK.