Background: Researchers often report and discuss gender differences. However, recent research has drawn attention to interaction effects between gender and other social categories. Aims: This study analysed the development of disparities in students' reading-related self-concept, intrinsic motivation, and behaviour, as they relate to differences in gender and socio-economic family background. Drawing on expectancy-value theory, we regarded reading-related self-concept, motivation, and behaviour as key to explaining the growing differences between boys and girls in adolescence. Specifically, we focused on the interaction between gender and socio-economic background in children, which has been discussed in the context of moderating gender differences but not in the context of reading-related attitudes and behaviour. Sample: The investigation is based on a longitudinal sample of N = 717 German students between third and sixth grades. Method: We used questionnaire data from both students and parents. To compare students' development across time, we applied multigroup latent growth curve models. Results: We found evidence of increasing gender differences, which were also moderated by the socio-economic status (SES) of parents: a gender gap either already existed (intrinsic motivation and reading behaviour) or intensified (reading self-concept and reading behaviour) between third and sixth grades. The interaction of gender and SES seemed particularly important for reading self-concept, with the gender gap growing less substantially for higher-SES children. Moreover, this pattern persisted for reading self-concept, even when controlling for achievement differences. Conclusions: The results provide evidence that gender, social background, and the interaction of the two are relevant for development in the domain of reading, even in young children.