<p>For a successful academic career, scholars must be both productive and innovative in their research. This leads to an essential tension between exploitation and exploration. Qualitative research has highlighted some micro-level factors affecting scientists’ development of their research agenda, but there is scant knowledge about the role played by the institutional context and practices, and little quantitative evidence. This article provides a longitudinal comparative study of 4,785 scientists in four research areas (i.e., Sociology and Political Science, Economics and Econometrics, Immunology, Statistical and Nonlinear Physics) in four European university systems. The existence of a “protected space” has been identified as a crucial condition for exploration. We argue that interactions between a researcher’s seniority, scientific mobility, and – crucially – the discipline in which they work, affect the protected space and hence the capability to diversify their individual research agenda. We examined the textual content of the abstracts of 141,690 publications collected from Scopus and explored which factors affected research diversification over the career of individuals, namely a publication’s diversity compared to the most similar past publication of the same author. We reconstructed the spread of each scholar’s publications within an embedding space obtained via a Large Language Model. The results show that seniority positively affects diversification. Mobility is also positively associated with diversification, especially in disciplinary areas where individuals have more limited protected space. Finally, new scientific collaborations are the stronger predictor of an increase in diversification.</p>