Johannes Koenig's analysis shows that an additional qualification after the doctorate is in most cases associated with considerable financial expenditure and that the benefits are primarily non-monetary.
The quantitative analysis is based on the IIPED dataset (IAB INCHER Panel on Earned Doctorates).

Koenig, Johannes (2024): Costs and benefits of a formal academic qualification beyond the PhD. Higher Education https://doi.org/10.1007/s10734-024-01338-3