Are interdisciplinary dissertations an asset on the job market?
No. Interdisciplinary PhDs face a wage penalty on average.
There's a lot of talk about valuing interdisciplinary research at universities, but it's largely hot air. PhD students who write interdisciplinary dissertations tend to earn less in their first job after graduation, compared to those who work in a single discipline.
The pattern is driven by dissertations that span closely related fields—for instance, evolutionary biology and horticultural science, or sociology and political science. In part, that's because graduates who bridge proximate fields are more likely than their peers to take postdocs, rather than industry or faculty positions.
The story is a bit more complicated when PhD students bridge distant disciplines. For students in low-paying disciplines, adding a high-paying secondary field can pay off—say, adding a computer science element to a humanities dissertation. But the reverse is true, too: computer scientists who dabble in literature will tend to lose pay in the short term.
Spanning distant disciplines can also hurt researchers' publication records. Interdisciplinary papers draw a lot of citations, but interdisciplinary researchers tend to have fewer publications.
All things considered, interdisciplinary dissertations appear risky for PhDs on the job market. For students in high-paying disciplines, they might be especially costly.