Depth and breadth aren’t the same virtue wearing two names. They trade off, and the tradeoff shifts across a career.
“Well-rounded” advice usually asks for both depth and breadth at once, as if they were free to acquire together. Longitudinal research on creative careers finds a real tradeoff instead: knowledge depth pays off earlier in a career, when someone needs a solid structure to build on, while knowledge breadth pays off later, when flexibility across domains matters more than mastery of any one. Depth without breadth calcifies into rigidity; breadth without depth stays permanently shallow.
The related idea of “T-shaped” expertise — deep in one discipline, broadly literate across many — has become the standard way applied fields describe what deep knowledge should look like in practice: enough depth to actually solve hard problems, enough breadth to talk to people who solve different ones.
Source: Mannucci, P.V. & Yong, K. “The Differential Impact of Knowledge Depth and Knowledge Breadth on Creativity over Individual Careers,” Academy of Management Journal, 2017; Conley, S.N. et al. “Acquisition of T-shaped expertise: an exploratory study,” 2017.