I enjoyed this article in the Harvard Business Review on leading people when they know more than you do. It makes the distinction between specialist leaders and generalist leaders. "A specialist manager knows what to do; the generalist manager knows who to call. The specialist leader tells her staff the answer, the generalist brings them together to collectively find the answer."
This got me thinking: this doesn't just apply to leadership of teams, but also to contributing to teams, dealing with clients or counterparts, and, really, any sort of endeavour when you have the option of interacting with other people to try to solve a problem.
When we are trying to solve a problem, even as a leader, there are points in time where we need to act like generalists and get people's inputs, and times we need to shut down the review process to focus on the task at hand. The generalist may be the superior manager for a broader range of tasks, but a generalist's success as a manager depends on the existence of specialists in their team that actually know the industry or technical skill required to get the work done.
In the real world, as the authors hinted at, it's not as simple as "specialists are bad" and "generalists are good." The authors were responding to the very common problem of specialists struggling to make the transition to being generalists. In fact, there also exist generalists that struggle with a lack of specialist skills.
A common concept that is used as an ideal for consultants by many big firms is that of the T-shaped professional. You can read the Wikipedia page or google for more info, but the figure below illustrates the knowledge of a pure specialist, a pure generalist, and a t-shaped professional.
All of the knowledge maps above have the same area. Someone that is smarter or works harder to learn can increase their area, in terms of depth and breadth, but most of us are never going to fill in the square entirely.
And that's ok. In fact when a client hires you to do something for them, it's not usually because you know their industry better than them, it's because you can do something they can't and it's cheaper/better/faster to hire you in to do this one thing than it is to try to develop the capacity to do it in-house.
A lot of professional services firms tend to have a comparative advantage in knowledge of particular techniques, that can be applied across a fairly wide range of industries. For example, tax planning, financial modelling, business strategy, legal drafting, technical writing, project management, and so on.
The best consultants I know are very clear with their clients in what they know, and where the limits of their knowledge lie. This helps both client and consultant work best together to identify potential blind spots that one or both parties may have, to focus attention there, and have the best chance at coming to the right decision.
The key thing that creates value in team building is complementarity of skill sets. The problem for pure generalists is that when they're working within a single domain, a lot of their knowledge is either not related to the domain, or duplicated by others in a team. A pure specialist, at least can add a lot of value if deployed correctly.
So, to answer the question I pose in my title: how much do you need to know about a field to give advice about it?
The HBR article's answer is something like: Not much. You're a generalist manager, don't waste your time duplicating knowledge that exists elsewhere because you don't want to look ignorant and ask a question. Rely on your specialist minions to deliver and get out of their way!
I think that's pretty solid advice, but I might add that you need to learn as much as you need to know how to leverage your specialist knowledge in the field. If you are truly the "leader without expertise" that the HBR article mentions in its opening paragraphs, then you'd better develop some expertise soon, or you're probably not long for leadership!
So, in conclusion: know thyself! Are you a specialist that needs to generalise, or a generalist that needs to specialise? The right answer will depend on you, your personality, your abilities, and the career and life you envision for yourself. Finding the right balance for you will give you the best chance at developing a satisfying career working with, and managing others.