It's a bit easier in math than other subjects to know when you're right and when you're not. That makes it a bit easier to know when you understand something and when you don't. And then it quickly becomes clear that pretending to understand something is counterproductive. It's much better to know and admit exactly how much you understand.
And the best mathematicians can be real masters of "not understanding". Even when they've reached the shallow or rote level of understanding that most of us consider "understanding", they are dissatisfied and say they don't understand - because they know the feeling of deep understanding, and they aren't content until they get that.
Gelfand was a great Russian mathematician who ran a seminar in Moscow for many years. Here's a little quote from Simon Gindikin about Gelfand's seminar, and Gelfand's gift for "not understanding":
One cannot avoid mentioning that the general attitude to the seminar was far from unanimous. Criticism mainly concerned its style, which was rather unusual for a scientific seminar. It was a kind of a theater with a unique stage director playing the leading role in the performance and organizing the supporting cast, most of whom had the highest qualifications. I use this metaphor with the utmost seriousness, without any intention to mean that the seminar was some sort of a spectacle. Gelfand had chosen the hardest and most dangerous genre: to demonstrate in public how he understood mathematics. It was an open lesson in the grasping of mathematics by one of the most amazing mathematicians of our time. This role could be only be played under the most favorable conditions: the genre dictates the rules of the game, which are not always very convenient for the listeners. This means, for example, that the leader follows only his own intuition in the final choice of the topics of the talks, interrupts them with comments and questions (a privilege not granted to other participants) [....] All this is done with extraordinary generosity, a true passion for mathematics.
Let me recall some of the stage director's strategems. An important feature were improvisations of various kinds. The course of the seminar could change dramatically at any moment. Another important mise en scene involved the "trial listener" game, in which one of the participants (this could be a student as well as a professor) was instructed to keep informing the seminar of his understanding of the talk, and whenever that information was negative, that part of the report would be repeated. A well-qualified trial listener could usually feel when the head of the seminar wanted an occasion for such a repetition. Also, Gelfand himself had the faculty of being "unable to understand" in situations when everyone around was sure that everything is clear. What extraordinary vistas were opened to the listeners, and sometimes even to the mathematician giving the talk, by this ability not to understand. Gelfand liked that old story of the professor complaining about his students: "Fantastically stupid students - five times I repeat proof, already I understand it myself, and still they don't get it."
In my time in the mathematical community I've formed the subjective impression that it's noticeably less common for mathematicians of the highest caliber to engage in status games than members of the general population do. This impression is consistent with the modesty that comes across in the writings of such mathematicians. I record some relevant quotations below and then discuss interpretations of the situation.
Acknowledgment - I learned of the Hironaka interview quoted below from my colleague Laurens Gunnarsen.
Edited 10/12/10 to remove the first portion of the Hironaka quote which didn't capture the phenomenon that I'm trying to get at here.
In a 2005 Interview for the Notices of the AMS, one of the reasons that Fields Medalist Heisuke Hironaka says
(I'll note in passing that the sense of the "genius" that Hironaka is using here is probably different than the sense of "genius" that Gowers uses in Mathematics: A Very Short Introduction.)
In his review of Haruzo Hida’s p-adic automorphic forms on Shimura varieties the originator of the Langlands program Robert Langlands wrote
For context, it's worthwhile to note that Langlands' own work is used in an essential way in Hida's book.
The 2009 Abel Prize Interview with Mikhail Gromov contains the following questions and answers:
In his MathOverflow self-summary, William Thurston wrote
I interpret the above quotations (and many others by similar such people) to point to a markedly lower than usual interest in status. As JoshuaZ points out, one could instead read the quotations as counter-signaling, but such an interpretation feels like a stretch to me. I doubt that in practice such remarks serve as an effective counter-signal. More to the point, there's a compelling alternate explanation for why one would see lower than usual levels of status signaling among mathematicians of the highest caliber. Gromov hints at this in the aforementioned interview:
In Récoltes et Semailles, Alexander Grothendieck offered a more detailed explanation:
The amount of focus on the subject itself which is required to do mathematical research of the highest caliber is very high. It's plausible that the focuses entailed by vanity and ambition are detrimental to subject matter focus. If this is true (as I strongly suspect to be the case based on my own experience, my observations of others, the remarks of colleagues, and the remarks of eminent figures like Gromov and Grothendieck), aspiring mathematicians would do well to work to curb their ambition and vanity and increase their attraction to mathematics for its own sake.