Okay, guys, this is getting a little ridiculous. I go to MIT, and I can assure everyone that although a very large fraction of the "best" students do come here if admitted, certainly not all of them do. It varies a lot year to year, by field, etc. Plus the admissions process isn't perfect, and since the department doesn't have a lot of funding it needs to rely a lot on the NSF. Good recruitment years tend to be ones where the NSF people do a good job and give money to the same people MIT wants anyway. Four years ago this happened with all MIT's current stars: Dell, Carroll, Chandrasekhar, Hendren. But it doesn't happen every year, and in the end MIT probably gets about 1/3 of the best students - maybe somewhat more at the very top.
And yes, the string of 4 MIT PhDs getting Clark medals in a row is somewhat unrepresentative. Yes, it reflects very well on MIT, but yes, partly it's just a lucky streak. What I don't get is why everyone feels the need to deem this some kind of grand conspiracy. Most of the choices have been pretty logical; there are some other strong candidates who have been passed over, but the winner has always been one of the two or three leading candidates in the mind of the profession. Take a look at this WSJ article on "handicapping the Clark medal" from two years ago:
They list four leading possibilities: Esther Duflo, Sendhil Mullainathan, Jonathan Levin, and Amy Finkelstein. (And go on to mention that Raj Chetty and Roland Fryer will probably get it someday, but are too young for now.)
Three Clark medals later, where are we? Well, the WSJ's predictions (presumably gathered by sniffing around and talking to various economists about who they think the good young prospects are) have been extremely accurate: of its four leading candidates to get the prize in 2010, three out of four ended up receiving it. Unless you think that the WSJ was also just talking to the MIT mafia or something, it sounds like the Clark medal results pretty accurately reflect the consensus in the profession.
Why did they give the award to Finkelstein and not Mullainathan? I don't know the story. Both are very good economists. Maybe the committee thought that Finkelstein is more prominent within her subfield than Mullainathan is in his; she is definitely the top young health economist, whereas Mullainathan is excellent but also just one of several successful young behavioral/development economists. (And they already gave a prize to Duflo two years ago.) She also has results from the Oregon experiment coming in, which figure to be just as important in the field of health economics as the RAND experiment was a few decades ago. These are things you can't gather from citation counts alone, which tend to be heavily biased toward particular subfields and don't reflect the general magnitude of someone's contribution to a particular subfield. But I don't really know; both were deserving, and the committee made a difficult and close choice.
Regardless, the MIT streak is surely coming to an end in the next few years, as leading candidates like Raj Chetty and Nick Bloom have no affiliation with MIT. Maybe then the slightly crazy people on this board will realize that the Clark medal is not a conspiracy, and that it mainly just represents the consensus within the profession. Of course, you can feel free to hate on that consensus...