Thus spake a very prominent colleague at another top law school, with many years of hiring experience:
Yale's disproportionately large number [in law teaching, relative to its size] is a selection effect. Yale ends up with so many more people in teaching because of the outdated myth "it's a good place to go if that's what you want to do" which keeps so many people going there. In fact, as I have long believed, it's a lousy place to go: most get dilettantish training, and way too many get pushed by their mentors to be second-rate versions of their mentors. Harvard is also bad--in HLS's case, because size means most don't get the kind of attention they need. But size and selection also keep the HLS numbers of people in teaching large. So HLS and YLS continue to have the appearance of being good places to go, and lots of kids (who would actually do better at a Stanford or Chicago) go to them and either don't get teaching jobs or get less good jobs as a result.
Far be it for me to disagree with this assessment!
ADDENDUM: A well-known legal scholar elsewhere, and a Yale Law grad, writes:
I have no inside information about Harvard, but completely agree with your informant’s observation about Yale Law grads: “most get dilettantish training, and way too many get pushed by their mentors to be second-rate versions of their mentors.” (Fewer than half the YLS alums in our business, by my rough estimate, had mentors when they were law students. I certainly had none and the faculty made it clear that nothing I did would get me any. The myth that once you get into that school, it’s all winks, secret handshakes, and unmerited interventions in your behalf is annoying. I’ll cop to being unjustly privileged by Yale Law on my resume, but not to any treatment from the faculty that gave me any help in my career. Peers who were mentored sure did get that push, though!) Dilettantish is a clement adjective for the training I received in the early 80s. Although it has improved since then, it’s still dilettantish.
But the suggestion that Stanford and Chicago would be better places to go: really? Both pump out a lot of law profs, especially Chicago. They’re not exactly deprived; shifting privilege into their ranks isn’t a change from the status quo. Be interesting for someone to investigate other high-ranked schools that don’t feed so much as those two (e.g. Penn, Michigan, Duke, Minnesota) to see which schools’ alums make especially good professors, or do particularly well on the entry market.
I agree that would be interesting, though I'm not quite sure how to come up with a suitable metric. I do think there is a significant difference in the intellectual and mentoring culture here at Chicago compared to Yale, but of course there's no simple way to quantify that either. But I agree with the basic point that the legal academy is ill-served by having 40% of professors coming from just six schools: Yale, Harvard, Chicago, Stanford, Columbia, and Michigan.