From 1940 to 1990, about one third of Supreme Court law clerks became law professors. But in recent years, Brian Leiter and Jeff Gordon note, that percentage has dropped considerably. Sarah Lawsky has some numbers of clerks entering legal academia in the last decade or so that Brian recently posted:
Even if Sarah is missing some former clerks in her numbers, that's a noticeable drop. What explains the trend? Over in the comments to Brian's post, Professor Dan Epps has a suggestion that I think explains a lot: The increasing separateness of the law clerk and law professor track.
