There are so many studies showing that it is definitely not true that all pre-1936 economists were advocates of laissez-faire that it is hard to know where to start listing them. On the other hand, whether or not Krugman is right depends critically on what you mean by "dominated" - what that word means here is just as problematic as "free market orthodoxy". I am not aware of any statistical analysis (which would be one way to do this) because the problem has so many dimensions - micro and macro for a start - and is riddled with difficulties. One starting point would be the literature on demand management before Keynes, which shows that interventionist policies were widely accepted in many quarters before 1936. It does not answer the question, but Michael Bernstein's book on the US economics profession might be worth a look. Roger Backhouse