Heh. Russell Roberts dislikes Paul Krugman's political views so much that he has completely refused to study in trade theory from the last 30 years, especially the New Trade Theory (NTT). Krugman started to research increasing-returns specialization in late 70's.
Scale Economies, Product Differentiation, and the Pattern of Trade
Paul Krugman, The American Economic Review
Vol. 70, No. 5 (Dec., 1980), pp. 950-959
Scale Economies, Product Differentiation, and the Pattern of Trade Paul Krugman, The American Economic Review Vol. 70, No. 5 (Dec., 1980), pp. 950-959
https://www.jstor.org/stable/1805774
edit: Later in the comments he confirms On Krugman, "I don’t know his work very well." It seem that he has missed the whole New Trade Theory (NTT).