title: Heckscher–Ohlin model
context: Bertil Ohlin first explained the theory in a book published in 1933. Ohlin wrote the book alone, but he credited Heckscher as co-developer of the model because of his earlier work on the problem, and because many of the ideas in the final model came from Ohlin's doctoral thesis, supervised by Heckscher.
question: When was the Heckscher–Ohlin model developed?
answers: 1933