There was a limited literature on external economies and international trade before 1978. It was never, however, very influential, because it seemed terminally messy; even the simplest models became bogged down in a taxonomy of possible outcomes. What has since become clear is that this messiness arose in large part because the modelers were asking their models to do what traditional trade models do, which is to predict a precise pattern of specialization and trade.

How I Work (1993)