“…21 To achieve our goal of offering a unifying perspective on the effects of Engel's Law on the patterns of structural change, innovation, and trade, we extended this model with many sectors producing gross complements, which differ only in the income elasticity, and allowing the two countries to differ both in the population size and in labor productivity. Although Krugman's model has been extended into many other directions, such as adding a competitive sector (Helpman and Krugman (1985), Chapter 10.4), many countries with different bilateral trade costs (Matsuyama (1999), Behrens, Lamorgese, Ottaviano, and Tabuchi (2009)), many sectors with different price elasticities and trade costs (Hanson and Xiang (2004)), non-CES demand systems (Costinot, Donaldson, Kyle, and Williams (2016)), etc., we abstain from such extensions in order to focus on Engel's Law.…”