The document discusses country similarity theory, which was developed by Swedish economist Steffan Linder. According to Linder, intra-industry trade occurs between countries with similar levels of economic development because they have similar consumer preferences. The theory suggests that companies first develop new products for their domestic market and then export those products to countries at a similar economic level once domestic demand is met. Location, culture, and political/economic interests can also influence trade between countries with similarities in these areas.