Economists have argued for more than two centuries that voluntary trade, whether domestic or international, is positive sum: it benefits both parties, or else the exchange wouldn't occur. Economists have also long argued that the economics of immigration -- immigrants coming here to exchange their labor for money that they then exchange for the products of other people's labor -- is positive sum. Yet our evolutionary intuition is that, because foreign workers gain from trade and immigrant workers gain from joining the U.S. economy, native-born workers must lose. This zero-sum thinking leads us to see trade and immigration as conflict ("trade wars," "immigrant invaders") when trade and immigration actually produce cooperation and mutual benefit, the exact opposite of conflict.
I wonder all the time why normally intelligent people can't see the benefits of free trade - maybe this is the answer.