Trade between countries creates gains overall (Specialization and the Terms of Trade). One market's supply and demand graph, with the surplus areas from Total Surplus and Deadweight Loss, shows who gains and who loses when a country opens to trade.
Key idea
A small country that opens to trade takes the world price $P_w$ as given. If $P_w$ is below its own equilibrium price, the country imports $Q_d - Q_s$ at $P_w$: consumers gain, domestic producers lose, and consumers gain more than producers lose. If $P_w$ is above, it exports $Q_s - Q_d$: producers gain more than consumers lose.
Why it works
With imports, the price falls to $P_w$. Buyers pay less on every unit and buy more, so consumer surplus grows a lot. Domestic sellers get less and sell less, so producer surplus shrinks. The rectangle between the old and new price on domestic sales just moves from producers to consumers. Consumers also gain a triangle on the extra imported units, and nobody loses that one: it's the net gain from trade.
Worked example: Opening to imports
$Q_d = 20 - 2P$, $Q_s = -4 + 2P$ (equilibrium \$6, 8 units). The country opens to trade at a world price of \$4. Find imports and the changes in consumer and producer surplus.
- At \$4: $Q_d = 12$, $Q_s = 4$, so imports are $12 - 4 = 8$.
- Consumer surplus: from $\tfrac{1}{2}(8)(4) = \$16$ to $\tfrac{1}{2}(12)(10 - 4) = \$36$, a gain of \$20.
- Producer surplus: from \$16 to $\tfrac{1}{2}(4)(4 - 2) = \$4$, a loss of \$12.
- Net gain: $\$20 - \$12 = \$8$, the triangle $\tfrac{1}{2} \times (6 - 4) \times (12 - 4)$.
Answer: Imports 8; consumers +\$20, producers −\$12, net +\$8.
Worked example: Opening to exports
Same market, but the world price is \$8. What happens?
- At \$8: $Q_d = 4$ and $Q_s = 12$. The country exports $12 - 4 = 8$.
- Domestic buyers now pay \$8 and buy less; producers sell more at a higher price.
Answer: Exports of 8; producers gain, domestic consumers lose, and the gain is larger.
Common mistake
Seeing only the loss to domestic producers and concluding the country as a whole is worse off. In the model, consumers' gain is bigger than producers' loss. But the losses are real and concentrated: particular workers and firms bear them, sometimes for years, while the gains are spread across many buyers. Check: compute both changes and add them.
Economists broadly agree on the net gain in this model. How large and lasting the losses are for workers in import-competing industries, and how best to help them, are actively studied and debated.