Econ Lab · Trade
Comparative advantage
Your country is good at making phones and only fair at growing food. A neighbour is the other way around. The puzzle is older than it looks. Even if you were better than them at both, you would still come out ahead by specialising and trading. The reason is opportunity cost, not raw skill.
Slide the trade price and watch the dashed line lift above your own frontier.
On its own your country gives up half a unit of food for each phone, the slope of its own frontier. A trading partner is clumsy at phones and will hand over up to food for one. The band between and is where a deal helps both of you.
At food per phone the price sits inside the band, so both countries come out ahead. The trade line lies above your frontier, which means you specialise in phones, sell them, and buy back food on terms your own farms could never match. The further the price climbs toward , the more of that gain is yours.
Drag the slider for the trade price. The solid line is what you can make alone. The dashed line is what trade lets you consume.
What it costs you, not what it costs them
Your frontier says you can make 120 phones or 60 food, so one phone costs you half a unit of food. That give-up rate is your opportunity cost.
Your partner is worse at phones. For them, one phone costs 2 food. Comparative advantage is about these costs, not about who is faster in absolute terms.
Specialise, then trade
Stop splitting your effort. Pour everything into phones, make all 120, and sell them for food at the trade price T. Your consumption now rides a new line.
Keep some phones for yourself and export the rest. Each exported phone brings home T units of food.
Above your own frontier
Here is the payoff. As long as T is above your own cost of half, the dashed trade line sits above the solid frontier. You can now consume bundles your own farms and factories could never reach at once. The gap between the two lines is the gain from trade, and it widens as the trade price improves.
Why both sides win
The trade only works if the price lands between the two opportunity costs, half a food per phone and 2 food per phone. Below half, you may as well grow your own. Above 2, the partner walks away. Anywhere in that band both countries end up beyond their own frontiers. That band is the whole story of why trade creates value rather than just moving it around.