I agree, but I don't see how that supports free trade in quite the way Krugman used it. When the countries are outwardly similar, the partner's "comparative advantage" is then often an economy of scale, which is a good reason to centralize, or a capital investment, which isn't necessarily. Then free trade is made a bit more transparent and we can start arguing about what we want to maximize - efficiency or [insert metric here].
But sadly, I'm pretty sure that sometimes there's no comparative advantage at all. Sometimes trade is profitable even if you don't have any advantage, so long as the cost of shipping is less than the profit margin at the market price. People are irrational, and squeezing even a little extra profit out of this fact would allow for all sorts of semi-useless trade.
Note that I agree that there is still "ordinary" comparative advantage out there, among this other stuff. But I think that this other stuff, while fairly indifferent to efficiency, can be harmful to [metric of choice] (say, assign negative value to burning fossil fuels).
Sometimes trade is profitable even if you don't have any advantage, so long as the cost of shipping is less than the profit margin at the market price.
If the cost of shipping is less than the profit margin, that means that someone is willing to buy something from you for more than it costs you to make it - which, in an ideal market, is pretty much equivalent to having a comparative advantage. (I think.)
I would like to learn more about economics but I don't know where to start. Can lesswrong suggest specific areas of economics that are particularly useful for understanding and optimising the world? Specific suggestions such as reading lists and resources would also be much appreciated.