Gravity equations have been widely used to infer trade flow effects of various institutional arrangements. We show that estimated gravity equations do not have a theoretical foundation. This implies both that estimation suffers from omitted variables bias and that comparative statics analysis is unfounded. We develop a method that (i) consistently and efficiently estimates a theoretical gravity equation and (ii) correctly calculates the comparative statics of trade frictions. We apply the method to solve the famous McCallum border puzzle. Applying our method, we find that national borders reduce trade between industrialized countries by moderate amounts of 20-50 percent.

This paper surveys the measurement of trade costs: what we know and don't know but may usefully attempt to learn. Partial and incomplete data on direct measures of costs go with inference on implicit costs from trade flows and prices. Total trade costs in rich countries are large. The ad valorem tax equivalent is about 170 percent when pushing the data hard. Poor countries face even higher trade costs. There is a lot of variation across countries and across goods within countries, much of which makes economic sense. In our survey, theory provides interpretation and perspective and suggests improvements for the future. Some new results are presented to properly apply and interpret gravity theory and handle aggregation.

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National money is a barrier to international trade. Accordingly, currency unions have lower trade barriers, more trade, and higher welfare. This paper uses empirical gravity models to quantify these effects using a large panel data set. We estimate that trade barriers associated with national borders are halved when countries join a currency union, significantly raising trade and welfare. EMU may lead to an increase in euroland's trade of over 50%, with comparable numbers for Mexican dollarization. JEL Classification Numbers: F15, F33

Empirical evidence shows that most exchange rate volatility at short to medium horizons is related to order flow and not to macroeconomic variables. We introduce symmetric information dispersion about future macroeconomic fundamentals in a dynamic rational expectations model in order to explain these stylized facts. Consistent with the evidence, the model implies that (a) observed fundamentals account for little of exchange rate volatility in the short to medium run, (b) over long horizons, the exchange rate is closely related to observed fundamentals, (c) exchange rate changes are a weak predictor of future fundamentals, and (d) the exchange rate is closely related to order flow. (JEL F3, F4, G0, G1, E0)