When international policymakers do not agree on the correct macroeconomic model, they will still be able to agree on a cooperative policy package that each believes will improve welfare; but the package may turn out to move the target variables in the wrong direction. Using ten leading econometric models that could represent U.S. beliefs, non-U.S. beliefs, and the true model, we find that monetary coordination improves U.S. welfare in only 546 cases out of 1,000.