Barry Eichengreen points out that while the differential between growth and interest rates between the U.S. and most other countries should have predicted a stronger dollar in 2017, it actually fell by 8% and is still falling so far in 2018. Explaining exchange rate changes after the fact is a lot like explaining stock market changes after the fact – they are easy to rationalize after the fact, but if anyone really knew how to predict them accurately, that person would be a trillionaire. Somewhat humorously, Mr. Eichengreen links to an article that gives 17 possible reasons (with links to sources for many of them), which is essentially the same as giving none.

Finally he says the most likely explanation is just uncertainty. Foreign investors just don’t know where the U.S. and its economy are headed, or that it will continue to be the rock solid safe haven it has been for the past 50 years. This sounds about right to me. Foreigners have been willing to stuff U.S. dollars under their mattresses for 50 years, in the last couple decades with low or even no returns, and some may have decided it is time to diversify.