The hypothesis that people cannot always make completely rational decisions by examining every possible choice but instead use of rules of thumb to guide them is known as;

Thomas Paine proved Common sense was a rule used to make rational decay

Currently, as of february 2016, an american traveling in france would be by the exchange rate. the weakness of the euro and the strength of the us dollar means that items in france cost less to an american than they otherwise would. of course, if this trend reverses and the us dollar weakens and the euro strengthens, such as was happening in early 2008, an american traveling in france would have an adverse experience because items would cost relatively more.