Dear Professor Ariely,
I work as a waiter in Waikiki, and sometimes to pass the time I conduct mini-experiments with customers, altering my behavior and attitude from day to day and seeing if it increases tips (in case you were wondering, seeming sad nets the most tips). I have noticed that those paying with credit cards leave bigger tips, but it varies by card: American Express AXP +0.31% users tip the most, those with Visas a little less. Discover DFS +0.99% card users are by far the worst. I can’t quite figure this out. —V.