Kevin Johnson, 29, sits in his Peachtree Street office. After returning from his honeymoon, American Express informed him it was lowering his credit limit.Johnson was surprised, since he has a perfect payment history and a high credit score. And he was floored by one of the reasons American Express cited: It didn’t like where he shopped.

“Other customers who have used their card at establishments where you recently shopped have a poor repayment history with American Express,” the letter said. Johnson complained to American Express by phone and letter.

Johnson checked his charges to try to figure out what might have raised a red flag in the American Express data-mining model. He didn’t see anything but typical transactions, including purchases at Amazon, Ruby Tuesday, Wal-Mart, Starbucks and Federal Express.