Final answer:
The Carnegie Mellon University study found that paying with cash causes the most pain and reluctance, due to the immediate realization of spending that is felt more acutely than when using cards.
Step-by-step explanation:
According to a Carnegie Mellon University study, paying with cash causes the most pain and reluctance. This phenomenon is often explained by the concept of the 'pain of paying,' which suggests that the act of physically handing over cash and immediately seeing less money in one's wallet triggers higher levels of emotional discomfort than less transparent forms of payment, like credit or debit cards. These findings have important implications for consumer psychology and spending behavior.