Forget shopping lists and budgets, the powerful emotions of revenge and gratitude are the true determinants of how we shop, a new study on consumer behaviour suggests.

The study, to be published in the December issue of the Journal of Consumer Research, revealed that when a shopper is surprised by a positive change such as a sale, they often ended up buying more in an attempt to reward the retailer.

Researchers have found that sales often inspire shoppers to spend more than budgeted.
(Associated Press)

But when faced with a negative change, such as a price hike or a decrease in product quality, consumers become angry and ended up purchasing less. Researchers found that the frustrated shoppers would even overlook other products offered at discounted prices.

"[In] the same way that a restaurant patron might express gratitude for a complimentary bottle of wine by ordering more or express anger for a long wait by ordering less, here we observe similar responses to unexpected product price and quality changes, even in settings where such actions would have no disciplining effect," the study said.

Lead researcher Narayan Janakiraman, a marketing professor at the University of Arizona, deemed this pattern of behaviour the "spillover effect" — referring to how one change can affect other purchases. 

The researchers noted that in some cases, consumers may believe that when they purchase less, they're forcing retailers to change course and offer better prices.

However, the authors noted that the study did not include parameters for retail rewards and punishment, and yet these two instincts prevailed. 

The authors of the study said this challenges research that suggests consumers make a mental budget and purchase according to their own sense of increased or diminished wealth.