You always seem to end up with more in your cart than you had anticipated, don’t you? As you heap heads of lettuce onto the conveyer belt and gingerly set down a carton of eggs, you place beside the regulars an unfamiliar face: a pint of ice cream perhaps, or a family-sized bag of potato chips that you’re certain will only satisfy one stomach. But don’t fret — the psychology of grocery shopping effectively demands that you stop thinking rationally about whatever it is you’re buying.

Grocery stores are huge today. There’s little debate about this. While the occasional farm stand punctuates a town’s landscape, the corporate grocer across the highway unquestionably dominates the space. Inside are two dozen rows containing thousands of different products and brands. Forty types of salad dressing. Twelve brands of ketchup. A cheese section. How can your brain handle all this sensory information? According to a growing body of research into the cognitive effects of shopping, unfortunately, it really can’t.

Being able to stick to a grocery list is, for many people, an exercise in self-control. Certain families choose to adhere to a strict budget, while others have a looser understanding of how much they’re spending each week. The prevailing theme remains the same: shoppers go into supermarkets with a plan, of some form, because they’ve responsibly decided at home that some things are necessary and some aren’t. As it turns out, people frequently abandon these guidelines within half an hour of shopping.

Researchers from Bangor University are currently running tests using MRI (magnetic resonance imaging) on a group of subjects that are told they have £80 ($129) to spend on a week’s worth of groceries. The research team is testing for the subjects’ ability to spot rip-off “special deals” from worthwhile discounts. Their preliminary results suggest that the subjects’ brains stop donating full effort around 23 minutes into the simulated shopping trip. At 40 minutes, nearly all the shoppers began purchasing emotionally, rather than rationally. And as any avid snack fan can attest, emotional shopping is dangerous shopping.

"We know from previous research that the brain behaves illogically when faced with the sort of information overload that shoppers are faced with in a typical supermarket," Phillip Adcock, managing director of Shopping Behaviour Xplained, told the BBC. "Now we have a reliable and scientific way of validating this research and understanding exactly what is happening in the brain during the weekly shop."

It’s also no accident that these difficulties arise. Grocery stores are strategically laid out in order to maximize the time shoppers spend in the store and to ensure they pass by as many products as possible on the way to what they actually came to buy. The Boomerang Effect, for instance, occurs as a result of the most popular products being stocked in the middle of the aisles, so that customers must pass the entire length of an aisle — dozens upon dozens of items — to reach the popular brands. It’s also no accident that the most popular items — milk, eggs, bread, and meat — are located in the far corners of the store. Or that sugary breakfast cereals are placed on lower shelves, at children’s eye-level.  

Perhaps the oldest trick in the book is the price-ending illusion of cheapness: the 99-cent trap. Foods that are priced at $4.99 rather than an even $5.00 are viewed as far more affordable than a simple penny difference.

"The bargain price is appealing to you because it challenges the status quo. The retailer appears not to be in complete control of the final price of the product, and this makes you feel that you are now in control,” Dr. Dimitri Tsivrikos, consumer psychologist from London Metropolitan University, told the BBC. "And because of that you feel you can negotiate the final price that you have to pay — whether that is the sale price or even a buy-one-get-one-free deal.”

This control can wreak havoc on our ability to spend rationally. A 2010 Cornell University study showed that consumers who were more incentivized to calculate their expected bill at checkout actually performed worse at the task. They tried harder, yet gave a worse estimate of what they were spending. Worse, the researchers found that the more shoppers underestimated the total cost of their basket items, the more they overspend relative to their budget. As the researchers note, spending more than a budget allows can have significant cumulative effects on consumer welfare.

“Because motivated shoppers appear to be most inclined to calculate the exact total basket price,” the authors explained, “the results indicate that shoppers with tight budgets, who are most motivated to be accurate, may also be most at risk for spending more than their budget.”

Herein lies the disconnect. Retailers want to maximize the time consumers spend in-store, yet the consumers (and their brains) are donating loads of effort to staying on task. The matter is complicated given that grocery stores provide a necessary good. In other words, people have to eat. So it’s in the store’s interest to keep shoppers stimulated with bright lights and colors, the pleasant smell of baked goods, and the calming sounds of easy-listening music played overhead.

The greatest defense against these tactics, the Cornell researchers point out, is to educate consumers. And given the present brain imaging study, scientists too will have greater insight into why people decide at the last minute that they desperately need, say, a pint of ice cream or a family-sized bag of potato chips.

"Using advanced brain imaging techniques we hope to get a better understanding of how shoppers respond to special offers," said Dr. Paul Mullins, Bangor University professor and researcher in the present study. "This also gives us the chance to bring our research on decision making into a real world context. We hope it'll tell us a lot about how we respond to different types of competing information in the world around us.”