Dec. 30, 2010 -- While Rachel Ferucci, a grandmother from Westport, Conn., wheels her shopping cart through her local Stop & Shop, Fern Grant tracks her every move.
Grant is "just trying to watch." She's studying grocery shoppers the way anthropologists observe their subjects in their natural habitats. The vice president of Strategic Planning for MARS Advertising, Grant is responsible for developing marketing strategies for clients who want to know what shoppers think about when they move products off the shelves and into their carts.
She tracks everyday grocery shoppers as they loop up and down the aisles, carefully grabbing essentials on their lists and making unplanned impulse purchases.
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"I'm just trying to see what she's doing, and why she's doing it,' she said.
Grant, along with her colleague Liz Crawford, uses this research to zero in on one habit in particular. They want to know why shoppers, like Ferucci, add items to their carts they didn't intend to buy.
Questioning the shoppers' every move -- what's going on in their mind, what are they trying to achieve going down every aisle, do they generally make lists? -- Grant and Crawford can draw conclusions about why we buy what we do on a whim.
"I think we are wired to impulse buy," Crawford said. "We are ready to take advantage of our environment probably like we did thousands of years ago."
Crawford and Grant's research helps big companies tap in to our most basic instincts and shows how we all can be tempted to spend. Most of us, without knowing it, budget between 20 percent and 30 percent of our grocery money on impulse, the researchers said.
On this particular shopping outing, Ferucci said she planned to spend between $100 and $150 for the items on her list. When she rolled up to the checkout line, ingredients for a chocolate cream pie, Oreo cookies and a box of pasta made it out of the cart and onto the conveyor belt.
Even with the Oreos and other impulse buys, Ferucci spent less than what she planned. "I'm surprised," she said.
Impulse Purchases Stay Within Budget
That shoppers' mental calculators keep them within their budget tells Crawford "that shoppers are even smarter than they think," even when they're not thinking about it.
ABC News' Jake Whitman contributed to this report.