As we scurry about shopping, our money habits are on full display. Yet the motivations behind how we spend are often hidden, stemming from our most ancient and fundamental human tendencies.

The relatively new academic field of consumer behavior takes a closer look at the odd beast that is the modern-day consumer. Its findings help explain our actions at the mall or supermarket and why we often make poor buying decisions.

The field finds fascinating answers to basic questions. Would you spend more or less cash if the bills in your wallet were new and crisp? Why do we prefer to buy matching brands of chips and salsa? How do sadness and feeling impoverished affect our spending?

Marketers know the answers to many of these questions. It can help if we do, too. Here are a few recent findings in the field of consumer behavior.

• Crisp bills: We’ve learned previously that consumers spend more with credit cards than cash. And when they spend cash, they are hesitant to spend large-denomination bills – they more readily spend four $5 bills than a $20 bill.

Now, we find shoppers are also hesitant to spend crisp bills – unless they’re showing off, according to the Journal of Consumer Research.

“Consumers tend to infer that worn bills are used and contaminated, whereas crisp bills give them a sense of pride in owning bills that can be spent around others,” write authors Fabrizio Di Muro of the University of Winnipeg and Theodore J. Noseworthy of the University of Guelph.

They found in experiments that consumers spent more with worn bills than with crisp bills and were more likely to break a worn larger bill than pay the exact amount in crisp lower denominations. But when consumers thought they were being socially monitored, they tended to spend crisp bills more than worn ones.

The reason? “People want to rid themselves of worn bills because they are disgusted by the contamination from others,” the authors write. Indeed, the authors note that most bills are often replaced not because they are physically worn but because of “soil content,” which refers to the amount of bacteria found on the bills. Meanwhile, spending crisp bills gives people a sense of pride.

So perhaps spendthrifts can help curb their impulses by visiting the ATM for some new $20s before their next shopping trip.

• Matching brands: Consumers prefer matching brands for products that are used together. Apparently, we believe they have been designed and tested to work in concert – even if it doesn’t make much sense.

Consider chips and salsa, write University of Minnesota authors Ryan Rahinel and Joseph P. Redden in the Journal of Consumer Research.

In one study, consumers ate Tostitos brand tortilla chips and Tostitos brand salsa but were told that the chips and salsa were various combinations of fictional brand names. The finding? Though it makes almost no common sense, consumers enjoyed the chips and salsa more when told they were the same brand.

So, perhaps when buying chips and salsa – or a television and DVD player – you’ll get more satisfaction if you buy matching brands. But realize that little effort might have gone into making them work well together. They might simply be marketed together. So, make purchasing decisions based on traditional – and probably more important – factors, such as the individual quality of each and their prices.

Of course, if you’re a product designer or marketer, perhaps you want to develop companion products under the same brand because you just might please your customers and sell more.

• Scarcity: Poor people often behave in ways that reinforce their poverty – playing the lottery, failing to enroll in assistance programs, saving too little and borrowing too much. A number of reasons account for that behavior, including education, health and living conditions.

But a recent study suggests a more general reason: that scarcity creates a mindset that changes how people view problems and make decisions, according to a recent paper in the magazine Science by Anuj K. Shah of the University of Chicago Booth School of Business, Sendhil Mullainathan of Harvard University and Eldar Shafir at Princeton University.

The idea is that scarcity creates overly intense focus on whatever is scarce. For example, a trip to the grocery store or paying the monthly rent is mundane for most people but an impending crisis for people without money.

That focus comes at the cost of paying attention to other, less urgent concerns, often in the future. “A scarcity mindset leads people to choose the most locally convenient response to pressing demands, leading to constant financial juggling,” the authors write.

So, for example, lack of money will create a tendency to borrow at outrageous interest rates, which quells the immediate crisis but leads to larger expenses in the future.

The scarcity theory doesn’t only hold for lack of money but also applies when you’re hungry or feeling pressed for time, the authors say.

“What these studies show is that if you take otherwise sophisticated people and put them in conditions of scarcity, they exhibit the same behaviors we see among the poor,” said Shafir, a member of the President’s Advisory Council on Financial Capability.

• Myopic misery: Another study about misdirected focus addresses the issue of being sad.

Sadness increases people’s impatience and creates a focus on obtaining money immediately instead of later, which can lead to financial losses, says a paper published in Psychological Science.

One obvious example arises when a loved one dies, the authors note. A survivor is sad at the death and at the same time might have to make huge financial decisions, from funeral arrangements to settling the estate.

That survivor is more likely to make choices that involve immediate gratification, which, when it comes to money, are often the wrong choices.

Simply being aware of your tendencies during periods of sadness might cause you to take a more thoughtful and long-term view when making money decisions.