For many of us, the holiday season is when we go from being human beings to something called consumers slack-jawed robots with one purpose: to buy stuff. We pour into stores and onto the web to buy gifts for friends and family (and, heck, while we re there, a little something for ourselves). To stores, we re just walking, talking, browsing cash pi atas.

With that in mind, here are a few things you should know about your consumer brain before retailers start whacking at you with plastic bats:

Look, but Don t Try On

Shopping for a new sweater? A new jacket? A new pair of jeans? Beware the dressing room because once you re trying on an item of clothing, something called the endowment effect may start to set in.

The endowment effect is when a person s ownership of an object (real or imagined) makes it suddenly worth much more to them. Take, for example, a study conducted by economist Dan Ariely, and recounted in his book, "Predictably Irrational." At Duke, people are fanatical about their basketball team, and especially about tickets to games in the university s small basketball stadium. During the Final Four, Ariely s research team, posing as scalpers, called up students who had been in a ticket raffle. For students who had not won tickets, they asked what the most was the students would pay for a ticket; the average was $170. For students who had won tickets, they asked what was the smallest amount for which the students would sell the tickets; the average was $2,400.

The lesson: Once we own something, it becomes much more valuable to us. The students who had won the tickets valued them 14 times more than those who wanted to buy them. We grow attached to things quickly thinking about them, imagining ourselves enjoying them, and focusing on the pain we d feel if we lost them.

The second you try on that sweater or take that car for a test drive or start using that trial version of an expensive piece of software you start to feel ownership of it. And it s that much harder to give it back.

In Fact, Don t Even Touch

What s more, recent research shows that it doesn t even take an interaction as intense as trying on to make you begin to feel the endowment effect. A paper by Joann Peck of the Wisconsin School of Business, published this year in the Journal of Consumer Research, shows that merely touching an object can induce feelings of ownership and make a person willing to pay more for the object in question. No wonder Apple wants to give you every chance to play with the iPhone in their stores.

For online retailers, Peck found that encouraging ownership imagery basically, having people imagine owning and touching something can have a similar effect to actually touching. So, don t fondle the goods in your mind too much either.

Are You a Man? Beware of Shopping Near Women

The most important thing we do, biologically, is reproduce. So, it s no wonder that sex factors into shopping. In 2007, a study in the Journal of Personality and Social Psychology reported that romantically primed men (that is, men induced to think about sex by means of being shown pictures of attractive women) were more likely to splurge on items of conspicuous consumption, like a new car, a fancy watch or a new cellphone. At the same time, romantically primed men were not likely to increase their spending on inconspicuous items like an alarm clock or household cleaners.

The researchers conclusion: Men use purchases as a signal to potential mates, to show that they re fit and have access to resources.

(Do women display the same behavior when romantically primed? At least in this study, no. Instead, women responded to romantic priming by reporting more willingness to spend time volunteering.)

So, guys, watch out for that cute salesgirl. And if she touches you on the arm, be especially careful human touch releases a brain chemical called oxytocin, which is important to social bonding and trust. In the lab, oxytocin has been shown to make people more generous with their money.

The 99-Cent Effect

So, will knowing about these biases will help us act more rationally? Hopefully, but maybe not. After all, look at the and-ninety-nine-cents effect. You already knew about this one, right? Who do these retailers think they re kidding with a price like $99.99 or $199.99?

But if everyone gets it, why do retailers keep doing it? Because it still works.

A recent study in the Journal of Consumer Research found that when pens were priced at $1.99 and $4.00, only 18% of the participants chose the higher-priced pen; but when the pens were priced at $2.00 and $3.99, 44% of the participants selected the higher-priced pen. That one-cent price drop makes the $4 pen seem a lot cheaper.

For whatever reason, we can t take our eye off that leftmost digit. But we can at least try.

Ryan Sager writes the blog Neuroworld at TrueSlant.com.