Recently, this column took a look at some basic shopping psychology mind quirks people should be aware of before trudging off to the malls for Christmas. For instance, we looked at why touching items you re thinking about buying can make you irrationally attached to them and why shopping in front of an attractive woman might make you more likely to splurge on status goods (if you re a man, that is).

The ticks that might affect your buying decisions, however, are virtually limitless. So, without further ado, here are a few more biases you should watch out for while you shop or even before you leave the house or fire up the browser:

The Lure of the Middle Option

Nothing makes the price of a $200 ice-cream maker look more reasonable than placing it next to a $400 ice-cream maker. It s called the compromise effect, and retailers are experts at manipulating it or, rather, using it to manipulate you.

Take the following experiment conducted by cognitive science pioneers Amos Tversky and Itamar Simonson. Sixty undergraduates were asked to choose between two microwave ovens: an Emerson priced at $110 and a Panasonic priced at $180. The more expensive oven had a few more features; both were on sale for one-third off. In this scenario, 57% of the students chose the less expensive Emerson oven and 43% chose the more expensive Panasonic.

In the experiment, however, another group of students was asked to choose between three options: the $110 Emerson, the $180 Panasonic, and a $200 Panasonic (with only a 10% discount). The two higher-priced Panasonics appeared very similar, so there was very little reason for anyone to choose the most expensive option. And, in fact, few of the students did only 13%. The mere presence of this higher-priced option though increased the proportion of people picking the $180 Panasonic, the middle option, to 60% (from 43% in the group that only had two options).

This effect is obviously not rational. The presence of a higher-priced option obviously doesn t make the middle-priced option a better deal. But it certainly makes it feel like a better deal a fact to which retailers have long been hip. Williams-Sonoma, for instance, was able to increase sales of its $275 bread machine in the 1990s by adding a second, larger model to its catalogue priced at more than $400. Xerox Corp. pulled a similar trick boosting high-volume copier sales by rolling out a higher-priced model with some extra bells and whistles that managers could congratulate themselves about forgoing.

So, when you re standing in front of a rack of ridiculous doodads and congratulating yourself on not choosing the most expensive one, take a step back by the way, literally taking a step backward has been found to help people improve cognitive control and make sure you re evaluating a potential purchase on its own merits not just relative to overpriced alternatives.

Don t Shop Depressed

This one may be unavoidable. These are, after all, the holidays. But research shows that it may not be the best idea to go shopping when you re depressed.

A study published in Psychological Science in 2008, titled Misery Is Not Miserly, found that individuals feeling sad and self-focused will actually spend more on a given item than their non-sad counterparts. Researchers paid participants $10 when they came in for the experiment and then showed them a sad or neutral video clip. (The sad clip was of a boy s mentor dying in the movie "The Champ;" the neutral clip was from a National Geographic special about the Great Barrier Reef.) Next, they asked the subjects to write essays about themselves.

The study found that the people who were shown the sad video who were exhibiting a high-degree of self-focus were willing to spend roughly five times as much on a sporty, insulated water bottle, which they had been offered a chance to purchase at various price points. The researchers explanation is that people who are sad and self-focused crave the boost of self-enhancement.

So, don t shop sad unless you re willing to pay a lot to feel a little better.

Don t Let Your Brain Play Tricks on You

See something you want on sale but suddenly feel hesitant? Don t let your brain s usual shortcut ( expensive equals good ) short circuit your buying decision. Experiments have consistently shown that we can be fooled into thinking something is better just because it s more expensive.

Take, for example, an experiment conducted by Stanford neuroeconomist Baba Shiv. Shiv had a group of people solve word puzzles after having them purchase the energy drink SoBe Adrenaline Rush (which claims on its package to improve mental acuity). But there was a catch: Some people bought the drink at full price, and some bought it at a discount. Despite the fact that the drink was the same with or without the discount, the people who got the discount version solved about 30% fewer puzzles.

What s particularly interesting about the Shiv result is that people not only thought the discount SoBe was worse, they actually derived less benefit from it.

The brain wants what it wants. And, often, what it wants is to waste your money.

Ryan Sager writes the blog Neuroworld at TrueSlant.com.