During an interview, your potential new boss asks you to briefly describe your qualifications. At this moment, you have a single objective: be impressive. So you begin to rattle off your list of accomplishments: your degrees from Harvard and Yale, your prestigious internships, your intimate knowledge of essential software and statistical analysis. “Oh,” you add. “And I took two semesters of Spanish in college.” Not technically an impressive accomplishment, but since the company does a lot of business in Latin America, you figure some Spanish is better than none at all.

Or is it?

Actually, it isn’t. You’ve just fallen victim to a phenomenon that psychologists have recently discovered, called the “Presenter’s Paradox.” It’s another fascinating example of how our instincts about selling — ourselves, our company, or our products — can be surprisingly bad.

The problem, in a nutshell, is this: We assume when we present someone with a list of our accomplishments (or with a bundle of services or products), that they will see what we’re offering additively. If going to Harvard, a prestigious internship, and mad statistical skills are all a “10” on the scale of impressiveness, and two semesters of Spanish is a “2,” then we reason that added together, this is a 10 + 10 + 10 + 2, or a “32” in impressiveness. So it makes sense to mention your minimal Spanish skills — they add to the overall picture. More is better.

Only more is not in fact better to the interviewer (or the client or buyer), because this is not how other people see what we’re offering. They don’t add up the impressiveness, they average it. They see the Big Picture — looking at the package as a whole, rather than focusing on the individual parts.

To them, this is a (10+ 10+ 10+ 2)/4 package, or an “8” in impressiveness. And if you had left off the bit about Spanish, you would have had a (10 + 10+ 10)/3, or a “10” in impressiveness. So even though logically it seems like a little Spanish is better than none, mentioning it makes you a less attractive candidate than if you’d said nothing at all.

More is actually not better, if what you are adding is of lesser quality than the rest of your offerings. Highly favorable or positive things are diminished or diluted in the eye of the beholder when they are presented in the company of only moderately favorable or positive things.

Psychologists Kimberlee Weaver, Stephen Garcia, and Norbert Schwarz recently illustrated the Presenter’s Paradox in an elegant series of studies. For example, they showed that when buyers were presented with an iPod Touch package that contained either an iPod, cover, and one free song download, or just an iPod and cover, they were willing to pay an average of $177 for the package with the download, and $242 for the one without the download. So the addition of the low-value free song download brought down the perceived value of the package by a whopping $65! Perhaps most troubling, when a second set of participants were asked to play the role of marketer and choose which of the two packages they thought would be more attractive to buyers, 92% of them chose the package with the free download.

More just seems like it must be better when you are on the presenter’s end, even though it doesn’t seem that way at all when you are on the consumer’s end. And somehow, despite the fact that we are all both presenters and consumers in our everyday lives, we just don’t make the connection.

The same pattern emergences when you are creating deterrents or negative consequences to discourage bad behavior. In another study, participants were asked to choose between two punishments to give for littering: a $750 fine plus two hours of community service, or a $750 fine. 86% of participants felt that the fine plus community service would be the stronger deterrent. But they were wrong — in fact, a separate set of participants rated the $750 with the two hours of community service as significantly less severe than the fine alone. Once again, they reasoned that the overall punishment was on average less awful because two hours of community service isn’t so bad.

If the bias in presenter thinking is so pervasive, how can we stop ourselves from making this kind of mistake? The short answer is that we need to remind ourselves when making any kind of presentation to think of the big picture. What does the package I am presenting look like taken as a whole, and are there any components that are actually bringing down its overall value or impact? Three 10’s and a 2 is not better than three 10’s. A free carwash with the purchase of any new car is not going to make your cars seem more valuable. If your very expensive luxury hotel rooms offer ocean views, silk sheets, and a Jacuzzi, don’t mention the ironing board in the closet or the coffeepot. And unless you speak Spanish well, keep your ability to count to ocho and ask where la biblioteca is to yourself.