The human brain engages in all sorts of mental shortcuts in order to make quick judgements about the world, and some of them make us vulnerable to marketing. For example, consumers will readily attach weight to completely fictitious product statistics, preferring items with the most bogomips, even if they have no idea of what the significance of that figure is. That may be disappointing, but apparently it's even worse than that—even when they do know what the units are, people tend to prefer a bigger number. As a newly released study shows, people would rather pay for expedited service to get things in 31 days than they would to get it in one month.

The researchers involved in the new work cite an extensive list of some of the counterintuitive judgements people make when it comes to numbers. For example, we do dumb things when it comes to foreign exchange. When we shop in currencies with a favorable exchange rate, we tend to spend less, because handing over a large quantity of currency strikes us as overspending, regardless of the currency's relative value. In the same way, we tend to overspend currencies with a high value.

The authors argue that this may occur simply because we don't understand the units. It's hard to do some conversions in our heads, or we may not have all the relevant information to do them. But what happens when we do know the units—is bigger still better? The authors devised a series of tests to find out.

In the first set of tests, the authors used two different scales, either 1-10 or 1-1,000, to describe television quality and the success rate of a medical procedure; they also fed the subjects warranty lengths in either months or years. Even when the length of time was the same—seven versus nine years, or 94 versus 108 months—the subjects tended to prefer the bigger number. The same thing happened for medical procedures, where an equivalent effectiveness was preferred when it was rated on a thousand-point scale.

The same sort of thing occurred when they offered students a snack and listed its energy content in either kilocalories or a much smaller unit, the kilojoule. Depending on whether they were interested in watching their energy intake, the alternate figures had the expected impact: if you wanted fewer calories, you tended to prefer seeing things expressed in kilocalories, since that produced a smaller number.

The clearer indication of the unit effect came when the subjects were asked to consider paying for expedited service. Told there was a wait time of several months, the students were asked if they were willing to pay more to get an item in a shorter amount of time: a month or, alternately, 31 days. More students were interested in paying to get the item in 31 days; a month-long wait for delivery wasn't as appealing. All the researchers had to do was insert a reminder—tell the students that a month is roughly 31 days—and the unit effect vanished.

(The one oddity of this experiment is that the students were asked to envision waiting for an item like a CD or cell phone. Unless their hearts were set on a white iPhone, however, these sort of extended waiting times seem very artificial.)

The fact that we humans aren't especially brilliant when we make snap judgements shouldn't surprise anyone. But it is a bit of a shock that so many people seemed to need a reminder to consider that a month lasts about 31 days. The authors think that, rather than being unable to interpret the units involved (as is the case with currency), most people simply choose to ignore them entirely.

What's really striking, however, is that the researchers can't really explain why reminding anyone that a month is 31 days seems to wipe the effect out. The best guess they can offer is that it simply compels people to slow down and think a little bit.

Journal of Consumer Research, 2011. DOI: Not yet available.