Yet another reason why you should sleep on it before making an important decision

But according to a new study, sleeping on it can help us avoid judging a book solely by its cover.

In research published Sept. 9 in the Journal of Experimental Psychology: General, a team of researchers at Duke University started with an age-old question: Is it better to start strong with a good first impression, or end on a good note?

To shed some light on the issue, they did a study involving an imaginary garage sale. In a series of experiments conducted online, the researchers asked participants to look through virtual boxes of unwanted goods for items to include in the sale.

Most of the items inside each box weren't worth much -- an old alarm clock, for example, or a potted plant. A few special objects, like a nice lamp or a teddy bear, were worth more.

The participants earned real cash based on the boxes they chose, so they were motivated to figure out which boxes were most valuable.

Unbeknown to the participants, however, the combined total value of the 20 items in each box was the same. It was the sequence of the "junk" versus the "gems" that varied.

In some of the boxes, all the valuable items were on top, so as the participants unpacked the box they spotted those items first. Other boxes had their valuable items clustered in the middle or at the bottom, and in some boxes they were intermixed.

After the participants had opened the different boxes, the researchers asked them to estimate the value of each one and choose their favorites. Some participants judged the boxes immediately, but others "slept on it" and decided after an overnight delay.

A pattern quickly emerged: When the participants had to make a decision right away, they tended to remember and judge boxes not by the entirety of their contents, but rather by the first few items they came across.

"We found that people are strongly biased by first impressions," said lead author Allie Sinclair, who did the research as part of her Ph.D. in the lab of Dr. Alison Adcock, a Duke professor of psychiatry and behavioral sciences.

Over and over again, the participants went for boxes with valuable items on top. When they spotted these "treasures" first before the low-priced items, they were more likely to pick that box than if they had seen the cheap stuff first.

Not only did the participants consistently go for the boxes that "started strong" over the others, they also tended to overestimate their value -- guessing they were worth 10% more money than they actually were.

This is an example of a psychological phenomenon called primacy bias, said Sinclair, who is now a postdoctoral fellow at the University of Pennsylvania.

When it comes to forming an overall opinion of something, it turns out we are unduly influenced by the first information we encounter, even when new facts come to light.

In the case of the garage sale experiment, this bias prevented participants from comparing the boxes rationally, and even led them to believe that some boxes were more valuable than they really were. At the same time, ironically, they were less able to recall specifics when asked which items in these preferred boxes were the "treasures."

However, participants who weren't asked to decide until the next day were less likely to fall into these traps.

"They made more rational choices, equally favoring boxes with clusters of valuable items at the beginning, middle, or end," Sinclair said.

Participants who "slept on it" no longer overwhelmingly preferred the boxes that made a good first impression. Boxes that saved the best for last were weighted equally favorably in their mental calculus.

"Judging from first impressions may actually be a good thing for choices in the moment," Adcock said. Say you're watching the opening scene of a movie or skimming the first few pages of a book. Quick snap judgements based on these initial impressions can help us decide when it might be better to move on before we invest too much time and effort.

But when it comes to situations with longer-term stakes -- for example, going back to a restaurant, or hiring or dating, -- "there's wisdom in the idea of 'sleeping on it' before making a decision," Sinclair said.

"This is an exciting first look at how our brains summarize a rewarding experience," Adcock added. "When it's over, our brain knits it all together in memory to help us make better choices -- and that neat trick happens overnight."

This research was supported by a Duke University Duke Health Scholars Award to Alison Adcock.