TORONTO, Ontario (Reuters) -- Internet users can give Web sites a thumbs up or thumbs down in less than the blink of an eye, according to a study by Canadian researchers.

In just a brief one-twentieth of a second -- less than half the time it takes to blink -- people make aesthetic judgments that influence the rest of their experience with an Internet site.

The study was published in the latest issue of the Behaviour and Information Technology journal. The author said the findings had powerful implications for the field of Web site design.

"It really is just a physiological response," Gitte Lindgaard told Reuters on Tuesday. "So Web designers have to make sure they're not offending users visually.

"If the first impression is negative, you'll probably drive people off."

In the study, researchers discovered that people could rate the visual appeal of sites after seeing them for just one-twentieth of a second. These judgments were not random, the researchers found -- sites that were flashed up twice were given similar ratings both times.

They also matched the responses given by subjects who were shown the sites for longer.

But the results did not show how to win a positive reaction from users, said Lindgaard, a psychology professor at Carleton University in Ottawa. "When we looked at the Web sites that we tested, there is really nothing there that tells us what leads to dislike or to like."

And while further research may offer more clues, she said the vagaries of personal taste would always be a limiting factor.

"If design were reducible to a set of principles, wouldn't we find an awful lot of similar houses, gardens, cars, rooms?" said Lindgaard. "You'd have no variety."