"Visual appeal can be assessed within 50 milliseconds, suggesting that web designers have about 50 milliseconds to make a good impression," the Canadian researchers report in the March/April issue of the journal Behaviour & Information Technology.

Dr Gitte Lindgaard and colleagues from Carleton University in Ottawa confirm that internet users are a fickle lot.

The team did this by flashing up websites for 50 milliseconds and asking participants to rate them for visual appeal.

When they repeated the exercise after a longer viewing period the ratings were consistent.

The medium is the message
The finding comes as bad news to anyone hoping to convey information, says Sue Burgess, an Australian researcher who evaluates website usability and senior lecturer in information management at the University of Technology Sydney.

"There's no doubt that people do respond very quickly to websites and decide very quickly whether to stay on them," she says.

The appeal of a website is usually tied to color, movement and interactivity, she says, with the way the information is structured coming second.

Burgess says it's unclear whether the internet is changing our ability to concentrate for long periods our if we are adapting to the medium.

"There's so much information and ... there's always going to be a lot of clicking around just to see what's there," she says.

The halo effect
Australian associate professor of psychology Bill von Hippel, from the University of New South Wales, says it takes about 50 milliseconds to read one word, making this a "stunningly remarkable" timeframe in which to process the complex stimuli on a website.

"It's quite remarkable that people do it that fast and that it holds up in their later judgment," he says.

"This may be because we have an affective or emotional system that [works] independently of our cognitive system."

In evolutionary terms, this ability helped us respond rapidly to dangerous situations, he says.

The study also reflects the so-called halo effect, von Hippel says, where an initial bias towards something drives subsequent judgments.

"This suggests that we make very quick judgments based on some sort of emotional reaction and our more considered judgments still reflect that first impression."