Internet users can take just one-twentieth of a second to decide whether they like the look of a website, researchers say.
Dr Gitte Lindgaard and colleagues from Carleton University in Ottawa flashed up websites for 50 milliseconds and asked participants to rate them for visual appeal.
When they repeated the exercise after a longer viewing period, the participants' ratings were consistent.
"Visual appeal can be assessed within 50 milliseconds, suggesting that web designers have about 50 milliseconds to make a good impression," the Canadians report in the journal Behaviour & Information Technology.
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 judgement," he said.
"This may be because we have an affective or emotional system that [works] independently of our cognitive system."
He says that in evolutionary terms, this ability helped us respond rapidly to dangerous situations.
Professor von Hippel says the study also reflects the so-called halo effect, where an initial bias towards something drives subsequent judgements.
"This suggests that we make very quick judgements based on some sort of emotional reaction and our more considered judgements still reflect that first impression," he said.
Australian researcher Sue Burgess, who evaluates website useability and is a senior lecturer in information management at the University of Technology Sydney, says the finding comes as bad news to anyone hoping to convey information.
"There's no doubt that people do respond very quickly to websites and decide very quickly whether to stay on them," she said.
She says the appeal of a website is usually tied to colour, movement and interactivity, with the way the information is structured coming second.
Ms Burgess says it is unclear whether the Internet is changing our ability to concentrate for long periods or 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 said.