It takes less than 50 milliseconds for users to make a snap judgement about whether they like the look of a website. First impressions count and the message is - keep it simple and don't be afraid of looking like other sites.

These findings come from an experimental study reported on the Google Research blog and conducted by psychologists at the University of Basel, Switzerland together with Javier Bargas-Avila, Senior User Experience Researcher at YouTube UX Research. It set out to investigate how users' first impressions of websites are influenced by two design factors:

1. Visual complexity: how complex the visual design of a website looks 
2. Prototypicality: how representative a design looks for the category of websites in belongs to

Two experiments were carried out using screenshots of existing company web pages that varied in both visual complexity and prototypicality with participants being asked to rate their "beauty", a dimension related to their "gut feeling" about whether they wanted to visit the site or continue surfing to other sites.

The results showed that both visual complexity and prototypicality play crucial roles in the process of forming an aesthetic judgment and that the two factors are interrelated: if the visual complexity of a website is high, users perceive it as less beautiful, even if the design is familiar. And if the design is unfamiliar, i.e. low prototypicality, users judge it as uglier, even if it’s simple. Overall, websites with low visual complexity and high prototypicality were perceived as highly appealing.

In the first experiment presentation times of 50, 500 and 1000 milliseconds were used and results revealed that participants made their judgments within the first 50 milliseconds. In the second, presentation times were shortened to 17, 33 and 50 milliseconds and at the shortest times the effect of prototypicality is less pronounced than that of visual complexity.

Full details of the study are published in the International Journal of Human-Computer Studies, vol. 70(11) (2012), pp. 794-811.